Prelude
Role models are important.
-- Officer Alex J. Murphy / RoboCop
One thing has always bothered me as a Ruby developer—Python developers have a great programming style reference (PEP-8) and we never got an official guide, documenting Ruby coding style and best practices. And I do believe that style matters. I also believe that a great hacker community, such as Ruby has, should be quite capable of producing this coveted document.
This guide started its life as our internal company Ruby coding guidelines (written by yours truly). At some point I decided that the work I was doing might be interesting to members of the Ruby community in general and that the world had little need for another internal company guideline. But the world could certainly benefit from a community-driven and community-sanctioned set of practices, idioms and style prescriptions for Ruby programming.
Since the inception of the guide I've received a lot of feedback from members of the exceptional Ruby community around the world. Thanks for all the suggestions and the support! Together we can make a resource beneficial to each and every Ruby developer out there.
By the way, if you're into Rails you might want to check out the complementary Ruby on Rails Style Guide.
The Ruby Style Guide
This Ruby style guide recommends best practices so that real-world Ruby programmers can write code that can be maintained by other real-world Ruby programmers. A style guide that reflects real-world usage gets used, while a style guide that holds to an ideal that has been rejected by the people it is supposed to help risks not getting used at all—no matter how good it is.
The guide is separated into several sections of related rules. I've tried to add the rationale behind the rules (if it's omitted I've assumed it's pretty obvious).
I didn't come up with all the rules out of nowhere—they are mostly based on my extensive career as a professional software engineer, feedback and suggestions from members of the Ruby community and various highly regarded Ruby programming resources, such as "Programming Ruby" and "The Ruby Programming Language".
There are some areas in which there is no clear consensus in the Ruby community regarding a particular style (like string literal quoting, spacing inside hash literals, dot position in multi-line method chaining, etc.). In such scenarios all popular styles are acknowledged and it's up to you to pick one and apply it consistently.
This style guide evolves over time as additional conventions are identified and past conventions are rendered obsolete by changes in Ruby itself.
You can generate a PDF or an HTML copy of this guide using Pandoc.
RuboCop is a static code analyzer (linter) and formatter, based on this style guide.
Translations of the guide are available in the following languages:
- Chinese Simplified
- Chinese Traditional
- Egyptian Arabic
- French
- Japanese
- Korean
- Portuguese (pt-BR)
- Russian
- Spanish
- Vietnamese
Table of Contents
- Source Code Layout
- Syntax
- Naming
- Comments
- Classes & Modules
- Exceptions
- Collections
- Numbers
- Strings
- Date & Time
- Regular Expressions
- Percent Literals
- Metaprogramming
- Misc
- Tools
Source Code Layout
Nearly everybody is convinced that every style but their own is ugly and unreadable. Leave out the "but their own" and they're probably right...
-- Jerry Coffin (on indentation)
-
Use
UTF-8
as the source file encoding. [link] -
Use two spaces per indentation level (aka soft tabs). No hard tabs. [link]
# bad - four spaces def some_method do_something end # good def some_method do_something end
-
Use Unix-style line endings. (*BSD/Solaris/Linux/macOS users are covered by default, Windows users have to be extra careful.) [link]
-
If you're using Git you might want to add the following configuration setting to protect your project from Windows line endings creeping in:
$ git config --global core.autocrlf true
-
-
Don't use
;
to separate statements and expressions. As a corollary—use one expression per line. [link]# bad puts 'foobar'; # superfluous semicolon puts 'foo'; puts 'bar' # two expressions on the same line # good puts 'foobar' puts 'foo' puts 'bar' puts 'foo', 'bar' # this applies to puts in particular
-
Prefer a single-line format for class definitions with no body. [link]
# bad class FooError < StandardError end # okish class FooError < StandardError; end # good FooError = Class.new(StandardError)
-
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 be no more than one expression in a single-line method. [link]
# 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 ; makes 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
-
Use spaces around operators, after commas, colons and semicolons. Whitespace might be (mostly) irrelevant to the Ruby interpreter, but its proper use is the key to writing easily readable code. [link]
sum = 1 + 2 a, b = 1, 2 class FooError < StandardError; end
There are a few exceptions. One is the exponent operator:
# bad e = M * c ** 2 # good e = M * c**2
Another exception is the slash in rational literals:
# bad o_scale = 1 / 48r # good o_scale = 1/48r
Another exception is the safe navigation operator:
# bad foo &. bar foo &.bar foo&. bar # good foo&.bar
-
No spaces after
(
,[
or before]
,)
. Use spaces around{
and before}
. [link]# bad some( arg ).other [ 1, 2, 3 ].each{|e| puts e} # good some(arg).other [1, 2, 3].each { |e| puts e }
{
and}
deserve a bit of clarification, since they are used for block and hash literals, as well as string interpolation.For hash literals two styles are considered acceptable. The first variant is slightly more readable (and arguably more popular in the Ruby community in general). The second variant has the advantage of adding visual difference between block and hash literals. Whichever one you pick—apply it consistently.
