README.adoc

    The Ruby Style Guide

    Introduction

    Role models are important.

    — Officer Alex J. Murphy / RoboCop

    This Ruby style guide recommends best practices so that real-world Ruby programmers can write code that can be maintained by other real-world Ruby programmers. A style guide that reflects real-world usage gets used, while a style guide that holds to an ideal that has been rejected by the people it is supposed to help risks not getting used at all - no matter how good it is.

    The guide is separated into several sections of related guidelines. We’ve tried to add the rationale behind the guidelines (if it’s omitted we’ve assumed it’s pretty obvious).

    We didn’t come up with all the guidelines out of nowhere - they are mostly based on the professional experience of the editors, feedback and suggestions from members of the Ruby community and various highly regarded Ruby programming resources, such as "Programming Ruby" and "The Ruby Programming Language".

    This style guide evolves over time as additional conventions are identified and past conventions are rendered obsolete by changes in Ruby itself.

    If you’re into Rails or RSpec you might want to check out the complementary Ruby on Rails Style Guide and RSpec Style Guide.

    RuboCop is a static code analyzer (linter) and formatter, based on this style guide.

    Guiding Principles

    Programs must be written for people to read, and only incidentally for machines to execute.

    — Harold Abelson
    Structure and Interpretation of Computer Programs

    It’s common knowledge that code is read much more often than it is written. The guidelines provided here are intended to improve the readability of code and make it consistent across the wide spectrum of Ruby code. They are also meant to reflect real-world usage of Ruby instead of a random ideal. When we had to choose between a very established practice and a subjectively better alternative we’ve opted to recommend the established practice.[1]

    There are some areas in which there is no clear consensus in the Ruby community regarding a particular style (like string literal quoting, spacing inside hash literals, dot position in multi-line method chaining, etc.). In such scenarios all popular styles are acknowledged and it’s up to you to pick one and apply it consistently.

    Ruby had existed for over 15 years by the time the guide was created, and the language’s flexibility and lack of common standards have contributed to the creation of numerous styles for just about everything. Rallying people around the cause of community standards took a lot of time and energy, and we still have a lot of ground to cover.

    Ruby is famously optimized for programmer happiness. We’d like to believe that this guide is going to help you optimize for maximum programmer happiness.

    A Note about Consistency

    A foolish consistency is the hobgoblin of little minds, adored by little statesmen and philosophers and divines.

    — Ralph Waldo Emerson

    A style guide is about consistency. Consistency with this style guide is important. Consistency within a project is more important. Consistency within one class or method is the most important.

    However, know when to be inconsistent — sometimes style guide recommendations just aren’t applicable. When in doubt, use your best judgment. Look at other examples and decide what looks best. And don’t hesitate to ask!

    In particular: do not break backwards compatibility just to comply with this guide!

    Some other good reasons to ignore a particular guideline:

    • When applying the guideline would make the code less readable, even for someone who is used to reading code that follows this style guide.

    • To be consistent with surrounding code that also breaks it (maybe for historic reasons) — although this is also an opportunity to clean up someone else’s mess (in true XP style).

    • Because the code in question predates the introduction of the guideline and there is no other reason to be modifying that code.

    • When the code needs to remain compatible with older versions of Ruby that don’t support the feature recommended by the style guide.

    Translations

    Translations of the guide are available in the following languages:

    These translations are not maintained by our editor team, so their quality and level of completeness may vary. The translated versions of the guide often lag behind the upstream English version.

    Source Code Layout

    Nearly everybody is convinced that every style but their own is ugly and unreadable. Leave out the "but their own" and they’re probably right…​

    — Jerry Coffin (on indentation)

    Source Encoding

    Use UTF-8 as the source file encoding.

    UTF-8 has been the default source file encoding since Ruby 2.0.

    Tabs or Spaces?

    Use only spaces for indentation. No hard tabs.

    Indentation

    Use two spaces per indentation level (aka soft tabs).

    # bad - four spaces
    def some_method
        do_something
    end
    
    # good
    def some_method
      do_something
    end

    Maximum Line Length

    Limit lines to 80 characters.

    Most editors and IDEs have configuration options to help you with that. They would typically highlight lines that exceed the length limit.
    Why Bother with 80 characters in a World of Modern Widescreen Displays?

    A lot of people these days feel that a maximum line length of 80 characters is just a remnant of the past and makes little sense today. After all - modern displays can easily fit 200+ characters on a single line. Still, there are some important benefits to be gained from sticking to shorter lines of code.

    First, and foremost - numerous studies have shown that humans read much faster vertically and very long lines of text impede the reading process. As noted earlier, one of the guiding principles of this style guide is to optimize the code we write for human consumption.

    Additionally, limiting the required editor window width makes it possible to have several files open side-by-side, and works well when using code review tools that present the two versions in adjacent columns.

    The default wrapping in most tools disrupts the visual structure of the code, making it more difficult to understand. The limits are chosen to avoid wrapping in editors with the window width set to 80, even if the tool places a marker glyph in the final column when wrapping lines. Some web based tools may not offer dynamic line wrapping at all.

    Some teams strongly prefer a longer line length. For code maintained exclusively or primarily by a team that can reach agreement on this issue, it is okay to increase the line length limit up to 100 characters, or all the way up to 120 characters. Please, restrain the urge to go beyond 120 characters.

    No Trailing Whitespace

    Avoid trailing whitespace.

    Most editors and IDEs have configuration options to visualize trailing whitespace and to remove it automatically on save.

    Line Endings

    Use Unix-style line endings.[2]

    If you’re using Git you might want to add the following configuration setting to protect your project from Windows line endings creeping in:

    $ git config --global core.autocrlf true

    Should I Terminate Files with a Newline?

    End each file with a newline.

    This should be done via editor configuration, not manually.

    Should I Terminate Expressions with ;?

    Don’t use ; to terminate statements and expressions.

    # bad
    puts 'foobar'; # superfluous semicolon
    
    # good
    puts 'foobar'

    One Expression Per Line

    Use one expression per line.

    # bad
    puts 'foo'; puts 'bar' # two expressions on the same line
    
    # good
    puts 'foo'
    puts 'bar'
    
    puts 'foo', 'bar' # this applies to puts in particular

    Spaces and Operators

    Use spaces around operators, after commas, colons and semicolons. Whitespace might be (mostly) irrelevant to the Ruby interpreter, but its proper use is the key to writing easily readable code.

    # bad
    sum=1+2
    a,b=1,2
    class FooError<StandardError;end
    
    # good
    sum = 1 + 2
    a, b = 1, 2
    class FooError < StandardError; end

    There are a few exceptions:

    • Exponent operator:

    # bad
    e = M * c ** 2
    
    # good
    e = M * c**2
    • Slash in rational literals:

    # bad
    o_scale = 1 / 48r
    
    # good
    o_scale = 1/48r
    • Safe navigation operator:

    # bad
    foo &. bar
    foo &.bar
    foo&. bar
    
    # good
    foo&.bar

    Spaces and Braces

    No spaces after (, [ or before ], ). Use spaces around { and before }.

    # bad
    some( arg ).other
    [ 1, 2, 3 ].each{|e| puts e}
    
    # good
    some(arg).other
    [1, 2, 3].each { |e| puts e }

    { and } deserve a bit of clarification, since they are used for block and hash literals, as well as string interpolation.

    For hash literals two styles are considered acceptable. The first variant is slightly more readable (and arguably more popular in the Ruby community in general). The second variant has the advantage of adding visual difference between block and hash literals. Whichever one you pick - apply it consistently.

    # good - space after { and before }
    { one: 1, two: 2 }
    
    # good - no space after { and before }
    {one: 1, two: 2}

    With interpolated expressions, there should be no padded-spacing inside the braces.

    # bad
    "From: #{ user.first_name }, #{ user.last_name }"
    
    # good
    "From: #{user.first_name}, #{user.last_name}"

    No Space after Bang

    No space after !.

    # bad
    ! something
    
    # good
    !something

    No Space inside Range Literals

    No space inside range literals.

    # bad
    1 .. 3
    'a' ... 'z'
    
    # good
    1..3
    'a'...'z'

    Indent when to case

    Indent when as deep as case.

    # bad
    case
      when song.name == 'Misty'
        puts 'Not again!'
      when song.duration > 120
        puts 'Too long!'
      when Time.now.hour > 21
        puts "It's too late"
      else
        song.play
    end
    
    # good
    case
    when song.name == 'Misty'
      puts 'Not again!'
    when song.duration > 120
      puts 'Too long!'
    when Time.now.hour > 21
      puts "It's too late"
    else
      song.play
    end
    A Bit of History

    This is the style established in both "The Ruby Programming Language" and "Programming Ruby". Historically it is derived from the fact that case and switch statements are not blocks, hence should not be indented, and the when and else keywords are labels (compiled in the C language, they are literally labels for JMP calls).

    Indent Conditional Assignment

    When assigning the result of a conditional expression to a variable, preserve the usual alignment of its branches.

    # bad - pretty convoluted
    kind = case year
    when 1850..1889 then 'Blues'
    when 1890..1909 then 'Ragtime'
    when 1910..1929 then 'New Orleans Jazz'
    when 1930..1939 then 'Swing'
    when 1940..1950 then 'Bebop'
    else 'Jazz'
    end
    
    result = if some_cond
      calc_something
    else
      calc_something_else
    end
    
    # good - it's apparent what's going on
    kind = case year
           when 1850..1889 then 'Blues'
           when 1890..1909 then 'Ragtime'
           when 1910..1929 then 'New Orleans Jazz'
           when 1930..1939 then 'Swing'
           when 1940..1950 then 'Bebop'
           else 'Jazz'
           end
    
    result = if some_cond
               calc_something
             else
               calc_something_else
             end
    
    # good (and a bit more width efficient)
    kind =
      case year
      when 1850..1889 then 'Blues'
      when 1890..1909 then 'Ragtime'
      when 1910..1929 then 'New Orleans Jazz'
      when 1930..1939 then 'Swing'
      when 1940..1950 then 'Bebop'
      else 'Jazz'
      end
    
    result =
      if some_cond
        calc_something
      else
        calc_something_else
      end

    Empty Lines between Methods

    Use empty lines between method definitions and also to break up methods into logical paragraphs internally.

    # bad
    def some_method
      data = initialize(options)
      data.manipulate!
      data.result
    end
    def some_other_method
      result
    end
    
    # good
    def some_method
      data = initialize(options)
    
      data.manipulate!
    
      data.result
    end
    
    def some_other_method
      result
    end

    Two or More Empty Lines

    Don’t use several empty lines in a row.

    # bad - It has two empty lines.
    some_method
    
    
    some_method
    
    # good
    some_method
    
    some_method

    Empty Lines around Attribute Accessor

    Use empty lines around attribute accessor.

    # bad
    class Foo
      attr_reader :foo
      def foo
        # do something...
      end
    end
    
    # good
    class Foo
      attr_reader :foo
    
      def foo
        # do something...
      end
    end

    Empty Lines around Access Modifier

    Use empty lines around access modifier.

    # bad
    class Foo
      def bar; end
      private
      def baz; end
    end
    
    # good
    class Foo
      def bar; end
    
      private
    
      def baz; end
    end

    Empty Lines around Bodies

    Don’t use empty lines around method, class, module, block bodies.

    # bad
    class Foo
    
      def foo
    
        begin
    
          do_something do
    
            something
    
          end
    
        rescue
    
          something
    
        end
    
        true
    
      end
    
    end
    
    # good
    class Foo
      def foo
        begin
          do_something do
            something
          end
        rescue
          something
        end
      end
    end

    Trailing Comma in Method Arguments

    Avoid comma after the last parameter in a method call, especially when the parameters are not on separate lines.

    # bad - easier to move/add/remove parameters, but still not preferred
    some_method(
      size,
      count,
      color,
    )
    
    # bad
    some_method(size, count, color, )
    
    # good
    some_method(size, count, color)

    Spaces around Equals

    Use spaces around the = operator when assigning default values to method parameters:

    # bad
    def some_method(arg1=:default, arg2=nil, arg3=[])
      # do something...
    end
    
    # good
    def some_method(arg1 = :default, arg2 = nil, arg3 = [])
      # do something...
    end

    While several Ruby books suggest the first style, the second is much more prominent in practice (and arguably a bit more readable).

    Line Continuation in Expressions

    Avoid line continuation with \ where not required. In practice, avoid using line continuations for anything but string concatenation.

    # bad (\ is not needed here)
    result = 1 - \
             2
    
    # bad (\ is required, but still ugly as hell)
    result = 1 \
             - 2
    
    # good
    result = 1 -
             2
    
    long_string = 'First part of the long string' \
                  ' and second part of the long string'

    Multi-line Method Chains

    Adopt a consistent multi-line method chaining style. There are two popular styles in the Ruby community, both of which are considered good - leading . and trailing ..

    Leading .

    When continuing a chained method call on another line, keep the . on the second line.

    # bad - need to consult first line to understand second line
    one.two.three.
      four
    
    # good - it's immediately clear what's going on the second line
    one.two.three
      .four

    Trailing .

    When continuing a chained method call on another line, include the . on the first line to indicate that the expression continues.

    # bad - need to read ahead to the second line to know that the chain continues
    one.two.three
      .four
    
    # good - it's immediately clear that the expression continues beyond the first line
    one.two.three.
      four

    A discussion on the merits of both alternative styles can be found here.

    Method Arguments Alignment

    Align the arguments of a method call if they span more than one line. When aligning arguments is not appropriate due to line-length constraints, single indent for the lines after the first is also acceptable.

