ruby-style-guide 0,7,9,0,6,5,0,9

A community-driven Ruby coding style guide

Prelude

Role models are important.
– Officer Alex J. Murphy / RoboCop

One thing has always bothered me as a Ruby developer—Python developers have a great programming style reference ([PEP-8][]) and we never got an official guide, documenting Ruby coding style and best practices. And I do believe that style matters. I also believe that a great hacker community, such as Ruby has, should be quite capable of producing this coveted document.

This guide started its life as our internal company Ruby coding guidelines (written by yours truly). At some point I decided that the work I 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.

Since the inception of the guide I’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.

By the way, if you’re into Rails you might want to check out the complementary [Ruby on Rails Style Guide][rails-style-guide].

The Ruby Style Guide

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 rules. I’ve tried to add the rationale behind the rules (if it’s omitted I’ve assumed it’s pretty obvious).

I didn’t come up with all the rules out of nowhere—they are mostly based on my extensive career as a professional software engineer, feedback and suggestions from members of the Ruby community and various highly regarded Ruby programming resources, such as [“Programming Ruby”][pickaxe] and [“The Ruby Programming Language”][trpl].

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.

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

Many projects have their own coding style guidelines (often derived from this guide). In the event of any conflicts, such project-specific guides take precedence for that project.

You can generate a PDF or an HTML copy of this guide using [Pandoc][].

[RuboCop][] is a code analyzer, based on this style guide.

Translations of the guide are available in the following languages:

Table of Contents

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)

  • Use UTF-8 as the source file encoding. [link]

  • Use two spaces per indentation level (aka soft tabs). No hard tabs. [link]

  # bad - four spaces
  def some_method
      do_something
  end

  # good
  def some_method
    do_something
  end
  • Use Unix-style line endings. (*BSD/Solaris/Linux/macOS users are covered by default, Windows users have to be extra careful.) [link]

    • 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
    
  • Don’t use ; to separate statements and expressions. As a corollary—use one expression per line. [link]

  # bad
  puts 'foobar'; # superfluous semicolon

  puts 'foo'; puts 'bar' # two expressions on the same line

  # good
  puts 'foobar'

  puts 'foo'
  puts 'bar'

  puts 'foo', 'bar' # this applies to puts in particular
  • Prefer a single-line format for class definitions with no body. [link]
  # bad
  class FooError < StandardError
  end

  # okish
  class FooError < StandardError; end

  # good
  FooError = Class.new(StandardError)
  • 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&mdash;there should be no more than one expression in a single-line method. [link]
  # 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
  • 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. [link]
  sum = 1 + 2
  a, b = 1, 2
  class FooError < StandardError; end

The only exception, regarding operators, is the exponent operator:

  # bad
  e = M * c ** 2

  # good
  e = M * c**2
  • No spaces after (, [ or before ], ). Use spaces around { and before }. [link]
  # 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&mdash;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 !. [link]
  # bad
  ! something

  # good
  !something
  • No space inside range literals. [link]

    # bad
    1 .. 3
    'a' ... 'z'
    
    # good
    1..3
    'a'...'z'
    
  • Indent when as deep as case. This is the style established in both “The Ruby Programming Language” and “Programming Ruby”. [link]

  # 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
  • When assigning the result of a conditional expression to a variable, preserve the usual alignment of its branches. [link]
  # 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
  • Use empty lines between method definitions and also to break up methods into logical paragraphs internally. [link]
  def some_method
    data = initialize(options)

    data.manipulate!

    data.result
  end

  def some_method
    result
  end
  • Don’t use several empty lines in a row. [link]
  # bad - It has two empty lines.
  some_method


  some_method

  # good
  some_method

  some_method
  • Use empty lines around access modifiers. [link]
  # bad
  class Foo
    attr_reader :foo
    def foo
      # do something...
    end
  end

  # good
  class Foo
    attr_reader :foo

    def foo
      # do something...
    end
  end
  • Don’t use empty lines around method, class, module, block bodies. [link]
  # bad
  class Foo

    def foo

      begin

        do_something do

          something

        end

      rescue

        something

      end

    end

  end

  # good
  class Foo
    def foo
      begin
        do_something do
          something
        end
      rescue
        something
      end
    end
  end
  • Avoid comma after the last parameter in a method call, especially when the parameters are not on separate lines. [link]
  # 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)
  • Use spaces around the = operator when assigning default values to method parameters: [link]
  # 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).

