Ruby Style Guide

This guide is a modified version of https://github.com/bbatsov/ruby-style-guide.

The guide is a work in progress and a couple of things to note as we try and move towards using these guidelines:

• Be consistent
• Don't rewrite legacy code, unless you are working in that area
• If you don't like a rule or want to add a new rule, please bring it up during standup or via group email, get consensus from the team and then add it to this guide.

Table of Contents

• Source Code Layout
• Syntax
• Naming
• Comments
  • Comment Annotations
• Classes
• Methods
• Exceptions
• Collections
• Strings
• Regular Expressions
• Percent Literals
• Metaprogramming

Source Code Layout

• Use UTF-8 as the source file encoding.

• Use two spaces per indentation level. No hard tabs.

• Keep lines fewer than 80 characters.

• Never leaving trailing whitespace.

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

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

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

  # bad
  e = M * c ** 2
  
  # good
  e = M * c**2

• No spaces after (, [ or before ], ).

  some(arg).other
  [1, 2, 3].length

• Prefer a single-line format for class definitions with no body.

  # bad
  class FooError < StandardError
  end
  
  # good
  class FooError < StandardError; end

• 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 no more than one expression in a single-line method.

  # 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 ; make 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

• Indent when as deep as case. I know that many would disagree with this one, but it's the style established in both "The Ruby Programming Language" and "Programming Ruby".

  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

• 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).

• When continuing a chained method invocation on another line keep the . on the second line. And indent 2 spaces 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

• Align the parameters of a method call if they span more than one line.

  # 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 (normal indent)
  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

• 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

Syntax

• Use def with parentheses when there are arguments. Omit the parentheses when the method doesn't accept any arguments.

  def some_method
    # body omitted
  end
  
  def some_method_with_arguments(arg1, arg2)
    # body omitted
  end

• 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 }

• Never use then for multi-line if/unless.

  # bad
  if some_condition then
    # body omitted
  end
  
  # good
  if some_condition
    # body omitted
  end

• 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

• Use ! instead of not.

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

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

  # bad
  # boolean expression
  if some_condition and some_other_condition
    do_something
  end
  
  # control flow
  document.saved? or document.save!
  
  # good
  # boolean expression
  if some_condition && some_other_condition
    do_something
  end
  
  # control flow
  document.saved? || document.save!

• Favor 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

• 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

• 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)
    # body omitted
  end
  
  # good
  if x > 10
    # body omitted
  end
  
  # ok
  if (x = self.next_value)
    # body omitted
  end

• Omit parentheses around parameters for methods that are part of an internal DSL (e.g. Rake, Rails, RSpec), methods that have "keyword" status in Ruby (e.g. attr_reader, puts) and attribute access methods. Use parentheses around the arguments of all other method invocations.

  class Person
    attr_reader :name, :age
  
    # omitted
  end
  
  temperance = Person.new('Temperance', 30)
  temperance.name
  
  puts temperance.age
  
  x = Math.sin(y)
  array.delete(e)
  
  bowling.score.should == 0

• 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']
  
  # 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 { |name| name.upcase }

  Some will argue that multiline 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?

• 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

• Avoid self where not required. (It is only required when calling a self write accessor.)

  # 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.

  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.

  # bad (+ a warning)
  if (v = array.grep(/foo/))
    do_something(v)
    ...
  end
  
  # bad (+ a warning)
  if v = array.grep(/foo/)
    do_something(v)
    ...
  end
  
  # good
  v = array.grep(/foo/)
  if v
    do_something(v)
    ...
  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?

• Never put a space between a method name and the opening parenthesis.

  # bad
  f (3 + 2) + 1
  
  # good
  f(3 + 2) + 1

• If the first argument to a method begins with an open parenthesis, always use parentheses in the method invocation. For example, write f((3 + 2) + 1).

• Use the new lambda literal syntax.

  # bad
  lambda = lambda { |a, b| a + b }
  lambda.call(1, 2)
  
  # good
  lambda = ->(a, b) { a + b }
  lambda.(1, 2)

• Use _ for unused block parameters.