# good - space after { and before } { one: 1, two: 2 } # good - no space after { and before } {one: 1, two: 2}
With interpolated expressions, there should be no padded-spacing inside the braces.
# bad "From: #{ user.first_name }, #{ user.last_name }" # good "From: #{user.first_name}, #{user.last_name}"
-
No space after
!
. [link]# bad ! something # good !something
-
No space inside range literals. [link]
# bad 1 .. 3 'a' ... 'z' # good 1..3 'a'...'z'
-
Indent
when
as deep ascase
. This is the style established in both "The Ruby Programming Language" and "Programming Ruby". Historically it is derived from the fact thatcase
andswitch
statements are not blocks, hence should not be indented, and thewhen
andelse
keywords are labels (compiled in the C language, they are literally labels forJMP
calls). [link]# bad 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 # good 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
-
When assigning the result of a conditional expression to a variable, preserve the usual alignment of its branches. [link]
# bad - pretty convoluted 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 result = if some_cond calc_something else calc_something_else end # good - it's apparent what's going on 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 result = if some_cond calc_something else calc_something_else end # good (and a bit more width efficient) 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 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. [link]
def some_method data = initialize(options) data.manipulate! data.result end def some_method result end
-
Don't use several empty lines in a row. [link]
# bad - It has two empty lines. some_method some_method # good some_method some_method
-
Use empty lines around access modifiers. [link]
# bad class Foo attr_reader :foo def foo # do something... end end # good class Foo attr_reader :foo def foo # do something... end end
-
Don't use empty lines around method, class, module, block bodies. [link]
# bad class Foo def foo begin do_something do something end rescue something end end end # good class Foo def foo begin do_something do something end rescue something end end end
-
Avoid comma after the last parameter in a method call, especially when the parameters are not on separate lines. [link]
# bad - easier to move/add/remove parameters, but still not preferred some_method( size, count, color, ) # bad some_method(size, count, color, ) # good some_method(size, count, color)
-
Use spaces around the
=
operator when assigning default values to method parameters: [link]# 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).
-
Avoid line continuation
\
where not required. In practice, avoid using line continuations for anything but string concatenation. [link]# bad result = 1 - \ 2 # good (but still ugly as hell) result = 1 \ - 2 long_string = 'First part of the long string' \ ' and second part of the long string'
-
Adopt a consistent multi-line method chaining style. There are two popular styles in the Ruby community, both of which are considered good—leading
.
(Option A) and trailing.
(Option B). [link]-
(Option A) When continuing a chained method invocation on another line keep the
.
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
-
(Option B) When continuing a chained method invocation on another line, include the
.
on the first line to indicate that the expression continues.# bad - need to read ahead to the second line to know that the chain continues one.two.three .four # good - it's immediately clear that the expression continues beyond the first line one.two.three. four
A discussion on the merits of both alternative styles can be found here.