    # starting point (line is too long)
    def send_mail(source)
      Mailer.deliver(to: 'bob@example.com', from: 'us@example.com', subject: 'Important message', body: source.text)
    end
    
    # bad (double indent)
    def send_mail(source)
      Mailer.deliver(
          to: 'bob@example.com',
          from: 'us@example.com',
          subject: 'Important message',
          body: source.text)
    end
    
    # good
    def send_mail(source)
      Mailer.deliver(to: 'bob@example.com',
                     from: 'us@example.com',
                     subject: 'Important message',
                     body: source.text)
    end
    
    # good (normal indent)
    def send_mail(source)
      Mailer.deliver(
        to: 'bob@example.com',
        from: 'us@example.com',
        subject: 'Important message',
        body: source.text
      )
    end

    Implicit Options Hash

    As of Ruby 2.7 braces around an options hash are no longer optional.

    Omit the outer braces around an implicit options hash.

    # bad
    user.set({ name: 'John', age: 45, permissions: { read: true } })
    
    # good
    user.set(name: 'John', age: 45, permissions: { read: true })

    DSL Method Calls

    Omit both the outer braces and parentheses for methods that are part of an internal DSL.

    class Person < ActiveRecord::Base
      # bad
      validates(:name, { presence: true, length: { within: 1..10 } })
    
      # good
      validates :name, presence: true, length: { within: 1..10 }
    end

    Space in Method Calls

    Do not put a space between a method name and the opening parenthesis.

    # bad
    puts (x + y)
    
    # good
    puts(x + y)

    Space in Brackets Access

    Do not put a space between a receiver name and the opening brackets.

    # bad
    collection [index_or_key]
    
    # good
    collection[index_or_key]

    Multi-line Arrays Alignment

    Align the elements of array literals spanning multiple lines.

    # bad - single indent
    menu_item = %w[Spam Spam Spam Spam Spam Spam Spam Spam
      Baked beans Spam Spam Spam Spam Spam]
    
    # good
    menu_item = %w[
      Spam Spam Spam Spam Spam Spam Spam Spam
      Baked beans Spam Spam Spam Spam Spam
    ]
    
    # good
    menu_item =
      %w[Spam Spam Spam Spam Spam Spam Spam Spam
         Baked beans Spam Spam Spam Spam Spam]

    Naming Conventions

    The only real difficulties in programming are cache invalidation and naming things.

    — Phil Karlton

    English for Identifiers

    Name identifiers in English.

    # bad - identifier is a Bulgarian word, using non-ascii (Cyrillic) characters
    заплата = 1_000
    
    # bad - identifier is a Bulgarian word, written with Latin letters (instead of Cyrillic)
    zaplata = 1_000
    
    # good
    salary = 1_000

    Snake Case for Symbols, Methods and Variables

    Use snake_case for symbols, methods and variables.

    # bad
    :'some symbol'
    :SomeSymbol
    :someSymbol
    
    someVar = 5
    
    def someMethod
      # some code
    end
    
    def SomeMethod
      # some code
    end
    
    # good
    :some_symbol
    
    some_var = 5
    
    def some_method
      # some code
    end

    Identifiers with a Numeric Suffix

    Do not separate numbers from letters on symbols, methods and variables.

    # bad
    :some_sym_1
    
    some_var_1 = 1
    
    var_10 = 10
    
    def some_method_1
      # some code
    end
    
    # good
    :some_sym1
    
    some_var1 = 1
    
    var10 = 10
    
    def some_method1
      # some code
    end

    CapitalCase for Classes and Modules

    CapitalCase is also known as UpperCamelCase, `CapitalWords and PascalCase.

    Use CapitalCase for classes and modules. (Keep acronyms like HTTP, RFC, XML uppercase).

    # bad
    class Someclass
      # some code
    end
    
    class Some_Class
      # some code
    end
    
    class SomeXml
      # some code
    end
    
    class XmlSomething
      # some code
    end
    
    # good
    class SomeClass
      # some code
    end
    
    class SomeXML
      # some code
    end
    
    class XMLSomething
      # some code
    end

    Snake Case for Files

    Use snake_case for naming files, e.g. hello_world.rb.

    Snake Case for Directories

    Use snake_case for naming directories, e.g. lib/hello_world/hello_world.rb.

    One Class per File

    Aim to have just a single class/module per source file. Name the file name as the class/module, but replacing CapitalCase with snake_case.

    Screaming Snake Case for Constants

    Use SCREAMING_SNAKE_CASE for other constants (those that don’t refer to classes and modules).

    # bad
    SomeConst = 5
    
    # good
    SOME_CONST = 5

    Predicate Methods Suffix

    The names of predicate methods (methods that return a boolean value) should end in a question mark (i.e. Array#empty?). Methods that don’t return a boolean, shouldn’t end in a question mark.

    # bad
    def even(value)
    end
    
    # good
    def even?(value)
    end

    Predicate Methods Prefix

    Avoid prefixing predicate methods with the auxiliary verbs such as is, does, or can. These words are redundant and inconsistent with the style of boolean methods in the Ruby core library, such as empty? and include?.

    # bad
    class Person
      def is_tall?
        true
      end
    
      def can_play_basketball?
        false
      end
    
      def does_like_candy?
        true
      end
    end
    
    # good
    class Person
      def tall?
        true
      end
    
      def basketball_player?
        false
      end
    
      def likes_candy?
        true
      end
    end

    Dangerous Method Suffix

    The names of potentially dangerous methods (i.e. methods that modify self or the arguments, exit! (doesn’t run the finalizers like exit does), etc) should end with an exclamation mark if there exists a safe version of that dangerous method.

    # bad - there is no matching 'safe' method
    class Person
      def update!
      end
    end
    
    # good
    class Person
      def update
      end
    end
    
    # good
    class Person
      def update!
      end
    
      def update
      end
    end

    Relationship between Safe and Dangerous Methods

    Define the non-bang (safe) method in terms of the bang (dangerous) one if possible.

    class Array
      def flatten_once!
        res = []
    
        each do |e|
          [*e].each { |f| res << f }
        end
    
        replace(res)
      end
    
      def flatten_once
        dup.flatten_once!
      end
    end

    Unused Variables Prefix

    Prefix with _ unused block parameters and local variables. It’s also acceptable to use just _ (although it’s a bit less descriptive). This convention is recognized by the Ruby interpreter and tools like RuboCop will suppress their unused variable warnings.

    # bad
    result = hash.map { |k, v| v + 1 }
    
    def something(x)
      unused_var, used_var = something_else(x)
      # some code
    end
    
    # good
    result = hash.map { |_k, v| v + 1 }
    
    def something(x)
      _unused_var, used_var = something_else(x)
      # some code
    end
    
    # good
    result = hash.map { |_, v| v + 1 }
    
    def something(x)
      _, used_var = something_else(x)
      # some code
    end

    other Parameter

    When defining binary operators and operator-alike methods, name the parameter other for operators with "symmetrical" semantics of operands. Symmetrical semantics means both sides of the operator are typically of the same or coercible types.

    Operators and operator-alike methods with symmetrical semantics (the parameter should be named other): +, -, *, /, %, **, ==, >, <, |, &, ^, eql?, equal?.

    Operators with non-symmetrical semantics (the parameter should not be named other): <<, [] (collection/item relations between operands), === (pattern/matchable relations).

    Note that the rule should be followed only if both sides of the operator have the same semantics. Prominent exception in Ruby core is, for example, Array#*(int).

    # good
    def +(other)
      # body omitted
    end
    
    # bad
    def <<(other)
      @internal << other
    end
    
    # good
    def <<(item)
      @internal << item
    end
    
    # bad
    # Returns some string multiplied `other` times
    def *(other)
      # body omitted
    end
    
    # good
    # Returns some string multiplied `num` times
    def *(num)
      # body omitted
    end

    Flow of Control

    for Loops

    Do not use for, unless you know exactly why. Most of the time iterators should be used instead. for is implemented in terms of each (so you’re adding a level of indirection), but with a twist - for doesn’t introduce a new scope (unlike each) and variables defined in its block will be visible outside it.

    arr = [1, 2, 3]
    
    # bad
    for elem in arr do
      puts elem
    end
    
    # note that elem is accessible outside of the for loop
    elem # => 3
    
    # good
    arr.each { |elem| puts elem }
    
    # elem is not accessible outside each block
    elem # => NameError: undefined local variable or method `elem'

    then in Multi-line Expression

    Do not use then for multi-line if/unless/when/in.

    # bad
    if some_condition then
      # body omitted
    end
    
    # bad
    case foo
    when bar then
      # body omitted
    end
    
    # bad
    case expression
    in pattern then
      # body omitted
    end
    
    # good
    if some_condition
      # body omitted
    end
    
    # good
    case foo
    when bar
      # body omitted
    end
    
    # good
    case expression
    in pattern
      # body omitted
    end

    Condition Placement

    Always put the condition on the same line as the if/unless in a multi-line conditional.

    # bad
    if
      some_condition
      do_something
      do_something_else
    end
    
    # good
    if some_condition
      do_something
      do_something_else
    end

    Ternary Operator vs if

    Prefer the ternary operator(?:) over if/then/else/end constructs. It’s more common and obviously more concise.

    # bad
    result = if some_condition then something else something_else end
    
    # good
    result = some_condition ? something : something_else

    Nested Ternary Operators

    Use one expression per branch in a ternary operator. This also means that ternary operators must not be nested. Prefer if/else constructs in these cases.

    # bad
    some_condition ? (nested_condition ? nested_something : nested_something_else) : something_else
    
    # good
    if some_condition
      nested_condition ? nested_something : nested_something_else
    else
      something_else
    end

    Semicolon in if

    Do not use if x; …​. Use the ternary operator instead.

    # bad
    result = if some_condition; something else something_else end
    
    # good
    result = some_condition ? something : something_else

    case vs if-else

    Prefer case over if-elsif when compared value is the same in each clause.

    # bad
    if status == :active
      perform_action
    elsif status == :inactive || status == :hibernating
      check_timeout
    else
      final_action
    end
    
    # good
    case status
    when :active
      perform_action
    when :inactive, :hibernating
      check_timeout
    else
      final_action
    end

    Returning Result from if/case

    Leverage the fact that if and case are expressions which return a result.

    # bad
    if condition
      result = x
    else
      result = y
    end
    
    # good
    result =
      if condition
        x
      else
        y
      end

    One-line Cases

    Use when x then …​ for one-line cases.

    The alternative syntax when x: …​ has been removed as of Ruby 1.9.

    Semicolon in when

    Do not use when x; …​. See the previous rule.

    Semicolon in in

    Do not use in pattern; …​. Use in pattern then …​ for one-line in pattern branches.

    # bad
    case expression
    in pattern; do_something
    end
    
    # good
    case expression
    in pattern then do_something
    end

    ! vs not

    Use ! instead of not.

    # bad - parentheses are required because of op precedence
    x = (not something)
    
    # good
    x = !something

    Double Negation

    Avoid unnecessary uses of !!

    !! converts a value to boolean, but you don’t need this explicit conversion in the condition of a control expression; using it only obscures your intention.

    Consider using it only when there is a valid reason to restrict the result true or false. Examples include outputting to a particular format or API like JSON, or as the return value of a predicate? method. In these cases, also consider doing a nil check instead: !something.nil?.

    # bad
    x = 'test'
    # obscure nil check
    if !!x
      # body omitted
    end
    
    # good
    x = 'test'
    if x
      # body omitted
    end
    
    # good
    def named?
      !name.nil?
    end
    
    # good
    def banned?
      !!banned_until&.future?
    end

    and/or

    The and and or keywords are banned. The minimal added readability is just not worth the high probability of introducing subtle bugs. For boolean expressions, always use && and || instead. For flow control, use if and unless; && and || are also acceptable but less clear.

    # bad
    # boolean expression
    ok = got_needed_arguments and arguments_are_valid
    
    # control flow
    document.save or raise("Failed to save document!")
    
    # good
    # boolean expression
    ok = got_needed_arguments && arguments_are_valid
    
    # control flow
    raise("Failed to save document!") unless document.save
    
    # ok
    # control flow
    document.save || raise("Failed to save document!")
    Why Ban and and or?

    The main reason is very simple - they add a lot of cognitive overhead, as they don’t behave like similarly named operators in other languages.

    First of all, and and or operators have lower precedence than the = operator, whereas the && and || operators have higher precedence than the = operator, based on order of operations.

    foo = true and false # results in foo being equal to true. Equivalent to ( foo = true ) and false
    bar = false or true  # results in bar being equal to false. Equivalent to ( bar = false ) or true

    Also && has higher precedence than ||, where as and and or have the same one. Funny enough, even though and and or were inspired by Perl, they don’t have different precedence in Perl.

    foo = true or true and false # => false (it's effectively (true or true) and false)
    foz = true || true && false # => true (it's effectively true || (true && false)
    bar = false or true and false # => false (it's effectively (false or true) and false)
    baz = false || true && false # => false (it's effectively false || (true && false))

    Multi-line Ternary Operator

    Avoid multi-line ?: (the ternary operator); use if/unless instead.

    if as a Modifier

    Prefer modifier if/unless usage when you have a single-line body. Another good alternative is the usage of control flow &&/||.

    # bad
    if some_condition
      do_something
    end
    
    # good
    do_something if some_condition
    
    # another good option
    some_condition && do_something

    Multi-line if Modifiers

    Avoid modifier if/unless usage at the end of a non-trivial multi-line block.

    # bad
    10.times do
      # multi-line body omitted
    end if some_condition
    
    # good
    if some_condition
      10.times do
        # multi-line body omitted
      end
    end

    Nested Modifiers

    Avoid nested modifier if/unless/while/until usage. Prefer &&/|| if appropriate.

    # bad
    do_something if other_condition if some_condition
    
    # good
    do_something if some_condition && other_condition

    if vs unless

    Prefer unless over if for negative conditions (or control flow ||).

    # bad
    do_something if !some_condition
    
    # bad
    do_something if not some_condition
    
    # good
    do_something unless some_condition
    
    # another good option
    some_condition || do_something

    Using else with unless

    Do not use unless with else. Rewrite these with the positive case first.

