Ruby Style Guide

This is Airbnb's Ruby Style Guide.

It was inspired by Github's guide and Bozhidar Batsov's guide.

Airbnb also maintains a JavaScript Style Guide.

Whitespace

Indentation

Use soft-tabs with a two space-indent.

Indent when as deep as case.

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

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

Align function parameters either all on the same line or one per line.

# good
def self.create_translation(phrase_id,
                            phrase_key,
                            target_locale,
                            value,
                            user_id,
                            do_xss_check,
                            allow_verification)
  ...
end

# bad
def self.create_translation(phrase_id, phrase_key, target_locale,
                            value, user_id, do_xss_check, allow_verification)
  ...
end

Indent succeeding lines in multi-line boolean expressions.

# good
def is_eligible?(user)
  Trebuchet.current.launch?(ProgramEligibilityHelper::PROGRAM_TREBUCHET_FLAG) &&
    is_in_program?(user) &&
    program_not_expired
end

# bad
def is_eligible?(user)
  Trebuchet.current.launch?(ProgramEligibilityHelper::PROGRAM_TREBUCHET_FLAG) &&
  is_in_program?(user) &&
  program_not_expired
end

Inline

Never leave trailing whitespace.

Use spaces around operators; after commas, colons, and semicolons; and around { and before }.

sum = 1 + 2
a, b = 1, 2
1 > 2 ? true : false; puts 'Hi'
[1, 2, 3].each { |e| puts e }

No spaces after (, [ or before ], ).
```
some(arg).other
[1, 2, 3].length
```

Newlines

Add a new line after if conditions span multiple lines to help differentiate between the conditions and the body.

if @reservation_alteration.checkin == @reservation.start_date &&
   @reservation_alteration.checkout == (@reservation.start_date + @reservation.nights)

  redirect_to_alteration @reservation_alteration
end

Add a new line after conditionals, blocks, case statements, etc.

if robot.is_awesome?
  send_robot_present
end

robot.add_trait(:human_like_intelligence)

Line Length

Keep each line of code to a readable length. Unless you have a reason to, keep lines to fewer than 100 characters. Keeping code visually grouped together (as a 100-character line limit enforces) makes it easier to understand. For example, you don't have to scroll back and forth on one line to see what's going on -- you can view it all together.

Here are examples from our codebase showing several techniques for breaking complex statements into multiple lines that are all < 100 characters. Notice techniques like:

liberal use of linebreaks inside unclosed ( { [
chaining methods, ending unfinished chains with a .
composing long strings by putting strings next to each other, separated by a backslash-then-newline.
breaking long logical statements with linebreaks after operators like && and ||

scope = Translation::Phrase.includes(:phrase_translations).
  joins(:phrase_screenshots).
  where(:phrase_screenshots => {
    :controller => controller_name,
    :action => JAROMIR_JAGR_SALUTE,
  })

translation = FactoryGirl.create(
  :phrase_translation,
  :locale => :is,
  :phrase => phrase,
  :key => 'phone_number_not_revealed_time_zone',
  :value => 'Símanúmerið þitt verður ekki birt. Það er aðeins hægt að hringja á '\
            'milli 9:00 og 21:00 %{time_zone}.'
)

if @reservation_alteration.checkin == @reservation.start_date &&
   @reservation_alteration.checkout == (@reservation.start_date + @reservation.nights)

  redirect_to_alteration @reservation_alteration
end

<% if @presenter.guest_visa_russia? %>
  <%= icon_tile_for(I18n.t("email.reservation_confirmed_guest.visa.details_header",
                           :default => "Visa for foreign Travelers"),
                    :beveled_big_icon => "stamp") do %>
    <%= I18n.t("email.reservation_confirmed_guest.visa.russia.details_copy",
               :default => "Foreign guests travelling to Russia may need to obtain a visa...") %>
  <% end %>
<% end %>

These code snippets are very much more readable than the alternative:

scope = Translation::Phrase.includes(:phrase_translations).joins(:phrase_screenshots).where(:phrase_screenshots => { :controller => controller_name, :action => JAROMIR_JAGR_SALUTE })

translation = FactoryGirl.create(:phrase_translation, :locale => :is, :phrase => phrase, :key => 'phone_number_not_revealed_time_zone', :value => 'Símanúmerið þitt verður ekki birt. Það er aðeins hægt að hringja á milli 9:00 og 21:00 %{time_zone}.')