-
-
Align the parameters of a method call if they span more than one line. When aligning parameters is not appropriate due to line-length constraints, single indent for the lines after the first is also acceptable. [link]
# 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 # good (normal indent) def send_mail(source) Mailer.deliver( to: 'bob@example.com', from: 'us@example.com', subject: 'Important message', body: source.text ) end
-
Align the elements of array literals spanning multiple lines. [link]
# bad - single indent menu_item = %w[Spam Spam Spam Spam Spam Spam Spam Spam Baked beans Spam Spam Spam Spam Spam] # good menu_item = %w[ Spam Spam Spam Spam Spam Spam Spam Spam Baked beans Spam Spam Spam Spam Spam ] # good menu_item = %w[Spam Spam Spam Spam Spam Spam Spam Spam Baked beans Spam Spam Spam Spam Spam]
-
Add underscores to large numeric literals to improve their readability. [link]
# bad - how many 0s are there? num = 1000000 # good - much easier to parse for the human brain num = 1_000_000
-
Prefer lowercase letters for numeric literal prefixes.
0o
for octal,0x
for hexadecimal and0b
for binary. Do not use0d
prefix for decimal literals. [link]# bad num = 01234 num = 0O1234 num = 0X12AB num = 0B10101 num = 0D1234 num = 0d1234 # good - easier to separate digits from the prefix num = 0o1234 num = 0x12AB num = 0b10101 num = 1234
-
Limit lines to 80 characters. [link]
-
Avoid trailing whitespace. [link]
-
End each file with a newline. [link]
-
Don't use block comments. They cannot be preceded by whitespace and are not as easy to spot as regular comments. [link]
# bad =begin comment line another comment line =end # good # comment line # another comment line
Syntax
-
Use
::
only to reference constants (this includes classes and modules) and constructors (likeArray()
orNokogiri::HTML()
).- Do not use
::
for regular method invocation. [link]
# bad SomeClass::some_method some_object::some_method # good SomeClass.some_method some_object.some_method SomeModule::SomeClass::SOME_CONST SomeModule::SomeClass()
- Do not use
-
Do not use
::
to define class methods. [link]# bad class Foo def self::some_method end end # good class Foo def self.some_method end end
-
Use
def
with parentheses when there are parameters. Omit the parentheses when the method doesn't accept any parameters. [link]# bad def some_method() # body omitted end # good def some_method # body omitted end # bad def some_method_with_parameters param1, param2 # body omitted end # good def some_method_with_parameters(param1, param2) # body omitted end
-
Use parentheses around the arguments of method invocations, especially if the first argument begins with an open parenthesis
(
, as inf((3 + 2) + 1)
. [link]# bad x = Math.sin y # good x = Math.sin(y) # bad array.delete e # good array.delete(e) # bad temperance = Person.new 'Temperance', 30 # good temperance = Person.new('Temperance', 30)
Always omit parentheses for
-
Method calls with no arguments:
# bad Kernel.exit!() 2.even?() fork() 'test'.upcase() # good Kernel.exit! 2.even? fork 'test'.upcase
-
Methods that are part of an internal DSL (e.g., Rake, Rails, RSpec):
# bad validates(:name, presence: true) # good validates :name, presence: true
-
Methods that have "keyword" status in Ruby:
class Person # bad attr_reader(:name, :age) # good attr_reader :name, :age # body omitted end
Can omit parentheses for * Methods that have "keyword" status in Ruby, but are not declarative: ```Ruby # good puts(temperance.age) system('ls') # also good puts temperance.age system 'ls' ```
-
-
Define optional arguments at the end of the list of arguments. Ruby has some unexpected results when calling methods that have optional arguments at the front of the list. [link]
# bad def some_method(a = 1, b = 2, c, d) puts "#{a}, #{b}, #{c}, #{d}" end some_method('w', 'x') # => '1, 2, w, x' some_method('w', 'x', 'y') # => 'w, 2, x, y' some_method('w', 'x', 'y', 'z') # => 'w, x, y, z' # good def some_method(c, d, a = 1, b = 2) puts "#{a}, #{b}, #{c}, #{d}" end some_method('w', 'x') # => '1, 2, w, x' some_method('w', 'x', 'y') # => 'y, 2, w, x' some_method('w', 'x', 'y', 'z') # => 'y, z, w, x'
-
Use keyword arguments when passing boolean argument to a method. [link]
# bad def some_method(bar = false) puts bar end # bad - common hack before keyword args were introduced def some_method(options = {}) bar = options.fetch(:bar, false) puts bar end # good def some_method(bar: false) puts bar end some_method # => false some_method(bar: true) # => true
-
Prefer keyword arguments over optional arguments. [link]
# bad def some_method(a, b = 5, c = 1) # body omitted end # good def some_method(a, b: 5, c: 1) # body omitted end
-
Use keyword arguments instead of option hashes. [link]
