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Gem containing the rubocop.yml config that corresponds to the implementation of the Shopify's style guide for Ruby.
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Ruby Style Guide

Ruby is the main language at Shopify. We are primarily a Ruby shop and we are probably one of the largest out there. Ruby is the go-to language for new web projects and scripting.

We expect all developers at Shopify to have at least a passing understanding of Ruby. It's a great language. It will make you a better developer no matter what you work in day to day. What follows is a loose coding style to follow while developing in Ruby.

This Style Guide is the result of over a decade of Ruby development at Shopify. Much of its content is based on Bozhidar Batsov's Ruby Style Guide, adapted to Shopify by many contributors.

Adoption with RuboCop

We recommend using RuboCop in your Ruby projects to help you adopt this Style Guide. To know how to install and use RuboCop please refer to RuboCop's official documentation.

We offer a default RuboCop configuration you can inherit from and be in sync with this Style Guide. To use it, you can add this to your Gemfile:

gem 'rubocop-shopify', require: false

And add to the top of your project's RuboCop configuration file:

inherit_gem:
  rubocop-shopify: rubocop.yml

For more information about inheriting configuration from a gem please check RuboCop's documentation.

Table of Contents

  • General
  • Formatting
  • Syntax
  • Naming
  • Classes and Modules
  • Exceptions
  • Collections
  • Strings
  • Regular Expressions
  • Percent Literals
  • Testing

General

  • Make all lines of your methods operate on the same level of abstraction. (Single Level of Abstraction Principle)

  • Code in a functional way. Avoid mutation (side effects) when you can.

  • Avoid defensive programming

  • Avoid mutating arguments.

  • Avoid monkeypatching.

  • Avoid long methods.

  • Avoid long parameter lists.

  • Avoid needless metaprogramming.

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

  • Write ruby -w safe code.

  • Avoid more than three levels of block nesting.

Formatting

  • Use UTF-8 as the source file encoding.

  • Use 2 space indent, no tabs.

  • Use Unix-style line endings.

  • Avoid using ; to separate statements and expressions. Use one expression per line.

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

  • Avoid spaces after (, [ and before ], ).

  • Avoid space after the ! operator.

  • Avoid space inside range literals.

  • Indent when as deep as the case line.

  • When assigning the result of a conditional expression to a variable, align its branches with the variable that receives the return value.

    # bad
    result =
      if some_cond
        # ...
        # ...
        calc_something
      else
        calc_something_else
      end
    
    # good
    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.

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

  • Avoid line continuation \ where not required.

  • Align the parameters of a method call, if they span more than one line, with one level of indentation relative to the start of the line with the method call.

    # starting point (line is too long)
    def send_mail(source)
      Mailer.deliver(to: 'bob@example.com', from: 'us@example.com', subject: 'Important message', body: source.text)
    end
    
    # bad (double indent)
    def send_mail(source)
      Mailer.deliver(
          to: 'bob@example.com',
          from: 'us@example.com',
          subject: 'Important message',
          body: source.text)
    end
    
    # good
    def send_mail(source)
      Mailer.deliver(
        to: 'bob@example.com',
        from: 'us@example.com',
        subject: 'Important message',
        body: source.text,
      )
    end
  • When chaining methods on multiple lines, indent successive calls by one level of indentation.

    # bad (indented to the previous call)
    User.pluck(:name)
        .sort(&:casecmp)
        .chunk { |n| n[0] }
    
    # good
    User
      .pluck(:name)
      .sort(&:casecmp)
      .chunk { |n| n[0] }
  • Align the elements of array literals spanning multiple lines.

  • Limit lines to 120 characters.

  • Avoid trailing whitespace.

  • Avoid extra whitespace, except for alignment purposes.

  • End each file with a newline.

  • Avoid block comments:

    # bad
    =begin
    comment line
    another comment line
    =end
    
    # good
    # comment line
    # another comment line
  • Place the closing method call brace on the line after the last argument when opening brace is on a separate line from the first argument.