if @reservation_alteration.checkin == @reservation.start_date && @reservation_alteration.checkout == (@reservation.start_date + @reservation.nights)
  redirect_to_alteration @reservation_alteration
end

<% if @presenter.guest_visa_russia? %>
  <%= icon_tile_for(I18n.t("email.reservation_confirmed_guest.visa.details_header", :default => "Visa for foreign Travelers"), :beveled_big_icon => "stamp") do %>
    <%= I18n.t("email.reservation_confirmed_guest.visa.russia.details_copy", :default => "Foreign guests travelling to Russia may need to obtain a visa prior to...") %>
  <% end %>
<% end %>

Commenting

Though a pain to write, comments are absolutely vital to keeping our code readable. The following rules describe what you should comment and where. But remember: while comments are very important, the best code is self-documenting. Giving sensible names to types and variables is much better than using obscure names that you must then explain through comments.

When writing your comments, write for your audience: the next contributor who will need to understand your code. Be generous — the next one may be you!

—Google C++ Style Guide

Portions of this section borrow heavily from the Google C++ and Python style guides.

File/class-level comments

Every class definition should have an accompanying comment that describes what it is for and how it should be used.

A file that contains zero classes or more than one class should have a comment at the top describing its contents.

# Automatic conversion of one locale to another where it is possible, like
# American to British English.
module Translation
  # Class for converting between text between similar locales.
  # Right now only conversion between American English -> British, Canadian,
  # Australian, New Zealand variations is provided.
  class PrimAndProper
    def initialize
      @converters = { :en => { :"en-AU" => AmericanToAustralian.new,
                               :"en-CA" => AmericanToCanadian.new,
                               :"en-GB" => AmericanToBritish.new,
                               :"en-NZ" => AmericanToKiwi.new,
                             } }
    end

  ...

  # Applies transforms to American English that are common to
  # variants of all other English colonies.
  class AmericanToColonial
    ...
  end

  # Converts American to British English.
  # In addition to general Colonial English variations, changes "apartment"
  # to "flat".
  class AmericanToBritish < AmericanToColonial
    ...
  end

All files, including data and config files, should have file-level comments. From translation/config/colonial_spelling_variants.yml:

# List of American-to-British spelling variants.
#
# This list is made with
# lib/tasks/list_american_to_british_spelling_variants.rake.
#
# It contains words with general spelling variation patterns:
#   [trave]led/lled, [real]ize/ise, [flav]or/our, [cent]er/re, plus
# and these extras:
#   learned/learnt, practices/practises, airplane/aeroplane, ...

sectarianizes: sectarianises
neutralization: neutralisation
...

Function comments

Every function declaration should have comments immediately preceding it that describe what the function does and how to use it. These comments should be descriptive ("Opens the file") rather than imperative ("Open the file"); the comment describes the function, it does not tell the function what to do. In general, these comments do not describe how the function performs its task. Instead, that should be left to comments interspersed in the function's code.

Every function should mention what the inputs and outputs are, unless it meets all of the following criteria:

not externally visible
very short
obvious

You may use whatever format you wish. In Ruby, two popular function documentation schemes are TomDoc and YARD. You can also just write things out concisely:

# Returns the fallback locales for the_locale.
# If opts[:exclude_default] is set, the default locale, which is otherwise
# always the last one in the returned list, will be excluded.
#
# For example:
#   fallbacks_for(:"pt-BR")
#     => [:"pt-BR", :pt, :en]
#   fallbacks_for(:"pt-BR", :exclude_default => true)
#     => [:"pt-BR", :pt]
def fallbacks_for(the_locale, opts = {})
  ...
end

Block and inline comments

The final place to have comments is in tricky parts of the code. If you're going to have to explain it at the next code review, you should comment it now. Complicated operations get a few lines of comments before the operations commence. Non-obvious ones get comments at the end of the line.

def fallbacks_for(the_locale, opts = {})
  # dup() to produce an array that we can mutate.
  ret = @fallbacks[the_locale].dup

  # We make two assumptions here:
  # 1) There is only one default locale (that is, it has no less-specific
  #    children).
  # 1) The default locale is just a language. (Like :en, and not :"en-US".)
  if opts[:exclude_default] &&
      ret.last == default_locale &&
      ret.last != language_from_locale(the_locale)
    ret.pop
  end

  ret
end

On the other hand, never describe the code. Assume the person reading the code knows the language (though not what you're trying to do) better than you do.