    # bad
    unless success?
      puts 'failure'
    else
      puts 'success'
    end
    
    # good
    if success?
      puts 'success'
    else
      puts 'failure'
    end

    Parentheses around Condition

    Don’t use parentheses around the condition of a control expression.

    # bad
    if (x > 10)
      # body omitted
    end
    
    # good
    if x > 10
      # body omitted
    end
    There is an exception to this rule, namely safe assignment in condition.

    Multi-line while do

    Do not use while/until condition do for multi-line while/until.

    # bad
    while x > 5 do
      # body omitted
    end
    
    until x > 5 do
      # body omitted
    end
    
    # good
    while x > 5
      # body omitted
    end
    
    until x > 5
      # body omitted
    end

    while as a Modifier

    Prefer modifier while/until usage when you have a single-line body.

    # bad
    while some_condition
      do_something
    end
    
    # good
    do_something while some_condition

    while vs until

    Prefer until over while for negative conditions.

    # bad
    do_something while !some_condition
    
    # good
    do_something until some_condition

    Infinite Loop

    Use Kernel#loop instead of while/until when you need an infinite loop.

    # bad
    while true
      do_something
    end
    
    until false
      do_something
    end
    
    # good
    loop do
      do_something
    end

    loop with break

    Use Kernel#loop with break rather than begin/end/until or begin/end/while for post-loop tests.

    # bad
    begin
      puts val
      val += 1
    end while val < 0
    
    # good
    loop do
      puts val
      val += 1
      break unless val < 0
    end

    Explicit return

    Avoid return where not required for flow of control.

    # bad
    def some_method(some_arr)
      return some_arr.size
    end
    
    # good
    def some_method(some_arr)
      some_arr.size
    end

    Explicit self

    Avoid self where not required. (It is only required when calling a self write accessor, methods named after reserved words, or overloadable operators.)

    # bad
    def ready?
      if self.last_reviewed_at > self.last_updated_at
        self.worker.update(self.content, self.options)
        self.status = :in_progress
      end
      self.status == :verified
    end
    
    # good
    def ready?
      if last_reviewed_at > last_updated_at
        worker.update(content, options)
        self.status = :in_progress
      end
      status == :verified
    end

    Shadowing Methods

    As a corollary, avoid shadowing methods with local variables unless they are both equivalent.

    class Foo
      attr_accessor :options
    
      # ok
      def initialize(options)
        self.options = options
        # both options and self.options are equivalent here
      end
    
      # bad
      def do_something(options = {})
        unless options[:when] == :later
          output(self.options[:message])
        end
      end
    
      # good
      def do_something(params = {})
        unless params[:when] == :later
          output(options[:message])
        end
      end
    end

    Safe Assignment in Condition

    Don’t use the return value of = (an assignment) in conditional expressions unless the assignment is wrapped in parentheses. This is a fairly popular idiom among Rubyists that’s sometimes referred to as safe assignment in condition.

    # bad (+ a warning)
    if v = array.grep(/foo/)
      do_something(v)
      # some code
    end
    
    # good (MRI would still complain, but RuboCop won't)
    if (v = array.grep(/foo/))
      do_something(v)
      # some code
    end
    
    # good
    v = array.grep(/foo/)
    if v
      do_something(v)
      # some code
    end

    BEGIN Blocks

    Avoid the use of BEGIN blocks.

    END Blocks

    Do not use END blocks. Use Kernel#at_exit instead.

    # bad
    END { puts 'Goodbye!' }
    
    # good
    at_exit { puts 'Goodbye!' }

    Nested Conditionals

    Avoid use of nested conditionals for flow of control.

    Prefer a guard clause when you can assert invalid data. A guard clause is a conditional statement at the top of a function that bails out as soon as it can.

    # bad
    def compute_thing(thing)
      if thing[:foo]
        update_with_bar(thing[:foo])
        if thing[:foo][:bar]
          partial_compute(thing)
        else
          re_compute(thing)
        end
      end
    end
    
    # good
    def compute_thing(thing)
      return unless thing[:foo]
      update_with_bar(thing[:foo])
      return re_compute(thing) unless thing[:foo][:bar]
      partial_compute(thing)
    end

    Prefer next in loops instead of conditional blocks.

    # bad
    [0, 1, 2, 3].each do |item|
      if item > 1
        puts item
      end
    end
    
    # good
    [0, 1, 2, 3].each do |item|
      next unless item > 1
      puts item
    end

    Exceptions

    raise vs fail

    Prefer raise over fail for exceptions.

    # bad
    fail SomeException, 'message'
    
    # good
    raise SomeException, 'message'

    Raising Explicit RuntimeError

    Don’t specify RuntimeError explicitly in the two argument version of raise.

    # bad
    raise RuntimeError, 'message'
    
    # good - signals a RuntimeError by default
    raise 'message'

    Exception Class Messages

    Prefer supplying an exception class and a message as two separate arguments to raise, instead of an exception instance.

    # bad
    raise SomeException.new('message')
    # Note that there is no way to do `raise SomeException.new('message'), backtrace`.
    
    # good
    raise SomeException, 'message'
    # Consistent with `raise SomeException, 'message', backtrace`.

    return from ensure

    Do not return from an ensure block. If you explicitly return from a method inside an ensure block, the return will take precedence over any exception being raised, and the method will return as if no exception had been raised at all. In effect, the exception will be silently thrown away.

    # bad
    def foo
      raise
    ensure
      return 'very bad idea'
    end

    Implicit begin

    Use implicit begin blocks where possible.

    # bad
    def foo
      begin
        # main logic goes here
      rescue
        # failure handling goes here
      end
    end
    
    # good
    def foo
      # main logic goes here
    rescue
      # failure handling goes here
    end

    Contingency Methods

    Mitigate the proliferation of begin blocks by using contingency methods (a term coined by Avdi Grimm).

    # bad
    begin
      something_that_might_fail
    rescue IOError
      # handle IOError
    end
    
    begin
      something_else_that_might_fail
    rescue IOError
      # handle IOError
    end
    
    # good
    def with_io_error_handling
      yield
    rescue IOError
      # handle IOError
    end
    
    with_io_error_handling { something_that_might_fail }
    
    with_io_error_handling { something_else_that_might_fail }

    Suppressing Exceptions

    Don’t suppress exceptions.

    # bad
    begin
      do_something # an exception occurs here
    rescue SomeError
    end
    
    # good
    begin
      do_something # an exception occurs here
    rescue SomeError
      handle_exception
    end
    
    # good
    begin
      do_something # an exception occurs here
    rescue SomeError
      # Notes on why exception handling is not performed
    end
    
    # good
    do_something rescue nil

    Using rescue as a Modifier

    Avoid using rescue in its modifier form.

    # bad - this catches exceptions of StandardError class and its descendant classes
    read_file rescue handle_error($!)
    
    # good - this catches only the exceptions of Errno::ENOENT class and its descendant classes
    def foo
      read_file
    rescue Errno::ENOENT => e
      handle_error(e)
    end

    Using Exceptions for Flow of Control

    Don’t use exceptions for flow of control.

    # bad
    begin
      n / d
    rescue ZeroDivisionError
      puts 'Cannot divide by 0!'
    end
    
    # good
    if d.zero?
      puts 'Cannot divide by 0!'
    else
      n / d
    end

    Blind Rescues

    Avoid rescuing the Exception class. This will trap signals and calls to exit, requiring you to kill -9 the process.

    # bad
    begin
      # calls to exit and kill signals will be caught (except kill -9)
      exit
    rescue Exception
      puts "you didn't really want to exit, right?"
      # exception handling
    end
    
    # good
    begin
      # a blind rescue rescues from StandardError, not Exception as many
      # programmers assume.
    rescue => e
      # exception handling
    end
    
    # also good
    begin
      # an exception occurs here
    rescue StandardError => e
      # exception handling
    end

    Exception Rescuing Ordering

    Put more specific exceptions higher up the rescue chain, otherwise they’ll never be rescued from.

    # bad
    begin
      # some code
    rescue StandardError => e
      # some handling
    rescue IOError => e
      # some handling that will never be executed
    end
    
    # good
    begin
      # some code
    rescue IOError => e
      # some handling
    rescue StandardError => e
      # some handling
    end

    Reading from a file

    Use the convenience methods File.read or File.binread when only reading a file start to finish in a single operation.

    ## text mode
    # bad (only when reading from beginning to end - modes: 'r', 'rt', 'r+', 'r+t')
    File.open(filename).read
    File.open(filename, &:read)
    File.open(filename) { |f| f.read }
    File.open(filename) do |f|
      f.read
    end
    File.open(filename, 'r').read
    File.open(filename, 'r', &:read)
    File.open(filename, 'r') { |f| f.read }
    File.open(filename, 'r') do |f|
      f.read
    end
    
    # good
    File.read(filename)
    
    ## binary mode
    # bad (only when reading from beginning to end - modes: 'rb', 'r+b')
    File.open(filename, 'rb').read
    File.open(filename, 'rb', &:read)
    File.open(filename, 'rb') { |f| f.read }
    File.open(filename, 'rb') do |f|
      f.read
    end
    
    # good
    File.binread(filename)

    Writing to a file

    Use the convenience methods File.write or File.binwrite when only opening a file to create / replace its content in a single operation.

    ## text mode
    # bad (only truncating modes: 'w', 'wt', 'w+', 'w+t')
    File.open(filename, 'w').write(content)
    File.open(filename, 'w') { |f| f.write(content) }
    File.open(filename, 'w') do |f|
      f.write(content)
    end
    
    # good
    File.write(filename, content)
    
    ## binary mode
    # bad (only truncating modes: 'wb', 'w+b')
    File.open(filename, 'wb').write(content)
    File.open(filename, 'wb') { |f| f.write(content) }
    File.open(filename, 'wb') do |f|
      f.write(content)
    end
    
    # good
    File.binwrite(filename, content)

    Release External Resources

    Release external resources obtained by your program in an ensure block.

    f = File.open('testfile')
    begin
      # .. process
    rescue
      # .. handle error
    ensure
      f.close if f
    end

    Auto-release External Resources

    Use versions of resource obtaining methods that do automatic resource cleanup when possible.

    # bad - you need to close the file descriptor explicitly
    f = File.open('testfile')
    # some action on the file
    f.close
    
    # good - the file descriptor is closed automatically
    File.open('testfile') do |f|
      # some action on the file
    end

    Standard Exceptions

    Prefer the use of exceptions from the standard library over introducing new exception classes.

    Assignment & Comparison

    Parallel Assignment

    Avoid the use of parallel assignment for defining variables. Parallel assignment is allowed when it is the return of a method call, used with the splat operator, or when used to swap variable assignment. Parallel assignment is less readable than separate assignment.

    # bad
    a, b, c, d = 'foo', 'bar', 'baz', 'foobar'
    
    # good
    a = 'foo'
    b = 'bar'
    c = 'baz'
    d = 'foobar'
    
    # good - swapping variable assignment
    # Swapping variable assignment is a special case because it will allow you to
    # swap the values that are assigned to each variable.
    a = 'foo'
    b = 'bar'
    
    a, b = b, a
    puts a # => 'bar'
    puts b # => 'foo'
    
    # good - method return
    def multi_return
      [1, 2]
    end
    
    first, second = multi_return
    
    # good - use with splat
    first, *list = [1, 2, 3, 4] # first => 1, list => [2, 3, 4]
    
    hello_array = *'Hello' # => ["Hello"]
    
    a = *(1..3) # => [1, 2, 3]

    Values Swapping

    Use parallel assignment when swapping 2 values.

    # bad
    tmp = x
    x = y
    y = tmp
    
    # good
    x, y = y, x

    Dealing with Trailing Underscore Variables in Destructuring Assignment

    Avoid the use of unnecessary trailing underscore variables during parallel assignment. Named underscore variables are to be preferred over underscore variables because of the context that they provide. Trailing underscore variables are necessary when there is a splat variable defined on the left side of the assignment, and the splat variable is not an underscore.

    # bad
    foo = 'one,two,three,four,five'
    # Unnecessary assignment that does not provide useful information
    first, second, _ = foo.split(',')
    first, _, _ = foo.split(',')
    first, *_ = foo.split(',')
    
    # good
    foo = 'one,two,three,four,five'
    # The underscores are needed to show that you want all elements
    # except for the last number of underscore elements
    *beginning, _ = foo.split(',')
    *beginning, something, _ = foo.split(',')
    
    a, = foo.split(',')
    a, b, = foo.split(',')
    # Unnecessary assignment to an unused variable, but the assignment
    # provides us with useful information.
    first, _second = foo.split(',')
    first, _second, = foo.split(',')
    first, *_ending = foo.split(',')

    Self-assignment

    Use shorthand self assignment operators whenever applicable.

    # bad
    x = x + y
    x = x * y
    x = x**y
    x = x / y
    x = x || y
    x = x && y
    
    # good
    x += y
    x *= y
    x **= y
    x /= y
    x ||= y
    x &&= y

    Conditional Variable Initialization Shorthand

    Use ||= to initialize variables only if they’re not already initialized.

    # bad
    name = name ? name : 'Bozhidar'
    
    # bad
    name = 'Bozhidar' unless name
    
    # good - set name to 'Bozhidar', only if it's nil or false
    name ||= 'Bozhidar'

    Don’t use ||= to initialize boolean variables. (Consider what would happen if the current value happened to be false.)

    # bad - would set enabled to true even if it was false
    enabled ||= true
    
    # good
    enabled = true if enabled.nil?

    Existence Check Shorthand

    Use &&= to preprocess variables that may or may not exist. Using &&= will change the value only if it exists, removing the need to check its existence with if.