    # bad
    method(
      arg_1,
      arg_2)
    
    # good
    method(
      arg_1,
      arg_2,
    )

Syntax

  • Use :: only to reference constants (this includes classes and modules) and constructors (like Array() or Nokogiri::HTML()). Avoid :: for regular method invocation.

  • Avoid using :: for defining class and modules, or for inheritance, since constant lookup will not search in parent classes/modules.

    # bad
    module A
      FOO = 'test'
    end
    
    class A::B
      puts FOO  # this will raise a NameError exception
    end
    
    # good
    module A
      FOO = 'test'
    
      class B
        puts FOO
      end
    end
  • Use def with parentheses when there are parameters. Omit the parentheses when the method doesn't accept any parameters.

  • Avoid for.

  • Avoid then.

  • Favour the ternary operator(?:) over if/then/else/end constructs.

    # 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.

  • Avoid multiline ?: (the ternary operator); use if/unless instead.

  • Use when x then ... for one-line cases.

  • Use ! instead of not.

  • Prefer &&/|| over and/or.

  • Favour unless over if for negative conditions.

  • Avoid unless with else. Rewrite these with the positive case first.

  • Use parentheses around the arguments of method invocations. Omit parentheses when not providing arguments. Also omit parentheses when the invocation is single-line and the method:

    • is a class method call with implicit receiver
    • is called by syntactic sugar (e.g: 1 + 1 calls the + method, foo[bar] calls the [] method, etc)
    # bad
    class User
      include(Bar)
      has_many(:posts)
    end
    
    # good
    class User
      include Bar
      has_many :posts
      SomeClass.some_method(:foo)
    end
    • is one of the following methods:
      • require
      • require_relative
      • require_dependency
      • yield
      • raise
      • puts
  • Omit the outer braces around an implicit options hash.

  • Use the proc invocation shorthand when the invoked method is the only operation of a block.

    # bad
    names.map { |name| name.upcase }
    
    # good
    names.map(&:upcase)
  • Prefer {...} over do...end for single-line blocks.

  • Prefer do..end over {...} for multi-line blocks.

  • Omit return where possible.

  • Omit self where possible.

    # bad
    self.my_method
    
    # good
    my_method
    
    # also good
    attr_writer :name
    
    def my_method
      self.name = 'Rafael' # `self` is neeeded to reference the attribute writer.
    end
  • Wrap assignment in parentheses when using its return value in a conditional statement.

    if (value = /foo/.match(string))
  • Use ||= to initialize variables only if they're not already initialized.

  • Avoid using ||= to initialize boolean variables.

    # bad - would set enabled to true even if it was false
    @enabled ||= true
    
    # good
    @enabled = true if enabled.nil?
    
    # also valid - defined? workaround
    @enabled = true unless defined?(@enabled)
  • Avoid spaces between a method name and the opening parenthesis.

  • Prefer the lambda literal syntax over lambda.

    # bad
    l = lambda { |a, b| a + b }
    l.call(1, 2)
    
    l = lambda do |a, b|
      tmp = a * 7
      tmp * b / 50
    end
    
    # good
    l = ->(a, b) { a + b }
    l.call(1, 2)
    
    l = ->(a, b) do
      tmp = a * 7
      tmp * b / 50
    end
  • Prefer proc over Proc.new.

  • Prefix unused block parameters with _. It's also acceptable to use just _.

  • 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)
        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 keyword arguments over options hash.

  • Prefer map over collect, find over detect, select over find_all, size over length.

  • Prefer Time over DateTime.

  • Prefer Time.iso8601(foo) instead of Time.parse(foo) when expecting ISO8601 formatted time strings like "2018-03-20T11:16:39-04:00".

Naming

  • Use snake_case for symbols, methods, and variables.

  • Use CamelCase for classes and modules, but keep acronyms like HTTP, RFC, XML uppercase.

  • Use snake_case for naming files and directories, e.g. hello_world.rb.

