Shopify’s 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.
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.
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.
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.
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.
# bad
foo . bar
# good
foo.bar
Avoid space in lambda literals.
# bad
a = -> (x, y) { x + y }
# good
a = ->(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.
# bad
result =
if some_cond
# ...
# ...
calc_something
else
calc_something_else
end
# good
result = if some_cond
# ...
# ...
calc_something
else
calc_something_else
end
When assigning the result of a begin block, align rescue/ensure/end with the start of the line
# bad
host = begin
URI.parse(value).host
rescue URI::Error
nil
end
# good
host = begin
URI.parse(value).host
rescue URI::Error
nil
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: "[email protected]", from: "[email protected]", subject: "Important message", body: source.text)
end
# bad (double indent)
def send_mail(source)
Mailer.deliver(
to: "[email protected]",
from: "[email protected]",
subject: "Important message",
body: source.text)
end
# good
def send_mail(source)
Mailer.deliver(
to: "[email protected]",
from: "[email protected]",
subject: "Important message",
body: source.text,
)
end
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,
)
Place each element/argument on a new line when wrapping a method call, hash, or array
on multiple lines.
# bad
method(arg_1, arg_2,
arg_3
)
[
value_1, value_2,
value_3,
]
{
key1: value_1,
key2: value_2, key3: value_3,
}
# good
method(
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 call
method({
foo: bar,
})
# Last argument, itself is multiline
class User
after_save :method, if: -> {
do_some_checks
}
end
# Single value array
errors = [{
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 Person
class Person
# Some code
end
# bad
# frozen_string_literal: true
# Some documentation for Person
class Person
# Some code
end
Use empty lines around attribute accessor.
# bad
class Foo
attr_reader :foo
def foo
# do something...
end
end
# good
class Foo
attr_reader :foo
def foo
# do something...
end
end
Avoid empty lines around method, class, module, and block bodies.
# bad
class Foo
def foo
begin
do_something do
something
end
rescue
something
end
true
end
end
# good
class Foo
def foo
begin
do_something do
something
end
rescue
something
end
end
end
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:
1 + 1 calls the + method, foo[bar][] method, etc).# bad
class User
include(Bar)
has_many(:posts)
end
# good
class User
include Bar
has_many :posts
SomeClass.some_method(:foo)
end
requirerequire_relativerequire_dependencyyieldraiseputsOmit 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 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 = 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".
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.
# bad
def foo
@foo ||= begin
return 1 if flag?
2
end
end
# good
def foo
@foo ||= begin
if flag?
1
else
2
end
end
end
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.
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.
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.
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
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,
}
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}"
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.
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"
Indent heredoc contents and closing according to its opening.
# bad
class Foo
def bar
<<~SQL
'Hi'
SQL
end
end
# good
class Foo
def bar
<<~SQL
'Hi'
SQL
end
end
# bad
# heredoc contents is before closing heredoc.
foo arg,
<<~EOS
Hi
EOS
# good
foo arg,
<<~EOS
Hi
EOS
# good
foo arg,
<<~EOS
Hi
EOS
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.
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.
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: "[email protected]")
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