    # bad
    if something
      something = something.downcase
    end
    
    # bad
    something = something ? something.downcase : nil
    
    # ok
    something = something.downcase if something
    
    # good
    something = something && something.downcase
    
    # better
    something &&= something.downcase

    Identity Comparison

    Prefer equal? over == when comparing object_id. Object#equal? is provided to compare objects for identity, and in contrast Object#== is provided for the purpose of doing value comparison.

    # bad
    foo.object_id == bar.object_id
    
    # good
    foo.equal?(bar)

    Similarly, prefer using Hash#compare_by_identity than using object_id for keys:

    # bad
    hash = {}
    hash[foo.object_id] = :bar
    if hash.key?(baz.object_id) # ...
    
    # good
    hash = {}.compare_by_identity
    hash[foo] = :bar
    if hash.key?(baz) # ...

    Note that Set also has Set#compare_by_identity available.

    Explicit Use of the Case Equality Operator

    Avoid explicit use of the case equality operator ===. As its name implies it is meant to be used implicitly by case expressions and outside of them it yields some pretty confusing code.

    # bad
    Array === something
    (1..100) === 7
    /something/ === some_string
    
    # good
    something.is_a?(Array)
    (1..100).include?(7)
    some_string.match?(/something/)
    With direct subclasses of BasicObject, using is_a? is not an option since BasicObject doesn’t provide that method (it’s defined in Object). In those rare cases it’s OK to use ===.

    is_a? vs kind_of?

    Prefer is_a? over kind_of?. The two methods are synonyms, but is_a? is the more commonly used name in the wild.

    # bad
    something.kind_of?(Array)
    
    # good
    something.is_a?(Array)

    is_a? vs instance_of?

    Prefer is_a? over instance_of?.

    While the two methods are similar, is_a? will consider the whole inheritance chain (superclasses and included modules), which is what you normally would want to do. instance_of?, on the other hand, only returns true if an object is an instance of that exact class you’re checking for, not a subclass.

    # bad
    something.instance_of?(Array)
    
    # good
    something.is_a?(Array)

    instance_of? vs class comparison

    Use Object#instance_of? instead of class comparison for equality.

    # bad
    var.class == Date
    var.class.equal?(Date)
    var.class.eql?(Date)
    var.class.name == 'Date'
    
    # good
    var.instance_of?(Date)

    == vs eql?

    Do not use eql? when using == will do. The stricter comparison semantics provided by eql? are rarely needed in practice.

    # bad - eql? is the same as == for strings
    'ruby'.eql? some_str
    
    # good
    'ruby' == some_str
    1.0.eql? x # eql? makes sense here if want to differentiate between Integer and Float 1

    Blocks, Procs & Lambdas

    Proc Application Shorthand

    Use the Proc call shorthand when the called method is the only operation of a block.

    # bad
    names.map { |name| name.upcase }
    
    # good
    names.map(&:upcase)

    Single-line Blocks Delimiters

    Prefer {…​} over do…​end for single-line blocks. Avoid using {…​} for multi-line blocks (multi-line chaining is always ugly). Always use do…​end for "control flow" and "method definitions" (e.g. in Rakefiles and certain DSLs). Avoid do…​end when chaining.

    names = %w[Bozhidar Filipp Sarah]
    
    # bad
    names.each do |name|
      puts name
    end
    
    # good
    names.each { |name| puts name }
    
    # bad
    names.select do |name|
      name.start_with?('S')
    end.map { |name| name.upcase }
    
    # good
    names.select { |name| name.start_with?('S') }.map(&:upcase)

    Some will argue that multi-line chaining would look OK with the use of {…​}, but they should ask themselves - is this code really readable and can the blocks' contents be extracted into nifty methods?

    Explicit Block Argument

    Consider using explicit block argument to avoid writing block literal that just passes its arguments to another block.

    require 'tempfile'
    
    # bad
    def with_tmp_dir
      Dir.mktmpdir do |tmp_dir|
        Dir.chdir(tmp_dir) { |dir| yield dir }  # block just passes arguments
      end
    end
    
    # good
    def with_tmp_dir(&block)
      Dir.mktmpdir do |tmp_dir|
        Dir.chdir(tmp_dir, &block)
      end
    end
    
    with_tmp_dir do |dir|
      puts "dir is accessible as a parameter and pwd is set: #{dir}"
    end

    Trailing Comma in Block Parameters

    Avoid comma after the last parameter in a block, except in cases where only a single argument is present and its removal would affect functionality (for instance, array destructuring).

    # bad - easier to move/add/remove parameters, but still not preferred
    [[1, 2, 3], [4, 5, 6]].each do |a, b, c,|
      a + b + c
    end
    
    # good
    [[1, 2, 3], [4, 5, 6]].each do |a, b, c|
      a + b + c
    end
    
    # bad
    [[1, 2, 3], [4, 5, 6]].each { |a, b, c,| a + b + c }
    
    # good
    [[1, 2, 3], [4, 5, 6]].each { |a, b, c| a + b + c }
    
    # good - this comma is meaningful for array destructuring
    [[1, 2, 3], [4, 5, 6]].map { |a,| a }

    Nested Method Definitions

    Do not use nested method definitions, use lambda instead. Nested method definitions actually produce methods in the same scope (e.g. class) as the outer method. Furthermore, the "nested method" will be redefined every time the method containing its definition is called.

    # bad
    def foo(x)
      def bar(y)
        # body omitted
      end
    
      bar(x)
    end
    
    # good - the same as the previous, but no bar redefinition on every foo call
    def bar(y)
      # body omitted
    end
    
    def foo(x)
      bar(x)
    end
    
    # also good
    def foo(x)
      bar = ->(y) { ... }
      bar.call(x)
    end

    Multi-line Lambda Definition

    Use the new lambda literal syntax for single-line body blocks. Use the lambda method for multi-line blocks.

    # bad
    l = lambda { |a, b| a + b }
    l.call(1, 2)
    
    # correct, but looks extremely awkward
    l = ->(a, b) do
      tmp = a * 7
      tmp * b / 50
    end
    
    # good
    l = ->(a, b) { a + b }
    l.call(1, 2)
    
    l = lambda do |a, b|
      tmp = a * 7
      tmp * b / 50
    end

    Stabby Lambda Definition with Parameters

    Don’t omit the parameter parentheses when defining a stabby lambda with parameters.

    # bad
    l = ->x, y { something(x, y) }
    
    # good
    l = ->(x, y) { something(x, y) }

    Stabby Lambda Definition without Parameters

    Omit the parameter parentheses when defining a stabby lambda with no parameters.

    # bad
    l = ->() { something }
    
    # good
    l = -> { something }

    proc vs Proc.new

    Prefer proc over Proc.new.

    # bad
    p = Proc.new { |n| puts n }
    
    # good
    p = proc { |n| puts n }

    Proc Call

    Prefer proc.call() over proc[] or proc.() for both lambdas and procs.

    # bad - looks similar to Enumeration access
    l = ->(v) { puts v }
    l[1]
    
    # good - most compact form, but might be confusing for newcomers to Ruby
    l = ->(v) { puts v }
    l.(1)
    
    # good - a bit verbose, but crystal clear
    l = ->(v) { puts v }
    l.call(1)

    Methods

    Short Methods

    Avoid methods longer than 10 LOC (lines of code). Ideally, most methods will be shorter than 5 LOC. Empty lines do not contribute to the relevant LOC.

    Top-Level Methods

    Avoid top-level method definitions. Organize them in modules, classes or structs instead.

    It is fine to use top-level method definitions in scripts.
    # bad
    def some_method; end
    
    # good
    class SomeClass
      def some_method; end
    end

    No Single-line Methods

    Avoid single-line methods. Although they are somewhat popular in the wild, there are a few peculiarities about their definition syntax that make their use undesirable. At any rate - there should be no more than one expression in a single-line method.

    Ruby 3 introduced an alternative syntax for single-line method definitions, that’s discussed in the next section of the guide.
    # bad
    def too_much; something; something_else; end
    
    # okish - notice that the first ; is required
    def no_braces_method; body end
    
    # okish - notice that the second ; is optional
    def no_braces_method; body; end
    
    # okish - valid syntax, but no ; makes it kind of hard to read
    def some_method() body end
    
    # good
    def some_method
      body
    end

    One exception to the rule are empty-body methods.

    # good
    def no_op; end

    Endless Methods

    Only use Ruby 3.0’s endless method definitions with a single line body. Ideally, such method definitions should be both simple (a single expression) and free of side effects.

    It’s important to understand that this guideline doesn’t contradict the previous one. We still caution against the use of single-line method definitions, but if such methods are to be used, prefer endless methods.
    # bad
    def fib(x) = if x < 2
      x
    else
      fib(x - 1) + fib(x - 2)
    end
    
    # good
    def the_answer = 42
    def get_x = @x
    def square(x) = x * x
    
    # Not (so) good: has side effect
    def set_x(x) = (@x = x)
    def print_foo = puts("foo")

    Double Colons

    Use :: only to reference constants (this includes classes and modules) and constructors (like Array() or Nokogiri::HTML()). Do not use :: for regular method calls.

    # bad
    SomeClass::some_method
    some_object::some_method
    
    # good
    SomeClass.some_method
    some_object.some_method
    SomeModule::SomeClass::SOME_CONST
    SomeModule::SomeClass()

    Colon Method Definition

    Do not use :: to define class methods.

    # bad
    class Foo
      def self::some_method
      end
    end
    
    # good
    class Foo
      def self.some_method
      end
    end

    Method Definition Parentheses

    Use def with parentheses when there are parameters. Omit the parentheses when the method doesn’t accept any parameters.

    # bad
    def some_method()
      # body omitted
    end
    
    # good
    def some_method
      # body omitted
    end
    
    # bad
    def some_method_with_parameters param1, param2
      # body omitted
    end
    
    # good
    def some_method_with_parameters(param1, param2)
      # body omitted
    end

    Method Call Parentheses

    Use parentheses around the arguments of method calls, especially if the first argument begins with an open parenthesis (, as in f((3 + 2) + 1).

    # bad
    x = Math.sin y
    # good
    x = Math.sin(y)
    
    # bad
    array.delete e
    # good
    array.delete(e)
    
    # bad
    temperance = Person.new 'Temperance', 30
    # good
    temperance = Person.new('Temperance', 30)

    Method Call with No Arguments

    Always omit parentheses for method calls with no arguments.

    # bad
    Kernel.exit!()
    2.even?()
    fork()
    'test'.upcase()
    
    # good
    Kernel.exit!
    2.even?
    fork
    'test'.upcase

    Methods That are Part of an Internal DSL

    Always omit parentheses for methods that are part of an internal DSL (e.g., Rake, Rails, RSpec):

    # bad
    validates(:name, presence: true)
    # good
    validates :name, presence: true

    Methods That Have "keyword" Status in Ruby

    Always omit parentheses for methods that have "keyword" status in Ruby.

    Unfortunately, it’s not exactly clear which methods have "keyword" status. There is agreement that declarative methods have "keyword" status. However, there’s less agreement on which non-declarative methods, if any, have "keyword" status.
    Declarative Methods That Have "keyword" Status in Ruby

    Always omit parentheses for declarative methods (a.k.a. DSL methods or macro methods) that have "keyword" status in Ruby (e.g., various Module instance methods):

    class Person
      # bad
      attr_reader(:name, :age)
      # good
      attr_reader :name, :age
    
      # body omitted
    end
    Non-Declarative Methods That Have "keyword" Status in Ruby

    For non-declarative methods with "keyword" status (e.g., various Kernel instance methods), two styles are considered acceptable. By far the most popular style is to omit parentheses. Rationale: The code reads better, and method calls look more like keywords. A less-popular style, but still acceptable, is to include parentheses. Rationale: The methods have ordinary semantics, so why treat them differently, and it’s easier to achieve a uniform style by not worrying about which methods have "keyword" status. Whichever one you pick, apply it consistently.

    # good (most popular)
    puts temperance.age
    system 'ls'
    exit 1
    
    # also good (less popular)
    puts(temperance.age)
    system('ls')
    exit(1)

    Using super with Arguments

    Always use parentheses when calling super with arguments:

    # bad
    super name, age
    
    # good
    super(name, age)
    When calling super without arguments, super and super() mean different things. Decide what is appropriate for your usage.

    Too Many Params

    Avoid parameter lists longer than three or four parameters.

    Optional Arguments

    Define optional arguments at the end of the list of arguments. Ruby has some unexpected results when calling methods that have optional arguments at the front of the list.

    # bad
    def some_method(a = 1, b = 2, c, d)
      puts "#{a}, #{b}, #{c}, #{d}"
    end
    
    some_method('w', 'x') # => '1, 2, w, x'
    some_method('w', 'x', 'y') # => 'w, 2, x, y'
    some_method('w', 'x', 'y', 'z') # => 'w, x, y, z'
    
    # good
    def some_method(c, d, a = 1, b = 2)
      puts "#{a}, #{b}, #{c}, #{d}"
    end
    
    some_method('w', 'x') # => '1, 2, w, x'
    some_method('w', 'x', 'y') # => 'y, 2, w, x'
    some_method('w', 'x', 'y', 'z') # => 'y, z, w, x'

    Keyword Arguments Order

    Put required keyword arguments before optional keyword arguments. Otherwise, it’s much harder to spot optional arguments there, if they’re hidden somewhere in the middle.

    # bad
    def some_method(foo: false, bar:, baz: 10)
      # body omitted
    end
    
    # good
    def some_method(foo:, bar: false, baz: 10)
      # body omitted
    end

    Boolean Keyword Arguments

    Use keyword arguments when passing a boolean argument to a method.

    # bad
    def some_method(bar = false)
      puts bar
    end
    
    # bad - common hack before keyword args were introduced
    def some_method(options = {})
      bar = options.fetch(:bar, false)
      puts bar
    end
    
    # good
    def some_method(bar: false)
      puts bar
    end
    
    some_method            # => false
    some_method(bar: true) # => true

    Keyword Arguments vs Optional Arguments

    Prefer keyword arguments over optional arguments.