  • Define a single class or module per source file. Name the file name as the class or module, but replacing CamelCase with snake_case.

  • Use SCREAMING_SNAKE_CASE for other constants.

  • When using inject with short blocks, name the arguments according to what is being injected, e.g. |hash, e| (mnemonic: hash, element)

  • When defining binary operators, name the parameter other(<< and [] are exceptions to the rule, since their semantics are different).

  • Name predicate methods with a ?. Predicate methods are methods that return a boolean value.

  • Avoid ending method names with a ? if they don't return a boolean.

  • Avoid prefixing method names with is_.

    # bad
    def is_empty?
    end
    
    # good
    def empty?
    end
  • Avoid starting method names with get_.

  • Avoid ending method names with ! when there is no equivalent method without the bang. Bangs are to mark a more dangerous version of a method, e.g. save returns a boolean in ActiveRecord, whereas save! will throw an exception on failure.

  • Avoid magic numbers. Use a constant and give it a meaningful name.

  • Avoid nomenclature that has (or could be interpreted to have) discriminatory origins.

Comments

  • Include relevant context in comments, as readers might be missing it.

  • Keep comments in sync with code.

  • Write comments using proper capitalization and punctuation.

  • Avoid superfluous comments. Focus on why the code is the way it is if this is not obvious, not how the code works.

Classes and Modules

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

  • Prefer extend self over module_function.

    # bad
    module SomeModule
      module_function
    
      def some_method
      end
    
      def some_other_method
      end
    end
    
    # good
    module SomeModule
      extend self
    
      def some_method
      end
    
      def some_other_method
      end
    end
  • Use a class << self block over def self. when defining class methods, and group them together within a single block.

    # bad
    class SomeClass
      def self.method1
      end
    
      def method2
      end
    
      private
    
      def method3
      end
    
      def self.method4 # this is actually not private
      end
    end
    
    # good
    class SomeClass
      class << self
        def method1
        end
    
        private
    
        def method4
        end
      end
    
      def method2
      end
    
      private
    
      def method3
      end
    end
  • Respect the Liskov Substitution Principle when designing class hierarchies.

  • Use attr_accessor, attr_reader, and attr_writer to define trivial accessors and mutators.

    # bad
    class Person
      def initialize(first_name, last_name)
        @first_name = first_name
        @last_name = last_name
      end
    
      def first_name
        @first_name
      end
    
      def last_name
        @last_name
      end
    end
    
    # good
    class Person
      attr_reader :first_name, :last_name
    
      def initialize(first_name, last_name)
        @first_name = first_name
        @last_name = last_name
      end
    end
  • Prefer attr_reader and attr_accessor over attr.

  • Avoid class (@@) variables.

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

    class SomeClass
      def public_method
        # ...
      end
    
      private
    
      def private_method
        # ...
      end
    
      def another_private_method
        # ...
      end
    end
  • Prefer alias_method over alias.

Exceptions

  • Signal exceptions using the raise method.

  • Omit RuntimeError in the two argument version of raise.

    # bad
    raise RuntimeError, 'message'
    
    # good - signals a RuntimeError by default
    raise 'message'
  • Prefer supplying an exception class and a message as two separate arguments to raise instead of an exception instance.

    # bad
    raise SomeException.new('message')
    # Note that there is no way to do `raise SomeException.new('message'), backtrace`.
    
    # good
    raise SomeException, 'message'
    # Consistent with `raise SomeException, 'message', backtrace`.
  • Avoid returning from an ensure block. If you explicitly return from a method inside an ensure block, the return will take precedence over any exception being raised, and the method will return as if no exception had been raised at all. In effect, the exception will be silently thrown away.

    # bad
    def foo
      raise
    ensure
      return 'very bad idea'
    end
  • 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
  • Avoid empty rescue statements.

    # bad
    begin
      # an exception occurs here
    rescue SomeError
      # the rescue clause does absolutely nothing
    end
    
    # bad - `rescue nil` swallows all errors, including syntax errors, and
    # makes them hard to track down.
    do_something rescue nil
  • Avoid rescue in its modifier form.