# bad def some_method(options = {}) bar = options.fetch(:bar, false) puts bar end # good def some_method(bar: false) puts bar end
-
Avoid the use of parallel assignment for defining variables. Parallel assignment is allowed when it is the return of a method call, used with the splat operator, or when used to swap variable assignment. Parallel assignment is less readable than separate assignment. [link]
# bad a, b, c, d = 'foo', 'bar', 'baz', 'foobar' # good a = 'foo' b = 'bar' c = 'baz' d = 'foobar' # good - swapping variable assignment # Swapping variable assignment is a special case because it will allow you to # swap the values that are assigned to each variable. a = 'foo' b = 'bar' a, b = b, a puts a # => 'bar' puts b # => 'foo' # good - method return def multi_return [1, 2] end first, second = multi_return # good - use with splat first, *list = [1, 2, 3, 4] # first => 1, list => [2, 3, 4] hello_array = *'Hello' # => ["Hello"] a = *(1..3) # => [1, 2, 3]
-
Avoid the use of unnecessary trailing underscore variables during parallel assignment. Named underscore variables are to be preferred over underscore variables because of the context that they provide. Trailing underscore variables are necessary when there is a splat variable defined on the left side of the assignment, and the splat variable is not an underscore. [link]
# bad foo = 'one,two,three,four,five' # Unnecessary assignment that does not provide useful information first, second, _ = foo.split(',') first, _, _ = foo.split(',') first, *_ = foo.split(',') # good foo = 'one,two,three,four,five' # The underscores are needed to show that you want all elements # except for the last number of underscore elements *beginning, _ = foo.split(',') *beginning, something, _ = foo.split(',') a, = foo.split(',') a, b, = foo.split(',') # Unnecessary assignment to an unused variable, but the assignment # provides us with useful information. first, _second = foo.split(',') first, _second, = foo.split(',') first, *_ending = foo.split(',')
-
Do not use
for
, unless you know exactly why. Most of the time iterators should be used instead.for
is implemented in terms ofeach
(so you're adding a level of indirection), but with a twist—for
doesn't introduce a new scope (unlikeeach
) and variables defined in its block will be visible outside it. [link]arr = [1, 2, 3] # bad for elem in arr do puts elem end # note that elem is accessible outside of the for loop elem # => 3 # good arr.each { |elem| puts elem } # elem is not accessible outside each's block elem # => NameError: undefined local variable or method `elem'
-
Do not use
then
for multi-lineif
/unless
. [link]# bad if some_condition then # body omitted end # good if some_condition # body omitted end
-
Always put the condition on the same line as the
if
/unless
in a multi-line conditional. [link]# bad if some_condition do_something do_something_else end # good if some_condition do_something do_something_else end
-
Favor the ternary operator(
?:
) overif/then/else/end
constructs. It's more common and obviously more concise. [link]# 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. [link]# 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
-
Do not use
if x; ...
. Use the ternary operator instead. [link]# bad result = if some_condition; something else something_else end # good result = some_condition ? something : something_else
-
Leverage the fact that
if
andcase
are expressions which return a result. [link]# bad if condition result = x else result = y end # good result = if condition x else y end
-
Use
when x then ...
for one-line cases. The alternative syntaxwhen x: ...
has been removed as of Ruby 1.9. [link] -
Do not use
when x; ...
. See the previous rule. [link] -
Use
!
instead ofnot
. [link]# bad - parentheses are required because of op precedence x = (not something) # good x = !something
-
Avoid the use of
!!
. [link]!!
converts a value to boolean, but you don't need this explicit conversion in the condition of a control expression; using it only obscures your intention. If you want to do anil
check, usenil?
instead.# bad x = 'test' # obscure nil check if !!x # body omitted end # good x = 'test' if x # body omitted end
-
The
and
andor
keywords are banned. The minimal added readability is just not worth the high probability of introducing subtle bugs. For boolean expressions, always use&&
and||
instead. For flow control, useif
andunless
;&&
and||
are also acceptable but less clear. [link]# bad # boolean expression ok = got_needed_arguments and arguments_are_valid # control flow document.save or raise("Failed to save document!") # good # boolean expression ok = got_needed_arguments && arguments_are_valid # control flow raise("Failed to save document!") unless document.save # ok # control flow document.save || raise("Failed to save document!")