    # bad
    def some_method(a, b = 5, c = 1)
      # body omitted
    end
    
    # good
    def some_method(a, b: 5, c: 1)
      # body omitted
    end

    Keyword Arguments vs Option Hashes

    Use keyword arguments instead of option hashes.

    # bad
    def some_method(options = {})
      bar = options.fetch(:bar, false)
      puts bar
    end
    
    # good
    def some_method(bar: false)
      puts bar
    end

    Arguments Forwarding

    Use Ruby 2.7’s arguments forwarding.

    # bad
    def some_method(*args, &block)
      other_method(*args, &block)
    end
    
    # bad
    def some_method(*args, **kwargs, &block)
      other_method(*args, **kwargs, &block)
    end
    
    # bad
    # Please note that it can cause unexpected incompatible behavior
    # because `...` forwards block also.
    # https://github.com/rubocop/rubocop/issues/7549
    def some_method(*args)
      other_method(*args)
    end
    
    # good
    def some_method(...)
      other_method(...)
    end

    Private Global Methods

    If you really need "global" methods, add them to Kernel and make them private.

    Classes & Modules

    Consistent Classes

    Use a consistent structure in your class definitions.

    class Person
      # extend/include/prepend go first
      extend SomeModule
      include AnotherModule
      prepend YetAnotherModule
    
      # inner classes
      CustomError = Class.new(StandardError)
    
      # constants are next
      SOME_CONSTANT = 20
    
      # afterwards we have attribute macros
      attr_reader :name
    
      # followed by other macros (if any)
      validates :name
    
      # public class methods are next in line
      def self.some_method
      end
    
      # initialization goes between class methods and other instance methods
      def initialize
      end
    
      # followed by other public instance methods
      def some_method
      end
    
      # protected and private methods are grouped near the end
      protected
    
      def some_protected_method
      end
    
      private
    
      def some_private_method
      end
    end

    Mixin Grouping

    Split multiple mixins into separate statements.

    # bad
    class Person
      include Foo, Bar
    end
    
    # good
    class Person
      # multiple mixins go in separate statements
      include Foo
      include Bar
    end

    Single-line Classes

    Prefer a two-line format for class definitions with no body. It is easiest to read, understand, and modify.

    # bad
    FooError = Class.new(StandardError)
    
    # okish
    class FooError < StandardError; end
    
    # ok
    class FooError < StandardError
    end
    Many editors/tools will fail to understand properly the usage of Class.new. Someone trying to locate the class definition might try a grep "class FooError". A final difference is that the name of your class is not available to the inherited callback of the base class with the Class.new form. In general it’s better to stick to the basic two-line style.

    File Classes

    Don’t nest multi-line classes within classes. Try to have such nested classes each in their own file in a folder named like the containing class.

    # bad
    
    # foo.rb
    class Foo
      class Bar
        # 30 methods inside
      end
    
      class Car
        # 20 methods inside
      end
    
      # 30 methods inside
    end
    
    # good
    
    # foo.rb
    class Foo
      # 30 methods inside
    end
    
    # foo/bar.rb
    class Foo
      class Bar
        # 30 methods inside
      end
    end
    
    # foo/car.rb
    class Foo
      class Car
        # 20 methods inside
      end
    end

    Namespace Definition

    Define (and reopen) namespaced classes and modules using explicit nesting. Using the scope resolution operator can lead to surprising constant lookups due to Ruby’s lexical scoping, which depends on the module nesting at the point of definition.

    module Utilities
      class Queue
      end
    end
    
    # bad
    class Utilities::Store
      Module.nesting # => [Utilities::Store]
    
      def initialize
        # Refers to the top level ::Queue class because Utilities isn't in the
        # current nesting chain.
        @queue = Queue.new
      end
    end
    
    # good
    module Utilities
      class WaitingList
        Module.nesting # => [Utilities::WaitingList, Utilities]
    
        def initialize
          @queue = Queue.new # Refers to Utilities::Queue
        end
      end
    end

    Modules vs Classes

    Prefer modules to classes with only class methods. Classes should be used only when it makes sense to create instances out of them.

    # bad
    class SomeClass
      def self.some_method
        # body omitted
      end
    
      def self.some_other_method
        # body omitted
      end
    end
    
    # good
    module SomeModule
      module_function
    
      def some_method
        # body omitted
      end
    
      def some_other_method
        # body omitted
      end
    end

    module_function

    Prefer the use of module_function over extend self when you want to turn a module’s instance methods into class methods.

    # bad
    module Utilities
      extend self
    
      def parse_something(string)
        # do stuff here
      end
    
      def other_utility_method(number, string)
        # do some more stuff
      end
    end
    
    # good
    module Utilities
      module_function
    
      def parse_something(string)
        # do stuff here
      end
    
      def other_utility_method(number, string)
        # do some more stuff
      end
    end

    Liskov

    When designing class hierarchies make sure that they conform to the Liskov Substitution Principle.

    SOLID design

    Try to make your classes as SOLID as possible.

    Define to_s

    Always supply a proper to_s method for classes that represent domain objects.

    class Person
      attr_reader :first_name, :last_name
    
      def initialize(first_name, last_name)
        @first_name = first_name
        @last_name = last_name
      end
    
      def to_s
        "#{first_name} #{last_name}"
      end
    end

    attr Family

    Use the attr family of functions to define trivial accessors or mutators.

    # bad
    class Person
      def initialize(first_name, last_name)
        @first_name = first_name
        @last_name = last_name
      end
    
      def first_name
        @first_name
      end
    
      def last_name
        @last_name
      end
    end
    
    # good
    class Person
      attr_reader :first_name, :last_name
    
      def initialize(first_name, last_name)
        @first_name = first_name
        @last_name = last_name
      end
    end

    Accessor/Mutator Method Names

    For accessors and mutators, avoid prefixing method names with get_ and set_. It is a Ruby convention to use attribute names for accessors (readers) and attr_name= for mutators (writers).

    # bad
    class Person
      def get_name
        "#{@first_name} #{@last_name}"
      end
    
      def set_name(name)
        @first_name, @last_name = name.split(' ')
      end
    end
    
    # good
    class Person
      def name
        "#{@first_name} #{@last_name}"
      end
    
      def name=(name)
        @first_name, @last_name = name.split(' ')
      end
    end

    attr

    Avoid the use of attr. Use attr_reader and attr_accessor instead.

    # bad - creates a single attribute accessor (deprecated in Ruby 1.9)
    attr :something, true
    attr :one, :two, :three # behaves as attr_reader
    
    # good
    attr_accessor :something
    attr_reader :one, :two, :three

    Struct.new

    Consider using Struct.new, which defines the trivial accessors, constructor and comparison operators for you.

    # good
    class Person
      attr_accessor :first_name, :last_name
    
      def initialize(first_name, last_name)
        @first_name = first_name
        @last_name = last_name
      end
    end
    
    # better
    Person = Struct.new(:first_name, :last_name) do
    end

    Don’t Extend Struct.new

    Don’t extend an instance initialized by Struct.new. Extending it introduces a superfluous class level and may also introduce weird errors if the file is required multiple times.

    # bad
    class Person < Struct.new(:first_name, :last_name)
    end
    
    # good
    Person = Struct.new(:first_name, :last_name)

    Duck Typing

    Prefer duck-typing over inheritance.

    # bad
    class Animal
      # abstract method
      def speak
      end
    end
    
    # extend superclass
    class Duck < Animal
      def speak
        puts 'Quack! Quack'
      end
    end
    
    # extend superclass
    class Dog < Animal
      def speak
        puts 'Bau! Bau!'
      end
    end
    
    # good
    class Duck
      def speak
        puts 'Quack! Quack'
      end
    end
    
    class Dog
      def speak
        puts 'Bau! Bau!'
      end
    end

    No Class Vars

    Avoid the usage of class (@@) variables due to their "nasty" behavior in inheritance.

    class Parent
      @@class_var = 'parent'
    
      def self.print_class_var
        puts @@class_var
      end
    end
    
    class Child < Parent
      @@class_var = 'child'
    end
    
    Parent.print_class_var # => will print 'child'

    As you can see all the classes in a class hierarchy actually share one class variable. Class instance variables should usually be preferred over class variables.

    Leverage Access Modifiers (e.g. private and protected)

    Assign proper visibility levels to methods (private, protected) in accordance with their intended usage. Don’t go off leaving everything public (which is the default).

    Access Modifiers Indentation

    Indent the public, protected, and private methods as much as the method definitions they apply to. Leave one blank line above the visibility modifier and one blank line below in order to emphasize that it applies to all methods below it.

    # good
    class SomeClass
      def public_method
        # some code
      end
    
      private
    
      def private_method
        # some code
      end
    
      def another_private_method
        # some code
      end
    end

    Defining Class Methods

    Use def self.method to define class methods. This makes the code easier to refactor since the class name is not repeated.

    class TestClass
      # bad
      def TestClass.some_method
        # body omitted
      end
    
      # good
      def self.some_other_method
        # body omitted
      end
    
      # Also possible and convenient when you
      # have to define many class methods.
      class << self
        def first_method
          # body omitted
        end
    
        def second_method_etc
          # body omitted
        end
      end
    end

    Alias Method Lexically

    Prefer alias when aliasing methods in lexical class scope as the resolution of self in this context is also lexical, and it communicates clearly to the user that the indirection of your alias will not be altered at runtime or by any subclass unless made explicit.

    class Westerner
      def first_name
        @names.first
      end
    
      alias given_name first_name
    end

    Since alias, like def, is a keyword, prefer bareword arguments over symbols or strings. In other words, do alias foo bar, not alias :foo :bar.

    Also be aware of how Ruby handles aliases and inheritance: an alias references the method that was resolved at the time the alias was defined; it is not dispatched dynamically.

    class Fugitive < Westerner
      def first_name
        'Nobody'
      end
    end

    In this example, Fugitive#given_name would still call the original Westerner#first_name method, not Fugitive#first_name. To override the behavior of Fugitive#given_name as well, you’d have to redefine it in the derived class.

    class Fugitive < Westerner
      def first_name
        'Nobody'
      end
    
      alias given_name first_name
    end

    alias_method

    Always use alias_method when aliasing methods of modules, classes, or singleton classes at runtime, as the lexical scope of alias leads to unpredictability in these cases.

    module Mononymous
      def self.included(other)
        other.class_eval { alias_method :full_name, :given_name }
      end
    end
    
    class Sting < Westerner
      include Mononymous
    end

    Class and self

    When class (or module) methods call other such methods, omit the use of a leading self or own name followed by a . when calling other such methods. This is often seen in "service classes" or other similar concepts where a class is treated as though it were a function. This convention tends to reduce repetitive boilerplate in such classes.

    class TestClass
      # bad -- more work when class renamed/method moved
      def self.call(param1, param2)
        TestClass.new(param1).call(param2)
      end
    
      # bad -- more verbose than necessary
      def self.call(param1, param2)
        self.new(param1).call(param2)
      end
    
      # good
      def self.call(param1, param2)
        new(param1).call(param2)
      end
    
      # ...other methods...
    end

    Defining Constants within a Block

    Do not define constants within a block, since the block’s scope does not isolate or namespace the constant in any way.

    Define the constant outside of the block instead, or use a variable or method if defining the constant in the outer scope would be problematic.

    # bad - FILES_TO_LINT is now defined globally
    task :lint do
      FILES_TO_LINT = Dir['lib/*.rb']
      # ...
    end
    
    # good - files_to_lint is only defined inside the block
    task :lint do
      files_to_lint = Dir['lib/*.rb']
      # ...
    end

    Classes: Constructors

    Factory Methods

    Consider adding factory methods to provide additional sensible ways to create instances of a particular class.

    class Person
      def self.create(options_hash)
        # body omitted
      end
    end

    Disjunctive Assignment in Constructor

    In constructors, avoid unnecessary disjunctive assignment (||=) of instance variables. Prefer plain assignment. In ruby, instance variables (beginning with an @) are nil until assigned a value, so in most cases the disjunction is unnecessary.

    # bad
    def initialize
      @x ||= 1
    end
    
    # good
    def initialize
      @x = 1
    end

    Comments

    Good code is its own best documentation. As you’re about to add a comment, ask yourself, "How can I improve the code so that this comment isn’t needed?". Improve the code and then document it to make it even clearer.

    — Steve McConnell

    No Comments

    Write self-documenting code and ignore the rest of this section. Seriously!

    Rationale Comments

    If the how can be made self-documenting, but not the why (e.g. the code works around non-obvious library behavior, or implements an algorithm from an academic paper), add a comment explaining the rationale behind the code.

    # bad
    
    x = BuggyClass.something.dup
    
    def compute_dependency_graph
      ...30 lines of recursive graph merging...
    end
    
    # good
    
    # BuggyClass returns an internal object, so we have to dup it to modify it.
    x = BuggyClass.something.dup
    
    # This is algorithm 6.4(a) from Worf & Yar's _Amazing Graph Algorithms_ (2243).
    def compute_dependency_graph
      ...30 lines of recursive graph merging...
    end

    English Comments

    Write comments in English.

    Hash Space

    Use one space between the leading # character of the comment and the text of the comment.

    English Syntax

    Comments longer than a word are capitalized and use punctuation. Use one space after periods.

    No Superfluous Comments

    Avoid superfluous comments.

    # bad
    counter += 1 # Increments counter by one.

    Comment Upkeep

    Keep existing comments up-to-date. An outdated comment is worse than no comment at all.

    Refactor, Don’t Comment

    Good code is like a good joke: it needs no explanation.

    — old programmers maxim
    through Russ Olsen

    Avoid writing comments to explain bad code. Refactor the code to make it self-explanatory. ("Do or do not - there is no try." Yoda)

    Comment Annotations

    Annotations Placement

    Annotations should usually be written on the line immediately above the relevant code.