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

    # 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.
    rescue => error
      # exception handling
    end
  • Prefer exceptions from the standard library over introducing new exception classes.

  • Use meaningful names for exception variables.

    # bad
    begin
      # an exception occurs here
    rescue => e
      # exception handling
    end
    
    # good
    begin
      # an exception occurs here
    rescue => error
      # exception handling
    end

Collections

  • Use literal array and hash creation notation unless you need to pass parameters to their constructors.

    # bad
    arr = Array.new
    hash = Hash.new
    
    # good
    arr = []
    hash = {}
  • Prefer the literal array syntax over %w or %i.

    # bad
    STATES = %w(draft open closed)
    
    # good
    STATES = ['draft', 'open', 'closed']
  • Append a trailing comma in multi-line collection literals.

    # bad
    {
      foo: :bar,
      baz: :toto
    }
    
    # good
    {
      foo: :bar,
      baz: :toto,
    }
  • When accessing the first or last element from an array, prefer first or last over [0] or [-1].

  • Avoid mutable objects as hash keys.

  • Use shorthand hash literal syntax when all keys are symbols.

    # bad
    { :a => 1, :b => 2 }
    
    # good
    { a: 1, b: 2 }
  • Prefer hash rockets syntax over shorthand syntax when not all keys are symbols.

    # bad
    { a: 1, 'b' => 2 }
    
    # good
    { :a => 1, 'b' => 2 }
  • Prefer Hash#key? over Hash#has_key?.

  • Prefer Hash#value? over Hash#has_value?.

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

    heroes = { batman: 'Bruce Wayne', superman: 'Clark Kent' }
    # bad - if we make a mistake we might not spot it right away
    heroes[:batman] # => "Bruce Wayne"
    heroes[:supermann] # => nil
    
    # good - fetch raises a KeyError making the problem obvious
    heroes.fetch(:supermann)
  • Introduce default values for hash keys via Hash#fetch as opposed to using custom logic.

    batman = { name: 'Bruce Wayne', is_evil: false }
    
    # bad - if we just use || operator with falsy value we won't get the expected result
    batman[:is_evil] || true # => true
    
    # good - fetch work correctly with falsy values
    batman.fetch(:is_evil, true) # => false
  • Place ] and } on the line after the last element when opening brace is on a separate line from the first element.

    # bad
    [
      1,
      2]
    
    {
      a: 1,
      b: 2}
    
    # good
    [
      1,
      2,
    ]
    
    {
      a: 1,
      b: 2,
    }

Strings

  • Prefer string interpolation and string formatting instead of string concatenation:

    # bad
    email_with_name = user.name + ' <' + user.email + '>'
    
    # good
    email_with_name = "#{user.name} <#{user.email}>"
    
    # good
    email_with_name = format('%s <%s>', user.name, user.email)
  • Avoid padded-spacing inside braces in interpolated expressions.

    # bad
    "From: #{ user.first_name }, #{ user.last_name }"
    
    # good
    "From: #{user.first_name}, #{user.last_name}"
  • Adopt a consistent string literal quoting style.

  • Avoid the character literal syntax ?x.

  • Use {} around instance and global variables being interpolated into a string.

    class Person
      attr_reader :first_name, :last_name
    
      def initialize(first_name, last_name)
        @first_name = first_name
        @last_name = last_name
      end
    
      # bad - valid, but awkward
      def to_s
        "#@first_name #@last_name"
      end
    
      # good
      def to_s
        "#{@first_name} #{@last_name}"
      end
    end
    
    $global = 0
    # bad
    puts "$global = #$global"
    
    # fine, but don't use globals
    puts "$global = #{$global}"
  • Avoid Object#to_s on interpolated objects.

    # bad
    message = "This is the #{result.to_s}."
    
    # good - `result.to_s` is called implicitly.
    message = "This is the #{result}."
  • Avoid String#gsub in scenarios in which you can use a faster more specialized alternative.

