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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

Any Include or Exclude configuration provided will be merged with RuboCop's defaults.

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

Table of Contents

• General
• Layout
• 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

  Overly defensive programming may safeguard against errors that will never be encountered, thus incurring run-time and maintenance costs.

• 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.

Layout

• 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.

• Avoid space around method call operators.

  # badfoo.bar# goodfoo.bar

• Avoid space in lambda literals.

  # bada=->(x,y){x + y}# gooda=->(x,y){x + y}

• 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.

  # badresult=ifsome_cond# ...# ...calc_somethingelsecalc_something_elseend# goodresult=ifsome_cond# ...# ...calc_somethingelsecalc_something_elseend

• When assigning the result of a begin block, align rescue/ensure/end with the start of the line

  # badhost=beginURI.parse(value).hostrescueURI::Errornilend# goodhost=beginURI.parse(value).hostrescueURI::Errornilend

• 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)defsend_mail(source)Mailer.deliver(to: "bob@example.com",from: "us@example.com",subject: "Important message",body: source.text)end# bad (double indent)defsend_mail(source)Mailer.deliver(to: "bob@example.com",from: "us@example.com",subject: "Important message",body: source.text)end# gooddefsend_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]}# goodUser.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=begincomment lineanother 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.

  # badmethod(arg_1,arg_2)# goodmethod(arg_1,arg_2,)

• Place each element/argument on a new line when wrapping a method call, hash, or array on multiple lines.

  # badmethod(arg_1,arg_2,arg_3)[value_1,value_2,value_3,]{key1: value_1,key2: value_2,key3: value_3,}# goodmethod(arg_1,arg_2,arg_3,)[value_1,value_2,value_3,]{key1: value_1,key2: value_2,key3: value_3,}# good (special cases)# Single argument method callmethod({foo: bar,})# Last argument, itself is multilineclassUserafter_save:method,if: ->{do_some_checks}end# Single value arrayerrors=[{error_code: 1234,error_message: "This is an error",}]

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

  # good# frozen_string_literal: true# Some documentation for PersonclassPerson# Some codeend# bad# frozen_string_literal: true# Some documentation for PersonclassPerson# Some codeend

• Use empty lines around attribute accessor.

  # badclassFooattr_reader:foodeffoo# do something...endend# goodclassFooattr_reader:foodeffoo# do something...endend

• Avoid empty lines around method, class, module, and block bodies.

  # badclassFoodeffoobegindo_somethingdosomethingendrescuesomethingendtrueendend# goodclassFoodeffoobegindo_somethingdosomethingendrescuesomethingendendend

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.

  # badmoduleAFOO="test"endclassA::BputsFOO# this will raise a NameError exceptionend# goodmoduleAFOO="test"classBputsFOOendend

• 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.

  # badresult=ifsome_conditionthensomethingelsesomething_elseend# goodresult=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).

  # badclassUserinclude(Bar)has_many(:posts)end# goodclassUserincludeBarhas_many:postsSomeClass.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.

  # badnames.map{ |name| name.upcase}# goodnames.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.

  # badself.my_method# goodmy_method# also goodattr_writer:namedefmy_methodself.name="Rafael"# `self` is needed 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=trueif@enabled.nil?# also valid - defined? workaround@enabled=trueunlessdefined?(@enabled)

• Avoid spaces between a method name and the opening parenthesis.

• Prefer the lambda literal syntax over lambda.

  # badl=lambda{ |a,b| a + b}l.call(1,2)l=lambdado |a,b|
  tmp=a * 7tmp * b / 50end# goodl=->(a,b){a + b}l.call(1,2)l=->(a,b)dotmp=a * 7tmp * b / 50end

• 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.

  # baddefcompute_thing(thing)ifthing[:foo]update_with_bar(thing)ifthing[:foo][:bar]partial_compute(thing)elsere_compute(thing)endendend# gooddefcompute_thing(thing)returnunlessthing[:foo]update_with_bar(thing[:foo])returnre_compute(thing)unlessthing[: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".

• Avoid returning from a begin block in assignment contexts. If you return from a method inside a begin block, the return will prevent the assignment from taking place, potentially causing confusing memoization bugs.

  # baddeffoo@foo ||= beginreturn1ifflag?2endend# gooddeffoo@foo ||= beginifflag?1else2endendend

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_.

  # baddefis_empty?end# gooddefempty?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.

  # badmoduleSomeModulemodule_functiondefsome_methodenddefsome_other_methodendend# goodmoduleSomeModuleextendselfdefsome_methodenddefsome_other_methodendend

• Use a class << self block over def self. when defining class methods, and group them together within a single block.

  # badclassSomeClassdefself.method1enddefmethod2endprivatedefmethod3enddefself.method4# this is actually not privateendend# goodclassSomeClassclass << selfdefmethod1endprivatedefmethod4endenddefmethod2endprivatedefmethod3endend

• Respect the Liskov Substitution Principle when designing class hierarchies.