    # bad
    def bar
      baz(:quux) # FIXME: This has crashed occasionally since v3.2.1.
    end
    
    # good
    def bar
      # FIXME: This has crashed occasionally since v3.2.1.
      baz(:quux)
    end

    Annotations Keyword Format

    The annotation keyword is followed by a colon and a space, then a note describing the problem.

    # bad
    def bar
      # FIXME This has crashed occasionally since v3.2.1.
      baz(:quux)
    end
    
    # good
    def bar
      # FIXME: This has crashed occasionally since v3.2.1.
      baz(:quux)
    end

    Multi-line Annotations Indentation

    If multiple lines are required to describe the problem, subsequent lines should be indented three spaces after the # (one general plus two for indentation purposes).

    def bar
      # FIXME: This has crashed occasionally since v3.2.1. It may
      #   be related to the BarBazUtil upgrade.
      baz(:quux)
    end

    Inline Annotations

    In cases where the problem is so obvious that any documentation would be redundant, annotations may be left at the end of the offending line with no note. This usage should be the exception and not the rule.

    def bar
      sleep 100 # OPTIMIZE
    end

    TODO

    Use TODO to note missing features or functionality that should be added at a later date.

    FIXME

    Use FIXME to note broken code that needs to be fixed.

    OPTIMIZE

    Use OPTIMIZE to note slow or inefficient code that may cause performance problems.

    HACK

    Use HACK to note code smells where questionable coding practices were used and should be refactored away.

    REVIEW

    Use REVIEW to note anything that should be looked at to confirm it is working as intended. For example: REVIEW: Are we sure this is how the client does X currently?

    Document Annotations

    Use other custom annotation keywords if it feels appropriate, but be sure to document them in your project’s README or similar.

    Magic Comments

    Magic Comments First

    Place magic comments above all code and documentation in a file (except shebangs, which are discussed next).

    # bad
    # Some documentation about Person
    
    # frozen_string_literal: true
    class Person
    end
    
    # good
    # frozen_string_literal: true
    
    # Some documentation about Person
    class Person
    end

    Below Shebang

    Place magic comments below shebangs when they are present in a file.

    # bad
    # frozen_string_literal: true
    #!/usr/bin/env ruby
    
    App.parse(ARGV)
    
    # good
    #!/usr/bin/env ruby
    # frozen_string_literal: true
    
    App.parse(ARGV)

    One Magic Comment per Line

    Use one magic comment per line if you need multiple.

    # bad
    # -*- frozen_string_literal: true; encoding: ascii-8bit -*-
    
    # good
    # frozen_string_literal: true
    # encoding: ascii-8bit

    Separate Magic Comments from Code

    Separate magic comments from code and documentation with a blank line.

    # bad
    # frozen_string_literal: true
    # Some documentation for Person
    class Person
      # Some code
    end
    
    # good
    # frozen_string_literal: true
    
    # Some documentation for Person
    class Person
      # Some code
    end

    Collections

    Literal Array and Hash

    Prefer literal array and hash creation notation (unless you need to pass parameters to their constructors, that is).

    # bad
    arr = Array.new
    hash = Hash.new
    
    # good
    arr = []
    arr = Array.new(10)
    hash = {}
    hash = Hash.new(0)

    %w

    Prefer %w to the literal array syntax when you need an array of words (non-empty strings without spaces and special characters in them). Apply this rule only to arrays with two or more elements.

    # bad
    STATES = ['draft', 'open', 'closed']
    
    # good
    STATES = %w[draft open closed]

    %i

    Prefer %i to the literal array syntax when you need an array of symbols (and you don’t need to maintain Ruby 1.9 compatibility). Apply this rule only to arrays with two or more elements.

    # bad
    STATES = [:draft, :open, :closed]
    
    # good
    STATES = %i[draft open closed]

    No Trailing Array Commas

    Avoid comma after the last item of an Array or Hash literal, especially when the items are not on separate lines.

    # bad - easier to move/add/remove items, but still not preferred
    VALUES = [
               1001,
               2020,
               3333,
             ]
    
    # bad
    VALUES = [1001, 2020, 3333, ]
    
    # good
    VALUES = [1001, 2020, 3333]

    No Gappy Arrays

    Avoid the creation of huge gaps in arrays.

    arr = []
    arr[100] = 1 # now you have an array with lots of nils

    first and last

    When accessing the first or last element from an array, prefer first or last over [0] or [-1].

    Set vs Array

    Use Set instead of Array when dealing with unique elements. Set implements a collection of unordered values with no duplicates. This is a hybrid of Array's intuitive inter-operation facilities and Hash's fast lookup.

    Symbols as Keys

    Prefer symbols instead of strings as hash keys.

    # bad
    hash = { 'one' => 1, 'two' => 2, 'three' => 3 }
    
    # good
    hash = { one: 1, two: 2, three: 3 }

    No Mutable Keys

    Avoid the use of mutable objects as hash keys.

    Hash Literals

    Use the Ruby 1.9 hash literal syntax when your hash keys are symbols.

    # bad
    hash = { :one => 1, :two => 2, :three => 3 }
    
    # good
    hash = { one: 1, two: 2, three: 3 }

    Hash Literal as Last Array Item

    Wrap hash literal in braces if it is a last array item.

    # bad
    [1, 2, one: 1, two: 2]
    
    # good
    [1, 2, { one: 1, two: 2 }]

    No Mixed Hash Syntaxes

    Don’t mix the Ruby 1.9 hash syntax with hash rockets in the same hash literal. When you’ve got keys that are not symbols stick to the hash rockets syntax.

    # bad
    { a: 1, 'b' => 2 }
    
    # good
    { :a => 1, 'b' => 2 }

    Avoid Hash[] constructor

    Hash::[] was a pre-Ruby 2.1 way of constructing hashes from arrays of key-value pairs, or from a flat list of keys and values. It has an obscure semantic and looks cryptic in code. Since Ruby 2.1, Enumerable#to_h can be used to construct a hash from a list of key-value pairs, and it should be preferred. Instead of Hash[] with a list of literal keys and values, just a hash literal should be preferred.

    # bad
    Hash[ary]
    Hash[a, b, c, d]
    
    # good
    ary.to_h
    {a => b, c => d}

    Hash#key?

    Use Hash#key? instead of Hash#has_key? and Hash#value? instead of Hash#has_value?.

    # bad
    hash.has_key?(:test)
    hash.has_value?(value)
    
    # good
    hash.key?(:test)
    hash.value?(value)

    Hash#each

    Use Hash#each_key instead of Hash#keys.each and Hash#each_value instead of Hash#values.each.

    # bad
    hash.keys.each { |k| p k }
    hash.values.each { |v| p v }
    hash.each { |k, _v| p k }
    hash.each { |_k, v| p v }
    
    # good
    hash.each_key { |k| p k }
    hash.each_value { |v| p v }

    Hash#fetch

    Use Hash#fetch when dealing with hash keys that should be present.

    heroes = { batman: 'Bruce Wayne', superman: 'Clark Kent' }
    # bad - if we make a mistake we might not spot it right away
    heroes[:batman] # => 'Bruce Wayne'
    heroes[:supermann] # => nil
    
    # good - fetch raises a KeyError making the problem obvious
    heroes.fetch(:supermann)

    Hash#fetch defaults

    Introduce default values for hash keys via Hash#fetch as opposed to using custom logic.

    batman = { name: 'Bruce Wayne', is_evil: false }
    
    # bad - if we just use || operator with falsy value we won't get the expected result
    batman[:is_evil] || true # => true
    
    # good - fetch works correctly with falsy values
    batman.fetch(:is_evil, true) # => false

    Use Hash Blocks

    Prefer the use of the block instead of the default value in Hash#fetch if the code that has to be evaluated may have side effects or be expensive.

    batman = { name: 'Bruce Wayne' }
    
    # bad - if we use the default value, we eager evaluate it
    # so it can slow the program down if done multiple times
    batman.fetch(:powers, obtain_batman_powers) # obtain_batman_powers is an expensive call
    
    # good - blocks are lazy evaluated, so only triggered in case of KeyError exception
    batman.fetch(:powers) { obtain_batman_powers }

    Hash#values_at

    Use Hash#values_at when you need to retrieve several values consecutively from a hash.

    # bad
    email = data['email']
    username = data['nickname']
    
    # good
    email, username = data.values_at('email', 'nickname')

    Hash#transform_keys and Hash#transform_values

    Prefer transform_keys or transform_values over each_with_object or map when transforming just the keys or just the values of a hash.

    # bad
    {a: 1, b: 2}.each_with_object({}) { |(k, v), h| h[k] = v * v }
    {a: 1, b: 2}.map { |k, v| [k.to_s, v] }.to_h
    
    # good
    {a: 1, b: 2}.transform_values { |v| v * v }
    {a: 1, b: 2}.transform_keys { |k| k.to_s }

    Ordered Hashes

    Rely on the fact that as of Ruby 1.9 hashes are ordered.

    No Modifying Collections

    Do not modify a collection while traversing it.

    Accessing Elements Directly

    When accessing elements of a collection, avoid direct access via [n] by using an alternate form of the reader method if it is supplied. This guards you from calling [] on nil.

    # bad
    Regexp.last_match[1]
    
    # good
    Regexp.last_match(1)

    Provide Alternate Accessor to Collections

    When providing an accessor for a collection, provide an alternate form to save users from checking for nil before accessing an element in the collection.

    # bad
    def awesome_things
      @awesome_things
    end
    
    # good
    def awesome_things(index = nil)
      if index && @awesome_things
        @awesome_things[index]
      else
        @awesome_things
      end
    end

    map/find/select/reduce/include?/size

    Prefer map over collect, find over detect, select over find_all, reduce over inject, include? over member? and size over length. This is not a hard requirement; if the use of the alias enhances readability, it’s ok to use it. The rhyming methods are inherited from Smalltalk and are not common in other programming languages. The reason the use of select is encouraged over find_all is that it goes together nicely with reject and its name is pretty self-explanatory.

    count vs size

    Don’t use count as a substitute for size. For Enumerable objects other than Array it will iterate the entire collection in order to determine its size.

    # bad
    some_hash.count
    
    # good
    some_hash.size

    flat_map

    Use flat_map instead of map + flatten. This does not apply for arrays with a depth greater than 2, i.e. if users.first.songs == ['a', ['b','c']], then use map + flatten rather than flat_map. flat_map flattens the array by 1, whereas flatten flattens it all the way.

    # bad
    all_songs = users.map(&:songs).flatten.uniq
    
    # good
    all_songs = users.flat_map(&:songs).uniq

    reverse_each

    Prefer reverse_each to reverse.each because some classes that include Enumerable will provide an efficient implementation. Even in the worst case where a class does not provide a specialized implementation, the general implementation inherited from Enumerable will be at least as efficient as using reverse.each.

    # bad
    array.reverse.each { ... }
    
    # good
    array.reverse_each { ... }

    Numbers

    Underscores in Numerics

    Add underscores to large numeric literals to improve their readability.

    # bad - how many 0s are there?
    num = 1000000
    
    # good - much easier to parse for the human brain
    num = 1_000_000

    Numeric Literal Prefixes

    Prefer lowercase letters for numeric literal prefixes. 0o for octal, 0x for hexadecimal and 0b for binary. Do not use 0d prefix for decimal literals.

    # bad
    num = 01234
    num = 0O1234
    num = 0X12AB
    num = 0B10101
    num = 0D1234
    num = 0d1234
    
    # good - easier to separate digits from the prefix
    num = 0o1234
    num = 0x12AB
    num = 0b10101
    num = 1234

    Integer Type Checking

    Use Integer to check the type of an integer number. Since Fixnum is platform-dependent, checking against it will return different results on 32-bit and 64-bit machines.

    timestamp = Time.now.to_i
    
    # bad
    timestamp.is_a?(Fixnum)
    timestamp.is_a?(Bignum)
    
    # good
    timestamp.is_a?(Integer)

    Random Numbers

    Prefer to use ranges when generating random numbers instead of integers with offsets, since it clearly states your intentions. Imagine simulating a roll of a dice:

    # bad
    rand(6) + 1
    
    # good
    rand(1..6)

    Float Division

    When performing float-division on two integers, either use fdiv or convert one-side integer to float.

    # bad
    a.to_f / b.to_f
    
    # good
    a.to_f / b
    a / b.to_f
    a.fdiv(b)

    Float Comparison

    Avoid (in)equality comparisons of floats as they are unreliable.

    Floating point values are inherently inaccurate, and comparing them for exact equality is almost never the desired semantics. Comparison via the ==/!= operators checks floating-point value representation to be exactly the same, which is very unlikely if you perform any arithmetic operations involving precision loss.

    # bad
    x == 0.1
    x != 0.1
    
    # good - using BigDecimal
    x.to_d == 0.1.to_d
    
    # good - not an actual float comparison
    x == Float::INFINITY
    
    # good
    (x - 0.1).abs < Float::EPSILON
    
    # good
    tolerance = 0.0001
    (x - 0.1).abs < tolerance
    
    # Or some other epsilon based type of comparison:
    # https://www.embeddeduse.com/2019/08/26/qt-compare-two-floats/

    Exponential Notation

    When using exponential notation for numbers, prefer using the normalized scientific notation, which uses a mantissa between 1 (inclusive) and 10 (exclusive). Omit the exponent altogether if it is zero.

    The goal is to avoid confusion between powers of ten and exponential notation, as one quickly reading 10e7 could think it’s 10 to the power of 7 (one then 7 zeroes) when it’s actually 10 to the power of 8 (one then 8 zeroes). If you want 10 to the power of 7, you should do 1e7.

    power notation exponential notation output

    10 ** 7

    1e7

    10000000

    10 ** 6

    1e6

    1000000

    10 ** 7

    10e6

    10000000

    One could favor the alternative engineering notation, in which the exponent must always be a multiple of 3 for easy conversion to the thousand / million / …​ system.