Punctuation, spelling and grammar

Pay attention to punctuation, spelling, and grammar; it is easier to read well-written comments than badly written ones.

Comments should be as readable as narrative text, with proper capitalization and punctuation. In many cases, complete sentences are more readable than sentence fragments. Shorter comments, such as comments at the end of a line of code, can sometimes be less formal, but you should be consistent with your style.

Although it can be frustrating to have a code reviewer point out that you are using a comma when you should be using a semicolon, it is very important that source code maintain a high level of clarity and readability. Proper punctuation, spelling, and grammar help with that goal.

TODO comments

Use TODO comments for code that is temporary, a short-term solution, or good-enough but not perfect.

TODOs should include the string TODO in all caps, followed by the full name of the person who can best provide context about the problem referenced by the TODO, in parentheses. A colon is optional. A comment explaining what there is to do is required. The main purpose is to have a consistent TODO format that can be searched to find the person who can provide more details upon request. A TODO is not a commitment that the person referenced will fix the problem. Thus when you create a TODO, it is almost always your name that is given.

  # bad
  # TODO(RS): Use proper namespacing for this constant.

  # bad
  # TODO(drumm3rz4lyfe): Use proper namespacing for this constant.

  # good
  # TODO(Ringo Starr): Use proper namespacing for this constant.

Commented-out code

Never leave commented-out code in our codebase.

Methods

Method definitions

Use def with parentheses when there are parameters. Omit the parentheses when the method doesn't accept any parameters.
```
def some_method
  # body omitted
end

def some_method_with_parameters(arg1, arg2)
  # body omitted
end
```

Do not use default arguments. Use an options hash instead.

# bad
def obliterate(things, gently = true, except = [], at = Time.now)
  ...
end

# good
def obliterate(things, options = {})
  default_options = {
    :gently => true, # obliterate with soft-delete
    :except => [], # skip obliterating these things
    :at => Time.now, # don't obliterate them until later
  }
  options.reverse_merge!(default_options)

  ...
end

Method calls

Use parentheses for a method call:

If the method returns a value.

# bad
@current_user = User.find_by_id 1964192

# good
@current_user = User.find_by_id(1964192)

If the first argument to the method uses parentheses.

# bad
put! (x + y) % len, value

# good
put!((x + y) % len, value)

Never put a space between a method name and the opening parenthesis.
```
# bad
f (3 + 2) + 1

# good
f(3 + 2) + 1
```
Omit parentheses for a method call if the method accepts no arguments.
```
# bad
nil?()

# good
nil?
```
If the method doesn't return a value (or we don't care about the return), parentheses are optional. (Especially if the arguments overflow to multiple lines, parentheses may add readability.)
```
# okay
render(:partial => 'foo')

# okay
render :partial => 'foo'
```

In either case:

If a method accepts an options hash as the last argument, do not use { } during invocation.

# bad
get '/v1/reservations', { :id => 54875 }

# good
get '/v1/reservations', :id => 54875

Conditional Expressions

Conditional keywords

Never use then for multi-line if/unless.

# bad
if some_condition then
  ...
end

# good
if some_condition
  ...
end

The and, or, and not keywords are banned. It's just not worth it. Always use &&, ||, and ! instead.

Modifier if/unless usage is okay when the body is simple, the condition is simple, and the whole thing fits on one line. Otherwise, avoid modifier if/unless.

# bad - this doesn't fit on one line
add_trebuchet_experiments_on_page(request_opts[:trebuchet_experiments_on_page]) if request_opts[:trebuchet_experiments_on_page] && !request_opts[:trebuchet_experiments_on_page].empty?