  • Avoid line continuation \ where not required. In practice, avoid using line continuations for anything but string concatenation. [link]
  # bad
  result = 1 - \
           2

  # good (but still ugly as hell)
  result = 1 \
           - 2

  long_string = 'First part of the long string' \
                ' and second part of the long string'
  • Adopt a consistent multi-line method chaining style. There are two popular styles in the Ruby community, both of which are considered good&mdash;leading . (Option A) and trailing . (Option B). [link]

    • (Option A) When continuing a chained method invocation 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
    
    • (Option B) When continuing a chained method invocation 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.

  • Align the parameters of a method call if they span more than one line. When aligning parameters is not appropriate due to line-length constraints, single indent for the lines after the first is also acceptable. [link]
  # starting point (line is too long)
  def send_mail(source)
    Mailer.deliver(to: '[email protected]', from: '[email protected]', subject: 'Important message', body: source.text)
  end

  # bad (double indent)
  def send_mail(source)
    Mailer.deliver(
        to: '[email protected]',
        from: '[email protected]',
        subject: 'Important message',
        body: source.text)
  end

  # good
  def send_mail(source)
    Mailer.deliver(to: '[email protected]',
                   from: '[email protected]',
                   subject: 'Important message',
                   body: source.text)
  end

  # good (normal indent)
  def send_mail(source)
    Mailer.deliver(
      to: '[email protected]',
      from: '[email protected]',
      subject: 'Important message',
      body: source.text
    )
  end
  • Align the elements of array literals spanning multiple lines. [link]
  # bad - single indent
  menu_item = ['Spam', 'Spam', 'Spam', 'Spam', 'Spam', 'Spam', 'Spam', 'Spam',
    'Baked beans', 'Spam', 'Spam', 'Spam', 'Spam', 'Spam']

  # good
  menu_item = [
    'Spam', 'Spam', 'Spam', 'Spam', 'Spam', 'Spam', 'Spam', 'Spam',
    'Baked beans', 'Spam', 'Spam', 'Spam', 'Spam', 'Spam'
  ]

  # good
  menu_item =
    ['Spam', 'Spam', 'Spam', 'Spam', 'Spam', 'Spam', 'Spam', 'Spam',
     'Baked beans', 'Spam', 'Spam', 'Spam', 'Spam', 'Spam']
  • Add underscores to large numeric literals to improve their readability. [link]
  # bad - how many 0s are there?
  num = 1000000

  # good - much easier to parse for the human brain
  num = 1_000_000
  • Prefer smallcase letters for numeric literal prefixes. 0o for octal, 0x for hexadecimal and 0b for binary. Do not use 0d prefix for decimal literals. [link]
  # 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
  • Use [Rdoc][rdoc] and its conventions for API documentation. Don’t put an empty line between the comment block and the def. [link]

  • Limit lines to 80 characters. [link]

  • Avoid trailing whitespace. [link]

  • End each file with a newline. [link]

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

  # bad
  =begin
  comment line
  another comment line
  =end

  # good
  # comment line
  # another comment line

Syntax

  • Use :: only to reference constants(this includes classes and modules) and constructors (like Array() or Nokogiri::HTML()). Do not use :: for regular method invocation. [link]
  # bad
  SomeClass::some_method
  some_object::some_method

  # good
  SomeClass.some_method
  some_object.some_method
  SomeModule::SomeClass::SOME_CONST
  SomeModule::SomeClass()
  • Use def with parentheses when there are parameters. Omit the parentheses when the method doesn’t accept any parameters. [link]
   # 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
  • Use parentheses around the arguments of method invocations, especially if the first argument begins with an open parenthesis (, as in f((3 + 2) + 1). [link]
  # 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)