    # bad
    10e6
    0.3e4
    11.7e5
    3.14e0
    
    # good
    1e7
    3e3
    1.17e6
    3.14

    Alternative : engineering notation:

    # bad
    3.2e7
    0.1e5
    12e4
    
    # good
    1e6
    17e6
    0.98e9

    Strings

    String Interpolation

    Prefer string interpolation and string formatting to string concatenation:

    # bad
    email_with_name = user.name + ' <' + user.email + '>'
    
    # good
    email_with_name = "#{user.name} <#{user.email}>"
    
    # good
    email_with_name = format('%s <%s>', user.name, user.email)

    Consistent String Literals

    Adopt a consistent string literal quoting style. There are two popular styles in the Ruby community, both of which are considered good - single quotes by default and double quotes by default.

    The string literals in this guide are using single quotes by default.

    Single Quote

    Prefer single-quoted strings when you don’t need string interpolation or special symbols such as \t, \n, ', etc.

    # bad
    name = "Bozhidar"
    
    name = 'De\'Andre'
    
    # good
    name = 'Bozhidar'
    
    name = "De'Andre"

    Double Quote

    Prefer double-quotes unless your string literal contains " or escape characters you want to suppress.

    # bad
    name = 'Bozhidar'
    
    sarcasm = "I \"like\" it."
    
    # good
    name = "Bozhidar"
    
    sarcasm = 'I "like" it.'

    No Character Literals

    Don’t use the character literal syntax ?x. Since Ruby 1.9 it’s basically redundant - ?x would be interpreted as 'x' (a string with a single character in it).

    # bad
    char = ?c
    
    # good
    char = 'c'

    Curlies Interpolate

    Don’t leave out {} around instance and global variables being interpolated into a string.

    class Person
      attr_reader :first_name, :last_name
    
      def initialize(first_name, last_name)
        @first_name = first_name
        @last_name = last_name
      end
    
      # bad - valid, but awkward
      def to_s
        "#@first_name #@last_name"
      end
    
      # good
      def to_s
        "#{@first_name} #{@last_name}"
      end
    end
    
    $global = 0
    # bad
    puts "$global = #$global"
    
    # good
    puts "$global = #{$global}"

    No to_s

    Don’t use Object#to_s on interpolated objects. It’s called on them automatically.

    # bad
    message = "This is the #{result.to_s}."
    
    # good
    message = "This is the #{result}."

    String Concatenation

    Avoid using String#+ when you need to construct large data chunks. Instead, use String#<<. Concatenation mutates the string instance in-place and is always faster than String#+, which creates a bunch of new string objects.

    # bad
    html = ''
    html += '<h1>Page title</h1>'
    
    paragraphs.each do |paragraph|
      html += "<p>#{paragraph}</p>"
    end
    
    # good and also fast
    html = ''
    html << '<h1>Page title</h1>'
    
    paragraphs.each do |paragraph|
      html << "<p>#{paragraph}</p>"
    end

    Don’t Abuse gsub

    Don’t use String#gsub in scenarios in which you can use a faster and more specialized alternative.

    url = 'http://example.com'
    str = 'lisp-case-rules'
    
    # bad
    url.gsub('http://', 'https://')
    str.gsub('-', '_')
    
    # good
    url.sub('http://', 'https://')
    str.tr('-', '_')

    String#chars

    Prefer the use of String#chars over String#split with empty string or regexp literal argument.

    These cases have the same behavior since Ruby 2.0.
    # bad
    string.split(//)
    string.split('')
    
    # good
    string.chars

    sprintf

    Prefer the use of sprintf and its alias format over the fairly cryptic String#% method.

    # bad
    '%d %d' % [20, 10]
    # => '20 10'
    
    # good
    sprintf('%d %d', 20, 10)
    # => '20 10'
    
    # good
    sprintf('%<first>d %<second>d', first: 20, second: 10)
    # => '20 10'
    
    format('%d %d', 20, 10)
    # => '20 10'
    
    # good
    format('%<first>d %<second>d', first: 20, second: 10)
    # => '20 10'

    Named Format Tokens

    When using named format string tokens, favor %<name>s over %{name} because it encodes information about the type of the value.

    # bad
    format('Hello, %{name}', name: 'John')
    
    # good
    format('Hello, %<name>s', name: 'John')

    Long Strings

    Break long strings into multiple lines but don’t concatenate them with +. If you want to add newlines, use heredoc. Otherwise use \:

    # bad
    "Lorem Ipsum is simply dummy text of the printing and typesetting industry. " +
    "Lorem Ipsum has been the industry's standard dummy text ever since the 1500s, " +
    "when an unknown printer took a galley of type and scrambled it to make a type specimen book."
    
    # good
    <<~LOREM
      Lorem Ipsum is simply dummy text of the printing and typesetting industry.
      Lorem Ipsum has been the industry's standard dummy text ever since the 1500s,
      when an unknown printer took a galley of type and scrambled it to make a type specimen book.
    LOREM
    
    # good
    "Lorem Ipsum is simply dummy text of the printing and typesetting industry. "\
    "Lorem Ipsum has been the industry's standard dummy text ever since the 1500s, "\
    "when an unknown printer took a galley of type and scrambled it to make a type specimen book."

    Heredocs

    Squiggly Heredocs

    Use Ruby 2.3’s squiggly heredocs for nicely indented multi-line strings.

    # bad - using Powerpack String#strip_margin
    code = <<-RUBY.strip_margin('|')
      |def test
      |  some_method
      |  other_method
      |end
    RUBY
    
    # also bad
    code = <<-RUBY
    def test
      some_method
      other_method
    end
    RUBY
    
    # good
    code = <<~RUBY
      def test
        some_method
        other_method
      end
    RUBY

    Heredoc Delimiters

    Use descriptive delimiters for heredocs. Delimiters add valuable information about the heredoc content, and as an added bonus some editors can highlight code within heredocs if the correct delimiter is used.

    # bad
    code = <<~END
      def foo
        bar
      end
    END
    
    # good
    code = <<~RUBY
      def foo
        bar
      end
    RUBY
    
    # good
    code = <<~SUMMARY
      An imposing black structure provides a connection between the past and
      the future in this enigmatic adaptation of a short story by revered
      sci-fi author Arthur C. Clarke.
    SUMMARY

    Heredoc Method Calls

    Place method calls with heredoc receivers on the first line of the heredoc definition. The bad form has significant potential for error if a new line is added or removed.

    # bad
    query = <<~SQL
      select foo from bar
    SQL
    .strip_indent
    
    # good
    query = <<~SQL.strip_indent
      select foo from bar
    SQL

    Heredoc Argument Closing Parentheses

    Place the closing parenthesis for method calls with heredoc arguments on the first line of the heredoc definition. The bad form has potential for error if the new line before the closing parenthesis is removed.

    # bad
    foo(<<~SQL
      select foo from bar
    SQL
    )
    
    # good
    foo(<<~SQL)
      select foo from bar
    SQL

    Date & Time

    Time.now

    Prefer Time.now over Time.new when retrieving the current system time.

    No DateTime

    Don’t use DateTime unless you need to account for historical calendar reform - and if you do, explicitly specify the start argument to clearly state your intentions.

    # bad - uses DateTime for current time
    DateTime.now
    
    # good - uses Time for current time
    Time.now
    
    # bad - uses DateTime for modern date
    DateTime.iso8601('2016-06-29')
    
    # good - uses Date for modern date
    Date.iso8601('2016-06-29')
    
    # good - uses DateTime with start argument for historical date
    DateTime.iso8601('1751-04-23', Date::ENGLAND)

    Regular Expressions

    Some people, when confronted with a problem, think "I know, I’ll use regular expressions." Now they have two problems.

    — Jamie Zawinski

    Plain Text Search

    Don’t use regular expressions if you just need plain text search in string.

    foo = 'I am an example string'
    
    # bad - using a regular expression is an overkill here
    foo =~ /example/
    
    # good
    foo['example']

    Using Regular Expressions as String Indexes

    For simple constructions you can use regexp directly through string index.

    match = string[/regexp/]             # get content of matched regexp
    first_group = string[/text(grp)/, 1] # get content of captured group
    string[/text (grp)/, 1] = 'replace'  # string => 'text replace'

    Prefer Non-capturing Groups

    Use non-capturing groups when you don’t use the captured result.

    # bad
    /(first|second)/
    
    # good
    /(?:first|second)/

    Do not mix named and numbered captures

    Do not mix named captures and numbered captures in a Regexp literal. Because numbered capture is ignored if they’re mixed.

    # bad - There is no way to access `(BAR)` capturing.
    m = /(?<foo>FOO)(BAR)/.match('FOOBAR')
    p m[:foo] # => "FOO"
    p m[1]    # => "FOO"
    p m[2]    # => nil   - not "BAR"
    
    # good - Both captures are accessible with names.
    m = /(?<foo>FOO)(?<bar>BAR)/.match('FOOBAR')
    p m[:foo] # => "FOO"
    p m[:bar] # => "BAR"
    
    # good - `(?:BAR)` is non-capturing grouping.
    m = /(?<foo>FOO)(?:BAR)/.match('FOOBAR')
    p m[:foo] # => "FOO"
    
    # good - Both captures are accessible with numbers.
    m = /(FOO)(BAR)/.match('FOOBAR')
    p m[1] # => "FOO"
    p m[2] # => "BAR"

    Refer named regexp captures by name

    Prefer using names to refer named regexp captures instead of numbers.

    # bad
    m = /(?<foo>FOO)(?<bar>BAR)/.match('FOOBAR')
    p m[1] # => "FOO"
    p m[2] # => "BAR"
    
    # good
    m = /(?<foo>FOO)(?<bar>BAR)/.match('FOOBAR')
    p m[:foo] # => "FOO"
    p m[:bar] # => "BAR"

    Avoid Perl-style Last Regular Expression Group Matchers

    Don’t use the cryptic Perl-legacy variables denoting last regexp group matches ($1, $2, etc). Use Regexp.last_match(n) instead.

    /(regexp)/ =~ string
    ...
    
    # bad
    process $1
    
    # good
    process Regexp.last_match(1)

    Avoid Numbered Groups

    Avoid using numbered groups as it can be hard to track what they contain. Named groups can be used instead.

    # bad
    /(regexp)/ =~ string
    # some code
    process Regexp.last_match(1)
    
    # good
    /(?<meaningful_var>regexp)/ =~ string
    # some code
    process meaningful_var

    Limit Escapes

    Character classes have only a few special characters you should care about: ^, -, \, ], so don’t escape . or brackets in [].

    Caret and Dollar Regexp

    Be careful with ^ and $ as they match start/end of line, not string endings. If you want to match the whole string use: \A and \z (not to be confused with \Z which is the equivalent of /\n?\z/).

    string = "some injection\nusername"
    string[/^username$/]   # matches
    string[/\Ausername\z/] # doesn't match

    Multi-line Regular Expressions

    Use x (free-spacing) modifier for multi-line regexps.

    That’s known as free-spacing mode. In this mode leading and trailing whitespace is ignored.
    # bad
    regex = /start\
    \s\
    (group)\
    (?:alt1|alt2)\
    end/
    
    # good
    regexp = /
      start
      \s
      (group)
      (?:alt1|alt2)
      end
    /x

    Comment Complex Regular Expressions

    Use x modifier for complex regexps. This makes them more readable and you can add some useful comments.

    regexp = /
      start         # some text
      \s            # white space char
      (group)       # first group
      (?:alt1|alt2) # some alternation
      end
    /x

    Use gsub with a Block or a Hash for Complex Replacements

    For complex replacements sub/gsub can be used with a block or a hash.

    words = 'foo bar'
    words.sub(/f/, 'f' => 'F') # => 'Foo bar'
    words.gsub(/\w+/) { |word| word.capitalize } # => 'Foo Bar'

    Percent Literals

    %q shorthand

    Use %() (it’s a shorthand for %Q) for single-line strings which require both interpolation and embedded double-quotes. For multi-line strings, prefer heredocs.

    # bad (no interpolation needed)
    %(<div class="text">Some text</div>)
    # should be '<div class="text">Some text</div>'
    
    # bad (no double-quotes)
    %(This is #{quality} style)
    # should be "This is #{quality} style"
    
    # bad (multiple lines)
    %(<div>\n<span class="big">#{exclamation}</span>\n</div>)
    # should be a heredoc.
    
    # good (requires interpolation, has quotes, single line)
    %(<tr><td class="name">#{name}</td>)

    %q

    Avoid %() or the equivalent %q() unless you have a string with both ' and " in it. Regular string literals are more readable and should be preferred unless a lot of characters would have to be escaped in them.

    # bad
    name = %q(Bruce Wayne)
    time = %q(8 o'clock)
    question = %q("What did you say?")
    
    # good
    name = 'Bruce Wayne'
    time = "8 o'clock"
    question = '"What did you say?"'
    quote = %q(<p class='quote'>"What did you say?"</p>)

    %r

    Use %r only for regular expressions matching at least one / character.

    # bad
    %r{\s+}
    
    # good
    %r{^/(.*)$}
    %r{^/blog/2011/(.*)$}

    %x

    Avoid the use of %x unless you’re going to execute a command with backquotes in it (which is rather unlikely).

    # bad
    date = %x(date)
    
    # good
    date = `date`
    echo = %x(echo `date`)

    %s

    Avoid the use of %s. It seems that the community has decided :"some string" is the preferred way to create a symbol with spaces in it.

    Percent Literal Braces

    Use the braces that are the most appropriate for the various kinds of percent literals.

    • () for string literals (%q, %Q).

    • [] for array literals (%w, %i, %W, %I) as it is aligned with the standard array literals.

    • {} for regexp literals (%r) since parentheses often appear inside regular expressions. That’s why a less common character with { is usually the best delimiter for %r literals.