    url = 'http://example.com'
    str = 'lisp-case-rules'
    
    # bad
    url.gsub('http://', 'https://')
    str.gsub('-', '_')
    str.gsub(/[aeiou]/, '')
    
    # good
    url.sub('http://', 'https://')
    str.tr('-', '_')
    str.delete('aeiou')
  • When using heredocs for multi-line strings keep in mind the fact that they preserve leading whitespace. It's a good practice to employ some margin based on which to trim the excessive whitespace.

    code = <<-END.gsub(/^\s+\|/, '')
      |def test
      |  some_method
      |  other_method
      |end
    END
    # => "def test\n  some_method\n  other_method\nend\n"
    
    # In Rails you can use `#strip_heredoc` to achieve the same result
    code = <<-END.strip_heredoc
      def test
        some_method
        other_method
      end
    END
    # => "def test\n  some_method\n  other_method\nend\n"
  • In Ruby 2.3, prefer "squiggly heredoc" syntax, which has the same semantics as strip_heredoc from Rails:

    code = <<~END
      def test
        some_method
        other_method
      end
    END
    # => "def test\n  some_method\n  other_method\nend\n"

Regular Expressions

  • Prefer plain text search over regular expressions in strings.

    string['text']
  • Use non-capturing groups when you don't use the captured result.

    # bad
    /(first|second)/
    
    # good
    /(?:first|second)/
  • Prefer Regexp#match over Perl-legacy variables to capture group matches.

    # bad
    /(regexp)/ =~ string
    process $1
    
    # good
    /(regexp)/.match(string)[1]
  • Prefer named groups over numbered groups.

    # bad
    /(regexp)/ =~ string
    ...
    process Regexp.last_match(1)
    
    # good
    /(?<meaningful_var>regexp)/ =~ string
    ...
    process meaningful_var
  • Prefer \A and \z over ^ and $ when matching strings from start to end.

    string = "some injection\nusername"
    string[/^username$/] # `^` and `$` matches start and end of lines.
    string[/\Ausername\z/] # `\A` and `\z` matches start and end of strings.

Percent Literals

  • Use %() for single-line strings which require both interpolation and embedded double-quotes. For multi-line strings, prefer heredocs.

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

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

    # bad
    %r{\s+}
    
    # good
    %r{^/(.*)$}
    %r{^/blog/2011/(.*)$}
  • Avoid the use of %s. Use :"some string" to create a symbol with spaces in it.

  • Prefer () as delimiters for all % literals, except, as often occurs in regular expressions, when parentheses appear inside the literal. Use the first of (), {}, [], <> which does not appear inside the literal.

Testing

  • Treat test code like any other code you write. This means: keep readability, maintainability, complexity, etc. in mind.

  • Prefer Minitest as the test framework.

  • Limit each test case to cover a single aspect of your code.

  • Organize the setup, action, and assertion sections of the test case into paragraphs separated by empty lines.

    test 'sending a password reset email clears the password hash and set a reset token' do
      user = User.create!(email: 'bob@example.com')
      user.mark_as_verified
    
      user.send_password_reset_email
    
      assert_nil user.password_hash
      refute_nil user.reset_token
    end
  • Split complex test cases into multiple simpler tests that test functionality in isolation.

  • Prefer using test 'foo'-style syntax to define test cases over def test_foo.

  • Prefer using assertion methods that will yield a more descriptive error message.

    # bad
    assert user.valid?
    assert user.name == 'tobi'
    
    
    # good
    assert_predicate user, :valid?
    assert_equal 'tobi', user.name
  • Avoid using assert_nothing_raised. Use a positive assertion instead.

  • Prefer using assertions over expectations. Expectations lead to more brittle tests, especially in combination with singleton objects.

    # bad
    StatsD.expects(:increment).with('metric')
    do_something
    
    # good
    assert_statsd_increment('metric') do
      do_something
    end