# okay
if request_opts[:trebuchet_experiments_on_page] &&
     !request_opts[:trebuchet_experiments_on_page].empty?

  add_trebuchet_experiments_on_page(request_opts[:trebuchet_experiments_on_page])
end

# bad - this is complex and deserves multiple lines and a comment
parts[i] = part.to_i(INTEGER_BASE) if !part.nil? && [0, 2, 3].include?(i)

# okay
return if reconciled?

Never 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

Avoid unless with multiple conditions.

  # bad
  unless foo? && bar?
    ...
  end

  # okay
  if !(foo? && bar?)
    ...
  end

Don't use parentheses around the condition of an if/unless/while, unless the condition contains an assignment (see Using the return value of = below).
```
# bad
if (x > 10)
  ...
end

# good
if x > 10
  ...
end

# ok
if (x = next_value)
  ...
end
```

Ternary operator

Avoid the ternary operator (?:) except in cases where all expressions are extremely trivial. However, do use the ternary operator(?:) over if/then/else/end constructs for single line conditionals.
```
# 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.

# 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

Avoid multiple conditions in ternaries. Ternaries are best used with single conditions.

Avoid multi-line ?: (the ternary operator), use if/then/else/end instead.

# bad
some_really_long_condition_that_might_make_you_want_to_split_lines ?
  something : something_else

# good
if some_really_long_condition_that_might_make_you_want_to_split_lines
  something
else
  something_else
end

Syntax

Never 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

# good
arr.each { |elem| puts elem }
```
Prefer {...} over do...end for single-line blocks. Avoid using {...} for multi-line blocks (multiline 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 = ["Bozhidar", "Steve", "Sarah"]

# good
names.each { |name| puts name }

# bad
names.each do |name| puts name end

# good
names.select { |name| name.start_with?("S") }.map { |name| name.upcase }

# bad
names.select do |name|
  name.start_with?("S")
end.map { |name| name.upcase }
```
Some will argue that multiline chaining would look okay with the use of {...}, but they should ask themselves if this code is really readable and whether the block's content can be extracted into nifty methods.

Avoid return where not required.

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

# good
def some_method(some_arr)
  some_arr.size
end

Using the return value of = (an assignment) is ok, but surround the assignment with parenthesis.

# good - shows intended use of assignment
if (v = array.grep(/foo/))
  ...
end

# bad
if v = array.grep(/foo/)
  ...
end

# also good - shows intended use of assignment and has correct precedence
if (v = next_value) == "hello"
  ...
end

Use ||= freely to initialize variables.

# 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?

Avoid using Perl-style special variables (like $0-9, $, etc. ). They are quite cryptic and their use in anything but one-liner scripts is discouraged. Prefer long form versions such as $PROGRAM_NAME.

Use _ for unused block arguments.

# bad
result = hash.map { |k, v| v + 1 }

# good
result = hash.map { |_, v| v + 1 }

When a method block takes only one argument, and the body consists solely of reading an attribute or calling one method with no arguments, use the &: shorthand.

# bad
bluths.map { |bluth| bluth.occupation }
bluths.select { |bluth| bluth.blue_self? }

# good
bluths.map(&:occupation)
bluths.select(&:blue_self?)

Prefer some_method over self.some_method when calling a method on the current instance.
```
# bad
def end_date
  self.start_date + self.nights
end