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

  # badclassPersondefinitialize(first_name,last_name)@first_name=first_name@last_name=last_nameenddeffirst_name@first_nameenddeflast_name@last_nameendend# goodclassPersonattr_reader:first_name,:last_namedefinitialize(first_name,last_name)@first_name=first_name@last_name=last_nameendend

• 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.

  classSomeClassdefpublic_method# ...endprivatedefprivate_method# ...enddefanother_private_method# ...endend

• Prefer alias_method over alias.

Exceptions

• Signal exceptions using the raise method.

• Omit RuntimeError in the two argument version of raise.

  # badraiseRuntimeError,"message"# good - signals a RuntimeError by defaultraise"message"

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

  # badraiseSomeException.new("message")# Note that there is no way to do `raise SomeException.new("message"), backtrace`.# goodraiseSomeException,"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.

  # baddeffooraiseensurereturn"very bad idea"end

• Use implicit begin blocks where possible.

  # baddeffoobegin# main logic goes hererescue# failure handling goes hereendend# gooddeffoo# main logic goes hererescue# failure handling goes hereend

• Avoid empty rescue statements.

  # badbegin# an exception occurs hererescueSomeError# the rescue clause does absolutely nothingend# bad - `rescue nil` swallows all errors, including syntax errors, and# makes them hard to track down.do_somethingrescuenil

• Avoid rescue in its modifier form.

  # bad - this catches exceptions of StandardError class and its descendant# classes.read_filerescuehandle_error($!)# good - this catches only the exceptions of Errno::ENOENT class and its# descendant classes.deffooread_filerescueErrno::ENOENT=>errorhandle_error(error)end

• Avoid rescuing the Exception class.

  # badbegin# calls to exit and kill signals will be caught (except kill -9)exitrescueExceptionputs"you didn't really want to exit, right?"# exception handlingend# goodbegin# a blind rescue rescues from StandardError, not Exception.rescue=>error# exception handlingend

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

• Use meaningful names for exception variables.

  # badbegin# an exception occurs hererescue=>e# exception handlingend# goodbegin# an exception occurs hererescue=>error# exception handlingend

Collections

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

  # badarr=Array.newhash=Hash.new# goodarr=[]hash={}

• Prefer the literal array syntax over %w or %i.

  # badSTATES=%w(draftopenclosed)# goodSTATES=["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 awayheroes[:batman]# => "Bruce Wayne"heroes[:supermann]# => nil# good - fetch raises a KeyError making the problem obviousheroes.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 resultbatman[:is_evil] || true# => true# good - fetch work correctly with falsy valuesbatman.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:

  # bademail_with_name=user.name + " <" + user.email + ">"# goodemail_with_name="#{user.name} <#{user.email}>"# goodemail_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}"

• Use double-quoted strings.

  # bad'Just some text''No special chars or interpolation'# good"Just some text""No special chars or interpolation""Every string in #{project} uses double_quotes"

• Avoid the character literal syntax ?x.

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

  classPersonattr_reader:first_name,:last_namedefinitialize(first_name,last_name)@first_name=first_name@last_name=last_nameend# bad - valid, but awkwarddefto_s"#@first_name #@last_name"end# gooddefto_s"#{@first_name}#{@last_name}"endend
  $global =0# badputs"$global = #$global"# fine, but don't use globalsputs"$global = #{$global}"

• Avoid Object#to_s on interpolated objects.

  # badmessage="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"# badurl.gsub("http://","https://")str.gsub("-","_")str.gsub(/[aeiou]/,"")# goodurl.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 |endEND# => "def test\n some_method\n other_method\nend\n"# In Rails you can use `#strip_heredoc` to achieve the same resultcode=<<-END.strip_heredoc def test some_method other_method endEND# => "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 endEND# => "def test\n some_method\n other_method\nend\n"

• Indent heredoc contents and closing according to its opening.

  # badclassFoodefbar<<~SQL 'Hi' SQLendend# goodclassFoodefbar<<~SQL 'Hi' SQLendend# bad# heredoc contents is before closing heredoc.fooarg,<<~EOS Hi EOS# goodfooarg,<<~EOS HiEOS# goodfooarg,<<~EOS Hi EOS

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)/ =~ stringprocess $1
  # good/(regexp)/.match(string)[1]

• Prefer named groups over numbered groups.

  # bad/(regexp)/ =~ string
  ...
  processRegexp.last_match(1)# good/(?<meaningful_var>regexp)/ =~ string
  ...
  processmeaningful_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"douser=User.create!(email: "bob@example.com")user.mark_as_verifieduser.send_password_reset_emailassert_niluser.password_hashrefute_niluser.reset_tokenend

• 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.

  # badassertuser.valid?assertuser.name == "tobi"# goodassert_predicateuser,: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.

  # badStatsD.expects(:increment).with("metric")do_something# goodassert_statsd_increment("metric")dodo_somethingend