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 (e.g., Rake, Rails, RSpec):

    # bad
    validates(:name, presence: true)
    # good
    validates :name, presence: true
    
  • Methods that have “keyword” status in Ruby:

    class Person
      # bad
      attr_reader(:name, :age)
      # good
      attr_reader :name, :age
    
    
      # body omitted
    end
    

Can omit parentheses for

  • Methods that have “keyword” status in Ruby, but are not declarative:

    # good
    puts(temperance.age)
    system('ls')
    # also good
    puts temperance.age
    system 'ls'
    
  • 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. [link]

  # 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'
  • 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. [link]
  # 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]
  • 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. [link]
  # 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(',')
  • 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&mdash;for doesn’t introduce a new scope (unlike each) and variables defined in its block will be visible outside it. [link]
  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's block
  elem # => NameError: undefined local variable or method `elem'
  • Do not use then for multi-line if/unless. [link]
  # bad
  if some_condition then
    # body omitted
  end

  # good
  if some_condition
    # body omitted
  end
  • Always put the condition on the same line as the if/unless in a multi-line conditional. [link]
  # bad
  if
    some_condition
    do_something
    do_something_else
  end

  # good
  if some_condition
    do_something
    do_something_else
  end
  • Favor the ternary operator(?:) over if/then/else/end constructs. It’s more common and obviously more concise. [link]
  # bad
  result = if some_condition then something else something_else end

  # good
  result = some_condition ? something : something_else
  • 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. [link]
  # 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
  • Do not use if x; .... Use the ternary operator instead. [link]
  # bad
  result = if some_condition; something else something_else end

  # good
  result = some_condition ? something : something_else
  • Leverage the fact that if and case are expressions which return a result. [link]
  # bad
  if condition
    result = x
  else
    result = y
  end

  # good
  result =
    if condition
      x
    else
      y
    end
  • Use when x then ... for one-line cases. The alternative syntax when x: ... has been removed as of Ruby 1.9. [link]

  • Do not use when x; .... See the previous rule. [link]

  • Use ! instead of not. [link]

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

  # good
  x = !something
  • Avoid the use of !!. [link]

!! 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. If you want to do a nil check, use nil? instead.

  # bad
  x = 'test'
  # obscure nil check
  if !!x
    # body omitted
  end

  # good
  x = 'test'
  if x
    # body omitted
  end
  • 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. [link]
  # bad
  # boolean expression
  ok = got_needed_arguments and arguments_are_valid

  # control flow
  document.save or fail(RuntimeError, "Failed to save document!")

  # good
  # boolean expression
  ok = got_needed_arguments && arguments_are_valid

  # control flow
  fail(RuntimeError, "Failed to save document!") unless document.save

  # ok
  # control flow
  document.save || fail(RuntimeError, "Failed to save document!")
  • Avoid multi-line ?: (the ternary operator); use if/unless instead. [link]

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

  # bad
  if some_condition
    do_something
  end

  # good
  do_something if some_condition

  # another good option
  some_condition && do_something
  • Avoid modifier if/unless usage at the end of a non-trivial multi-line block. [link]
  # 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
  • Avoid nested modifier if/unless/while/until usage. Favor &&/|| if appropriate. [link]
  # bad
  do_something if other_condition if some_condition

  # good
  do_something if some_condition && other_condition
  • Favor unless over if for negative conditions (or control flow ||). [link]
  # 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
  • Do not use unless with else. Rewrite these with the positive case first. [link]
  # bad
  unless success?
    puts 'failure'
  else
    puts 'success'
  end

  # good
  if success?
    puts 'success'
  else
    puts 'failure'
  end
  • Don’t use parentheses around the condition of a control expression. [link]
  # bad
  if (x > 10)
    # body omitted
  end

  # good
  if x > 10
    # body omitted
  end

Note that there is an exception to this rule, namely safe assignment in condition.