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

• 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.

• Use ranges instead of complex comparison logic when possible.

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

Naming

• Name identifiers in English.

• Use snake_case for symbols, methods and variables.

• Use SCREAMING_SNAKE_CASE for other constants.

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

  # bad
  class Someclass
    ...
  end
  
  class Some_Class
    ...
  end
  
  class SomeXml
    ...
  end
  
  # good
  class SomeClass
    ...
  end
  
  class SomeXML
    ...
  end

• 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! (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 not 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

• 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

Comments

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

• Write comments in English.

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

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

• Avoid superfluous comments.

  # bad
  counter += 1 # increments counter by one

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

Good code is like a good joke - it needs no explanation.
-- 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 should usually be written on the line immediately above the relevant code.

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

• If multiple lines are required to describe the problem, subsequent lines should be indented two spaces after the #.

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

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

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

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

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

• 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?

Classes

• Try to make your classes as [SOLID](http://en.wikipedia.org/wiki/SOLID_(object-oriented_design\)) as possible.

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

• Use a consistent structure in your class definitions.

  class Person
    # extend and include go first
    extend SomeModule
    include AnotherModule
  
    # 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
  
    # followed by 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

• 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

• 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

• 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.

• 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).

• Use def self.method to define singleton 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 singleton methods.
    class << self
      def first_method
        # body omitted
      end
  
      def second_method_etc
        # body omitted
      end
    end
  end

Methods

• Methods should not be longer than five lines of code. Empty lines do not contribute to the relevant LOC.
• Avoid parameter lists longer than four parameters.

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. 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

• 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

• Don't suppress exceptions.

  # bad
  begin
    # an exception occurs here
  rescue SomeError
    # the rescue clause does absolutely nothing
  end
  
  # bad
  do_something rescue nil

• Avoid using rescue in its modifier form.

  # bad - this catches all StandardError exceptions
  do_something rescue nil

• Favor the use of exceptions for the standard library over introducing new exception classes.

Collections

• 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 = []
  hash = {}

• 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)

• 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.

• Prefer symbols instead of strings as hash keys.

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

• Avoid the use of mutable objects as hash keys.

• Use the 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 }

Strings

• Prefer string interpolation instead of string concatenation:

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

• Consider padding string interpolation code with space. It more clearly sets the code apart from the string.

  "#{ user.last_name }, #{ user.first_name }"

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

  # bad
  name = "Bozhidar"
  
  # good
  name = 'Bozhidar'

• 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

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

• 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/] # 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.

• 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>)

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

  # bad
  %r(\s+)
  
  # still bad
  %r(^/(.*)$)
  # should be /^\/(.*)$/
  
  # good
  %r(^/blog/2011/(.*)$)

Metaprogramming

• Avoid needless metaprogramming.

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

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

  • 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 is preferable to class_eval{ def ... }

• When using class_eval (or other eval) with string interpolation, add a comment block showing its appearance if interpolated (a practice I learned from the 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}(*args, &block)       # def capitalize(*args, &block)
          to_str.#{unsafe_method}(*args, &block)  #   to_str.capitalize(*args, &block)
        end                                       # end
  
        def #{unsafe_method}!(*args)              # def capitalize!(*args)
          @dirty = true                           #   @dirty = true
          super                                   #   super
        end                                       # end
      EOT
    end
  end

• avoid using method_missing for metaprogramming. Backtraces become messy; the behavior is not listed in #methods; misspelled method calls might silently work (nukes.launch_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, *args, &block)
      if /^find_by_(?<prop>.*)/ =~ meth
        # ... lots of code to do a find_by
      else
        super
      end
    end
    
    # good
    def method_missing?(meth, *args, &block)
      if /^find_by_(?<prop>.*)/ =~ meth
        find_by(prop, *args, &block)
      else
        super
      end
    end
    
    # best of all, though, would to define_method as each findable attribute is declared

Contributing

Nothing written in this guide is set in stone. If you disagree or want to add to it please bring it up during standup or send an email with your rule/justification.