    • () for all other literals (e.g. %s, %x)

    # bad
    %q{"Test's king!", John said.}
    
    # good
    %q("Test's king!", John said.)
    
    # bad
    %w(one two three)
    %i(one two three)
    
    # good
    %w[one two three]
    %i[one two three]
    
    # bad
    %r((\w+)-(\d+))
    %r{\w{1,2}\d{2,5}}
    
    # good
    %r{(\w+)-(\d+)}
    %r|\w{1,2}\d{2,5}|

    Metaprogramming

    No Needless Metaprogramming

    Avoid needless metaprogramming.

    No Monkey Patching

    Do not mess around in core classes when writing libraries (do not monkey-patch them).

    Block class_eval

    The block form of class_eval is preferable to the string-interpolated form.

    Supply Location

    When you use the string-interpolated form, always supply __FILE__ and __LINE__, so that your backtraces make sense:

    class_eval 'def use_relative_model_naming?; true; end', __FILE__, __LINE__

    define_method

    define_method is preferable to class_eval { def …​ }

    eval Comment Docs

    When using class_eval (or other eval) with string interpolation, add a comment block showing its appearance if interpolated (a practice used in Rails code):

    # from activesupport/lib/active_support/core_ext/string/output_safety.rb
    UNSAFE_STRING_METHODS.each do |unsafe_method|
      if 'String'.respond_to?(unsafe_method)
        class_eval <<-EOT, __FILE__, __LINE__ + 1
          def #{unsafe_method}(*params, &block)       # def capitalize(*params, &block)
            to_str.#{unsafe_method}(*params, &block)  #   to_str.capitalize(*params, &block)
          end                                         # end
    
          def #{unsafe_method}!(*params)              # def capitalize!(*params)
            @dirty = true                             #   @dirty = true
            super                                     #   super
          end                                         # end
        EOT
      end
    end

    No method_missing

    Avoid using method_missing for metaprogramming because backtraces become messy, the behavior is not listed in #methods, and misspelled method calls might silently work, e.g. nukes.luanch_state = false. Consider using delegation, proxy, or define_method instead. If you must use method_missing:

    • Be sure to also define respond_to_missing?

    • Only catch methods with a well-defined prefix, such as find_by_*--make your code as assertive as possible.

    • Call super at the end of your statement

    • Delegate to assertive, non-magical methods:

    # bad
    def method_missing(meth, *params, &block)
      if /^find_by_(?<prop>.*)/ =~ meth
        # ... lots of code to do a find_by
      else
        super
      end
    end
    
    # good
    def method_missing(meth, *params, &block)
      if /^find_by_(?<prop>.*)/ =~ meth
        find_by(prop, *params, &block)
      else
        super
      end
    end
    
    # best of all, though, would to define_method as each findable attribute is declared

    Prefer public_send

    Prefer public_send over send so as not to circumvent private/protected visibility.

    # We have an ActiveModel Organization that includes concern Activatable
    module Activatable
      extend ActiveSupport::Concern
    
      included do
        before_create :create_token
      end
    
      private
    
      def reset_token
        # some code
      end
    
      def create_token
        # some code
      end
    
      def activate!
        # some code
      end
    end
    
    class Organization < ActiveRecord::Base
      include Activatable
    end
    
    linux_organization = Organization.find(...)
    # BAD - violates privacy
    linux_organization.send(:reset_token)
    # GOOD - should throw an exception
    linux_organization.public_send(:reset_token)

    Prefer __send__

    Prefer __send__ over send, as send may overlap with existing methods.

    require 'socket'
    
    u1 = UDPSocket.new
    u1.bind('127.0.0.1', 4913)
    u2 = UDPSocket.new
    u2.connect('127.0.0.1', 4913)
    # Won't send a message to the receiver obj.
    # Instead it will send a message via UDP socket.
    u2.send :sleep, 0
    # Will actually send a message to the receiver obj.
    u2.__send__ ...

    API Documentation

    YARD

    Use YARD and its conventions for API documentation.

    RD (Block) Comments

    Don’t use block comments. They cannot be preceded by whitespace and are not as easy to spot as regular comments.

    # bad
    =begin
    comment line
    another comment line
    =end
    
    # good
    # comment line
    # another comment line
    From Perl’s POD to RD

    This is not really a block comment syntax, but more of an attempt to emulate Perl’s POD documentation system.

    There’s an rdtool for Ruby that’s pretty similar to POD. Basically rdtool scans a file for =begin and =end pairs, and extracts the text between them all. This text is assumed to be documentation in RD format. You can read more about it here.

    RD predated the rise of RDoc and YARD and was effectively obsoleted by them.[3]

    Gemfile and Gemspec

    No RUBY_VERSION in the gemspec

    The gemspec should not contain RUBY_VERSION as a condition to switch dependencies. RUBY_VERSION is determined by rake release, so users may end up with wrong dependency.

    # bad
    Gem::Specification.new do |s|
      if RUBY_VERSION >= '2.5'
        s.add_runtime_dependency 'gem_a'
      else
        s.add_runtime_dependency 'gem_b'
      end
    end

    Fix by either:

    • Post-install messages.

    • Add both gems as dependency (if permissible).

    • If development dependencies, move to Gemfile.

    Misc

    No Flip-flops

    Avoid the use of flip-flops.

    No non-nil Checks

    Don’t do explicit non-nil checks unless you’re dealing with boolean values.

    # bad
    do_something if !something.nil?
    do_something if something != nil
    
    # good
    do_something if something
    
    # good - dealing with a boolean
    def value_set?
      !@some_boolean.nil?
    end

    Global Input/Output Streams

    Use $stdout/$stderr/$stdin instead of STDOUT/STDERR/STDIN. STDOUT/STDERR/STDIN are constants, and while you can actually reassign (possibly to redirect some stream) constants in Ruby, you’ll get an interpreter warning if you do so.

    # bad
    STDOUT.puts('hello')
    
    hash = { out: STDOUT, key: value }
    
    def m(out = STDOUT)
      out.puts('hello')
    end
    
    # good
    $stdout.puts('hello')
    
    hash = { out: $stdout, key: value }
    
    def m(out = $stdout)
      out.puts('hello')
    end
    The only valid use-case for the stream constants is obtaining references to the original streams (assuming you’ve redirected some of the global vars).

    Warn

    Use warn instead of $stderr.puts. Apart from being more concise and clear, warn allows you to suppress warnings if you need to (by setting the warn level to 0 via -W0).

    # bad
    $stderr.puts 'This is a warning!'
    
    # good
    warn 'This is a warning!'

    Array#join

    Prefer the use of Array#join over the fairly cryptic Array#* with a string argument.

    # bad
    %w[one two three] * ', '
    # => 'one, two, three'
    
    # good
    %w[one two three].join(', ')
    # => 'one, two, three'

    Array Coercion

    Use Array() instead of explicit Array check or [*var], when dealing with a variable you want to treat as an Array, but you’re not certain it’s an array.

    # bad
    paths = [paths] unless paths.is_a?(Array)
    paths.each { |path| do_something(path) }
    
    # bad (always creates a new Array instance)
    [*paths].each { |path| do_something(path) }
    
    # good (and a bit more readable)
    Array(paths).each { |path| do_something(path) }

    Ranges or between

    Use ranges or Comparable#between? instead of complex comparison logic when possible.

    # bad
    do_something if x >= 1000 && x <= 2000
    
    # good
    do_something if (1000..2000).include?(x)
    
    # good
    do_something if x.between?(1000, 2000)

    Predicate Methods

    Prefer the use of predicate methods to explicit comparisons with ==. Numeric comparisons are OK.

    # bad
    if x % 2 == 0
    end
    
    if x % 2 == 1
    end
    
    if x == nil
    end
    
    # good
    if x.even?
    end
    
    if x.odd?
    end
    
    if x.nil?
    end
    
    if x.zero?
    end
    
    if x == 0
    end

    No Cryptic Perlisms

    Avoid using Perl-style special variables (like $:, $;, etc). They are quite cryptic and their use in anything but one-liner scripts is discouraged.

    # bad
    $:.unshift File.dirname(__FILE__)
    
    # good
    $LOAD_PATH.unshift File.dirname(__FILE__)

    Use the human-friendly aliases provided by the English library if required.

    # bad
    print $', $$
    
    # good
    require 'English'
    print $POSTMATCH, $PID

    Use require_relative whenever possible

    For all your internal dependencies, you should use require_relative. Use of require should be reserved for external dependencies

    # bad
    require 'set'
    require 'my_gem/spec/helper'
    require 'my_gem/lib/something'
    
    # good
    require 'set'
    require_relative 'helper'
    require_relative '../lib/something'

    This way is more expressive (making clear which dependency is internal or not) and more efficient (as require_relative doesn’t have to try all of $LOAD_PATH contrary to require).

    Always Warn

    Write ruby -w safe code.

    No Optional Hash Params

    Avoid hashes as optional parameters. Does the method do too much? (Object initializers are exceptions for this rule).

    Instance Vars

    Use module instance variables instead of global variables.

    # bad
    $foo_bar = 1
    
    # good
    module Foo
      class << self
        attr_accessor :bar
      end
    end
    
    Foo.bar = 1

    OptionParser

    Use OptionParser for parsing complex command line options and ruby -s for trivial command line options.

    No Param Mutations

    Do not mutate parameters unless that is the purpose of the method.

    Three is the Number Thou Shalt Count

    Avoid more than three levels of block nesting.

    Functional Code

    Code in a functional way, avoiding mutation when that makes sense.

    a = []; [1, 2, 3].each { |i| a << i * 2 }   # bad
    a = [1, 2, 3].map { |i| i * 2 }             # good
    
    a = {}; [1, 2, 3].each { |i| a[i] = i * 17 }                # bad
    a = [1, 2, 3].reduce({}) { |h, i| h[i] = i * 17; h }        # good
    a = [1, 2, 3].each_with_object({}) { |i, h| h[i] = i * 17 } # good

    No explicit .rb to require

    Omit the .rb extension for filename passed to require and require_relative.

    If the extension is omitted, Ruby tries adding '.rb', '.so', and so on to the name until found. If the file named cannot be found, a LoadError will be raised. There is an edge case where foo.so file is loaded instead of a LoadError if foo.so file exists when require 'foo.rb' will be changed to require 'foo', but that seems harmless.
    # bad
    require 'foo.rb'
    require_relative '../foo.rb'
    
    # good
    require 'foo'
    require 'foo.so'
    require_relative '../foo'
    require_relative '../foo.so'

    Avoid tap

    The method tap can be helpful for debugging purposes but should not be left in production code.

    # bad
    Config.new(hash, path).tap do |config|
      config.check if check
    end
    
    # good
    config = Config.new(hash, path)
    config.check if check
    config

    This is simpler and more efficient.

    Tools

    Here are some tools to help you automatically check Ruby code against this guide.

    RuboCop

    RuboCop is a Ruby static code analyzer and formatter, based on this style guide. RuboCop already covers a significant portion of the guide and has plugins for most popular Ruby editors and IDEs.

    RuboCop’s cops (code checks) have links to the guidelines that they are based on, as part of their metadata.

    RubyMine

    RubyMine's code inspections are partially based on this guide.

    History

    This guide started its life in 2011 as an internal company Ruby coding guidelines (written by Bozhidar Batsov). Bozhidar had always been bothered as a Ruby developer about one thing - Python developers had a great programming style reference (PEP-8) and Rubyists never got an official guide, documenting Ruby coding style and best practices. Bozhidar firmly believed that style matters. He also believed that a great hacker community, such as Ruby has, should be quite capable of producing this coveted document. The rest is history…​

    At some point Bozhidar decided that the work he was doing might be interesting to members of the Ruby community in general and that the world had little need for another internal company guideline. But the world could certainly benefit from a community-driven and community-sanctioned set of practices, idioms and style prescriptions for Ruby programming.

    Bozhidar served as the guide’s only editor for a few years, before a team of editors was formed once the project transitioned to RuboCop HQ.

    Since the inception of the guide we’ve received a lot of feedback from members of the exceptional Ruby community around the world. Thanks for all the suggestions and the support! Together we can make a resource beneficial to each and every Ruby developer out there.

    Sources of Inspiration

    Many people, books, presentations, articles and other style guides influenced the community Ruby style guide. Here are some of them:

    Contributing

    The guide is still a work in progress - some guidelines are lacking examples, some guidelines don’t have examples that illustrate them clearly enough. Improving such guidelines is a great (and simple way) to help the Ruby community!

    In due time these issues will (hopefully) be addressed - just keep them in mind for now.

    Nothing written in this guide is set in stone. It’s our desire to work together with everyone interested in Ruby coding style, so that we could ultimately create a resource that will be beneficial to the entire Ruby community.

    Feel free to open tickets or send pull requests with improvements. Thanks in advance for your help!

    You can also support the project (and RuboCop) with financial contributions via one of the following platforms:

    How to Contribute?

    It’s easy, just follow the contribution guidelines below:

    Colophon

    This guide is written in AsciiDoc and is published as HTML using AsciiDoctor. The HTML version of the guide is hosted on GitHub Pages.

    Originally the guide was written in Markdown, but was converted to AsciiDoc in 2019.

    Spread the Word

    A community-driven style guide is of little use to a community that doesn’t know about its existence. Tweet about the guide, share it with your friends and colleagues. Every comment, suggestion or opinion we get makes the guide just a little bit better. And we want to have the best possible guide, don’t we?


    1. Occasionally we might suggest to the reader to consider some alternatives, though.
    2. *BSD/Solaris/Linux/macOS users are covered by default, Windows users have to be extra careful.
    3. According to this Wikipedia article the format used to be popular until the early 2000s when it was superseded by RDoc.

    项目简介

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    源项目地址

    https://github.com/rubocop-hq/ruby-style-guide

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