  • Do not use while/until condition do for multi-line while/until. [link]
  # 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
  • Favor modifier while/until usage when you have a single-line body. [link]
  # bad
  while some_condition
    do_something
  end

  # good
  do_something while some_condition
  • Favor until over while for negative conditions. [link]
  # bad
  do_something while !some_condition

  # good
  do_something until some_condition
  • Use Kernel#loop instead of while/until when you need an infinite loop. [link]

    # bad
    while true
      do_something
    end
    
    
    until false
      do_something
    end
    
    # good
    loop do
      do_something
    end
    
  • Use Kernel#loop with break rather than begin/end/until or begin/end/while for post-loop tests. [link]

  # bad
  begin
    puts val
    val += 1
  end while val < 0

  # good
  loop do
    puts val
    val += 1
    break unless val < 0
  end
  • Omit the outer braces around an implicit options hash. [link]
  # bad
  user.set({ name: 'John', age: 45, permissions: { read: true } })

  # good
  user.set(name: 'John', age: 45, permissions: { read: true })
  • Omit both the outer braces and parentheses for methods that are part of an internal DSL. [link]
  class Person < ActiveRecord::Base
    # bad
    validates(:name, { presence: true, length: { within: 1..10 } })

    # good
    validates :name, presence: true, length: { within: 1..10 }
  end
  • Use the proc invocation shorthand when the invoked method is the only operation of a block. [link]
  # bad
  names.map { |name| name.upcase }

  # good
  names.map(&:upcase)
  • 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. [link]
  names = %w[Bozhidar Steve 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&mdash;is this code really readable and can the blocks’ contents be extracted into nifty methods?

  • Consider using explicit block argument to avoid writing block literal that just passes its arguments to another block. Beware of the performance impact, though, as the block gets converted to a Proc. [link]
  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
  • Avoid return where not required for flow of control. [link]
  # bad
  def some_method(some_arr)
    return some_arr.size
  end

  # good
  def some_method(some_arr)
    some_arr.size
  end
  • Avoid self where not required. (It is only required when calling a self write accessor, methods named after reserved words, or overloadable operators.) [link]
  # 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
  • As a corollary, avoid shadowing methods with local variables unless they are both equivalent. [link]
  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
  • 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. [link]
  # 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
  • Use shorthand self assignment operators whenever applicable. [link]
  # 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
  • Use ||= to initialize variables only if they’re not already initialized. [link]
  # 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.) [link]
  # bad - would set enabled to true even if it was false
  enabled ||= true

  # good
  enabled = true if enabled.nil?
  • 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. [link]
  # 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
  • 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. [link]
  # bad
  Array === something
  (1..100) === 7
  /something/ === some_string

  # good
  something.is_a?(Array)
  (1..100).include?(7)
  some_string =~ /something/
  • Do not use eql? when using == will do. The stricter comparison semantics provided by eql? are rarely needed in practice. [link]
  # 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
  • Avoid using Perl-style special variables (like $:, $;, etc. ). They are quite cryptic and their use in anything but one-liner scripts is discouraged. Use the human-friendly aliases provided by the English library. [link]
  # bad
  $:.unshift File.dirname(__FILE__)

  # good
  require 'English'
  $LOAD_PATH.unshift File.dirname(__FILE__)
  • Do not put a space between a method name and the opening parenthesis. [link]
  # bad
  f (3 + 2) + 1

  # good
  f(3 + 2) + 1
  • Always run the Ruby interpreter with the -w option so it will warn you if you forget either of the rules above! [link]

  • 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 invoked. [link]

  # 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
  • Use the new lambda literal syntax for single line body blocks. Use the lambda method for multi-line blocks. [link]
  # 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
  • Don’t omit the parameter parentheses when defining a stabby lambda with parameters. [link]
  # bad
  l = ->x, y { something(x, y) }

  # good
  l = ->(x, y) { something(x, y) }
  • Omit the parameter parentheses when defining a stabby lambda with no parameters. [link]
  # bad
  l = ->() { something }