# good
def end_date
  start_date + nights
end
```
In the following three common cases, self. is required by the language and is good to use:
1. When defining a class method: def self.some_method.
2. The left hand side when calling an assignment method, including assigning an attribute when self is an ActiveRecord model: self.guest = user.
3. Referencing the current instance's class: self.class.

Naming

Use snake_case for methods and variables.
Use CamelCase for classes and modules. (Keep acronyms like HTTP, RFC, XML uppercase.)
Use SCREAMING_SNAKE_CASE for other constants.
The names of predicate methods (methods that return a boolean value) should end in a question mark. (i.e. Array#empty?).
The names of potentially "dangerous" methods (i.e. methods that modify self or the arguments, exit!, etc.) should end with an exclamation mark. Bang methods should only exist if a non-bang method exists. (More on this.)

Name throwaway variables _.

payment, _ = Payment.complete_paypal_payment!(params[:token],
                                              native_currency,
                                              created_at)

Classes

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.

Use def self.method to define singleton methods. This makes the methods more resistant to refactoring changes.

class TestClass
  # bad
  def TestClass.some_method
    ...
  end

  # good
  def self.some_other_method
    ...
  end

Avoid class << self except when necessary, e.g. single accessors and aliased attributes.

class TestClass
  # bad
  class << self
    def first_method
      ...
    end

    def second_method_etc
      ...
    end
  end

  # good
  class << self
    attr_accessor :per_page
    alias_method :nwo, :find_by_name_with_owner
  end

  def self.first_method
    ...
  end

  def self.second_method_etc
    ...
  end
end

Indent the public, protected, and private methods as much the method definitions they apply to. Leave one blank line above and below them.

class SomeClass
  def public_method
    # ...
  end

  private

  def private_method
    # ...
  end
end

Exceptions

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

Avoid rescuing the Exception class.

# bad
begin
  # an exception occurs here
rescue Exception
  # exception handling
end

# good
begin
  # an exception occurs here
rescue StandardError
  # exception handling
end

# acceptable
begin
  # an exception occurs here
rescue
  # exception handling
end

Collections

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.

Use symbols instead of strings as hash keys.

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

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

Use multi-line hashes when it makes the code more readable, and use trailing commas to ensure that parameter changes don't cause extraneous diff lines when the logic has not otherwise changed.

hash = {
  :protocol => 'https',
  :only_path => false,
  :controller => :users,
  :action => :set_password,
  :redirect => @redirect_url,
  :secret => @secret,
}

Use a trailing comma in an Array that spans more than 1 line

# good
array = [1, 2, 3]

# good
array = [
  "car",
  "bear",
  "plane",
  "zoo",
]

Strings

Prefer string interpolation instead of string concatenation:

# bad
email_with_name = user.name + ' <' + user.email + '>'

# good
email_with_name = "#{user.name} <#{user.email}>"

Furthermore, keep in mind Ruby 1.9-style interpolation. Let's say you are composing cache keys like this:

CACHE_KEY = '_store'

cache.write(@user.id + CACHE_KEY)

Prefer string interpolation instead of string concatentation:

CACHE_KEY = '%d_store'

cache.write(CACHE_KEY % @user.id)

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.
```
# good and also fast
html = ''
html << '<h1>Page title</h1>'

paragraphs.each do |paragraph|
  html << "<p>#{paragraph}</p>"
end
```

Regular Expressions

Avoid using $1-9 as it can be hard to track what they contain. Named groups can be used instead.

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

# good
/(?<meaningful_var>regexp)/ =~ string
...
process meaningful_var

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

Use x modifier for complex regexps. This makes them more readable and you can add some useful comments. Just be careful as spaces are ignored.

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

Percent Literals

Use %w freely.
```
STATES = %w(draft open closed)
```

Use %() 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>)

Use %r only for regular expressions matching more than one '/' character.

# bad
%r(\s+)

# still bad
%r(^/(.*)$)
# should be /^\/(.*)$/

# good
%r(^/blog/2011/(.*)$)

Rails

When immediately returning after calling render or redirect_to, put return on the next line, not the same line.

# bad
render :text => 'Howdy' and return

# good
render :text => 'Howdy'
return

# still bad
render :text => 'Howdy' and return if foo.present?

# good
if foo.present?
  render :text => 'Howdy'
  return
end

Be Consistent

If you're editing code, take a few minutes to look at the code around you and determine its style. If they use spaces around all their arithmetic operators, you should too. If their comments have little boxes of hash marks around them, make your comments have little boxes of hash marks around them too.

The point of having style guidelines is to have a common vocabulary of coding so people can concentrate on what you're saying rather than on how you're saying it. We present global style rules here so people know the vocabulary, but local style is also important. If code you add to a file looks drastically different from the existing code around it, it throws readers out of their rhythm when they go to read it. Avoid this.

—Google C++ Style Guide

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Ruby Style Guide

Table of Contents

Whitespace

Indentation

Inline

Newlines

Line Length

Commenting

File/class-level comments

Function comments

Block and inline comments

Punctuation, spelling and grammar

TODO comments

Commented-out code

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