  # good
  l = -> { something }
  • Prefer proc over Proc.new. [link]
  # bad
  p = Proc.new { |n| puts n }

  # good
  p = proc { |n| puts n }
  • Prefer proc.call() over proc[] or proc.() for both lambdas and procs. [link]
  # bad - looks similar to Enumeration access
  l = ->(v) { puts v }
  l[1]

  # also bad - uncommon syntax
  l = ->(v) { puts v }
  l.(1)

  # good
  l = ->(v) { puts v }
  l.call(1)
  • 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 and will suppress their unused variable warnings. [link]
  # 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
  • 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. [link]

  • 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). [link]

  • Favor the use of sprintf and its alias format over the fairly cryptic String#% method. [link]

  # 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'
  • When using named format string tokens, favor %<name>s over %{name} because it encodes information about the type of the value. [link]
  # bad
  format('Hello, %{name}', name: 'John')

  # good
  format('Hello, %<name>s', name: 'John')
  • Favor the use of Array#join over the fairly cryptic Array#* with a string argument. [link]
  # bad
  %w[one two three] * ', '
  # => 'one, two, three'

  # good
  %w[one two three].join(', ')
  # => 'one, two, three'
  • 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. [link]
  # 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) }
  • Use ranges or Comparable#between? instead of complex comparison logic when possible. [link]
  # bad
  do_something if x >= 1000 && x <= 2000

  # good
  do_something if (1000..2000).include?(x)

  # good
  do_something if x.between?(1000, 2000)
  • Favor the use of predicate methods to explicit comparisons with ==. Numeric comparisons are OK. [link]
  # 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
  • Don’t do explicit non-nil checks unless you’re dealing with boolean values. [link]

    # bad
    do_something if !something.nil?
    do_something if something != nil
    
    # good
    do_something if something
    
    # good - dealing with a boolean
    def value_set?
      [email protected]_boolean.nil?
    end
    
  • Avoid the use of BEGIN blocks. [link]

  • Do not use END blocks. Use Kernel#at_exit instead. [link]

  # bad
  END { puts 'Goodbye!' }

  # good
  at_exit { puts 'Goodbye!' }
  • Avoid the use of flip-flops. [link]

  • Avoid use of nested conditionals for flow of control. [link]

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
  • Prefer map over collect, find over detect, select over find_all, reduce over inject 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. [link]

  • 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. [link]

  # bad
  some_hash.count

  # good
  some_hash.size
  • 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. [link]
  # bad
  all_songs = users.map(&:songs).flatten.uniq

  # good
  all_songs = users.flat_map(&:songs).uniq
  • 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. [link]
  # bad
  array.reverse.each { ... }

  # good
  array.reverse_each { ... }

Naming

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

  • Name identifiers in English. [link]
  # bad - identifier using non-ascii characters
  заплата = 1_000

  # bad - identifier is a Bulgarian word, written with Latin letters (instead of Cyrillic)
  zaplata = 1_000

  # good
  salary = 1_000
  • Use snake_case for symbols, methods and variables. [link]
  # bad
  :'some symbol'
  :SomeSymbol
  :someSymbol

  someVar = 5
  var_10  = 10

  def someMethod
    # some code
  end

  def SomeMethod
    # some code
  end

  # good
  :some_symbol

  some_var = 5
  var10    = 10

  def some_method
    # some code
  end
  • Do not separate numbers from letters on symbols, methods and variables. [link]
  # bad
  :some_sym_1

  some_var_1 = 1

  def some_method_1
    # some code
  end

  # good
  :some_sym1

  some_var1 = 1

  def some_method1
    # some code
  end
  • Use CamelCase for classes and modules. (Keep acronyms like HTTP, RFC, XML uppercase.) [link]
  # 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
  • Use snake_case for naming files, e.g. hello_world.rb. [link]

  • Use snake_case for naming directories, e.g. lib/hello_world/hello_world.rb. [link]

  • Aim to have just a single class/module per source file. Name the file name as the class/module, but replacing CamelCase with snake_case. [link]

  • Use SCREAMING_SNAKE_CASE for other constants. [link]

  # bad
  SomeConst = 5

  # good
  SOME_CONST = 5
  • 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. [link]

  • 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?. [link]

  # 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
  • 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. [link]
  # 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
  • Define the non-bang (safe) method in terms of the bang (dangerous) one if possible. [link]
  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
  • When defining binary operators, name the parameter other(<< and [] are exceptions to the rule, since their semantics are different). [link]
  def +(other)
    # body omitted
  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

  • Write self-documenting code and ignore the rest of this section. Seriously! [link]

  • Write comments in English. [link]

  • Use one space between the leading # character of the comment and the text of the comment. [link]

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

  • Avoid superfluous comments. [link]

  # bad
  counter += 1 # Increments counter by one.
  • Keep existing comments up-to-date. An outdated comment is worse than no comment at all. [link]

Good code is like a good joke: it needs no explanation.
&mdash; 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&mdash;there is no try.” Yoda) [link]

Comment Annotations

  • Annotations should usually be written on the line immediately above the relevant code. [link]

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

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

  def bar
    # FIXME: This has crashed occasionally since v3.2.1. It may
    #   be related to the BarBazUtil upgrade.
    baz(:quux)
  end
  • 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. [link]
  def bar
    sleep 100 # OPTIMIZE
  end
  • Use TODO to note missing features or functionality that should be added at a later date. [link]

  • Use FIXME to note broken code that needs to be fixed. [link]

  • Use OPTIMIZE to note slow or inefficient code that may cause performance problems. [link]

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

  • 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? [link]

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

Magic Comments

  • Place magic comments above all code and documentation. Magic comments should only go below shebangs if they are needed in your source file. [link]
  # good
  # frozen_string_literal: true
  # Some documentation about Person
  class Person
  end

  # bad
  # Some documentation about Person
  # frozen_string_literal: true
  class Person
  end
  # good
  #!/usr/bin/env ruby
  # frozen_string_literal: true
  App.parse(ARGV)

  # bad
  # frozen_string_literal: true
  #!/usr/bin/env ruby
  App.parse(ARGV)
  • Use one magic comment per line if you need multiple. [link]
  # good
  # frozen_string_literal: true
  # encoding: ascii-8bit

  # bad
  # -*- frozen_string_literal: true; encoding: ascii-8bit -*-
  • Separate magic comments from code and documentation with a blank line. [link]
  # good
  # frozen_string_literal: true

  # Some documentation for Person
  class Person
    # Some code
  end

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

Classes & Modules

  • Use a consistent structure in your class definitions. [link]
  class Person
    # extend and include go first
    extend SomeModule
    include AnotherModule

    # 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
  • Split multiple mixins into separate statements. [link]
  # bad
  class Person
    include Foo, Bar
  end

  # good
  class Person
    # multiple mixins go in separate statements
    include Foo
    include Bar
  end
  • 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. [link]
  # 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
  • Prefer modules to classes with only class methods. Classes should be used only when it makes sense to create instances out of them. [link]
  # 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
  • Favor the use of module_function over extend self when you want to turn a module’s instance methods into class methods. [link]
  # 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
  • When designing class hierarchies make sure that they conform to the Liskov Substitution Principle. [link]

  • Try to make your classes as SOLID as possible. [link]

  • Always supply a proper to_s method for classes that represent domain objects. [link]

  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
  • Use the attr family of functions to define trivial accessors or mutators. [link]
  # 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
  • 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). [link]
  # 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
  • Avoid the use of attr. Use attr_reader and attr_accessor instead. [link]
  # 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
  • Consider using Struct.new, which defines the trivial accessors, constructor and comparison operators for you. [link]
  # 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 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. [link]
  # bad
  class Person < Struct.new(:first_name, :last_name)
  end

  # good
  Person = Struct.new(:first_name, :last_name)
  • name=”

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