-
Avoid multi-line
?:
(the ternary operator); useif
/unless
instead. [link] -
Favor modifier
if
/unless
usage when you have a single-line body. Another good alternative is the usage of control flow&&
/||
. [link]# bad if some_condition do_something end # good do_something if some_condition # another good option some_condition && do_something
-
Avoid modifier
if
/unless
usage at the end of a non-trivial multi-line block. [link]# bad 10.times do # multi-line body omitted end if some_condition # good if some_condition 10.times do # multi-line body omitted end end
-
Avoid nested modifier
if
/unless
/while
/until
usage. Favor&&
/||
if appropriate. [link]# bad do_something if other_condition if some_condition # good do_something if some_condition && other_condition
-
Favor
unless
overif
for negative conditions (or control flow||
). [link]# bad do_something if !some_condition # bad do_something if not some_condition # good do_something unless some_condition # another good option some_condition || do_something
-
Do not use
unless
withelse
. Rewrite these with the positive case first. [link]# 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 a control expression. [link]
# bad if (x > 10) # body omitted end # good if x > 10 # body omitted end
Note that there is an exception to this rule, namely safe assignment in condition.
-
Do not use
while/until condition do
for multi-linewhile/until
. [link]# bad while x > 5 do # body omitted end until x > 5 do # body omitted end # good while x > 5 # body omitted end until x > 5 # body omitted end
-
Favor modifier
while/until
usage when you have a single-line body. [link]# bad while some_condition do_something end # good do_something while some_condition
-
Favor
until
overwhile
for negative conditions. [link]# bad do_something while !some_condition # good do_something until some_condition
-
Use
Kernel#loop
instead ofwhile/until
when you need an infinite loop. [link]# bad while true do_something end until false do_something end # good loop do do_something end
-
Use
Kernel#loop
withbreak
rather thanbegin/end/until
orbegin/end/while
for post-loop tests. [link]# bad begin puts val val += 1 end while val < 0 # good loop do puts val val += 1 break unless val < 0 end
-
Omit the outer braces around an implicit options hash. [link]
# bad user.set({ name: 'John', age: 45, permissions: { read: true } }) # good user.set(name: 'John', age: 45, permissions: { read: true })
-
Omit both the outer braces and parentheses for methods that are part of an internal DSL. [link]
class Person < ActiveRecord::Base # bad validates(:name, { presence: true, length: { within: 1..10 } }) # good validates :name, presence: true, length: { within: 1..10 } end
-
Use the proc invocation shorthand when the invoked method is the only operation of a block. [link]
# bad names.map { |name| name.upcase } # good names.map(&:upcase)
-
Prefer
{...}
overdo...end
for single-line blocks. Avoid using{...}
for multi-line blocks (multi-line chaining is always ugly). Always usedo...end
for "control flow" and "method definitions" (e.g. in Rakefiles and certain DSLs). Avoiddo...end
when chaining. [link]names = %w[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(&:upcase)
Some will argue that multi-line 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?
-
Consider using explicit block argument to avoid writing block literal that just passes its arguments to another block. Beware of the performance impact, though, as the block gets converted to a Proc. [link]
require 'tempfile' # bad def with_tmp_dir Dir.mktmpdir do |tmp_dir| Dir.chdir(tmp_dir) { |dir| yield dir } # block just passes arguments end end # good def with_tmp_dir(&block) Dir.mktmpdir do |tmp_dir| Dir.chdir(tmp_dir, &block) end end with_tmp_dir do |dir| puts "dir is accessible as a parameter and pwd is set: #{dir}" end
-
Avoid
return
where not required for flow of control. [link]# 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, methods named after reserved words, or overloadable operators.) [link]# 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. [link]
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 unless the assignment is wrapped in parentheses. This is a fairly popular idiom among Rubyists that's sometimes referred to as safe assignment in condition. [link]# bad (+ a warning) if v = array.grep(/foo/) do_something(v) # some code end # good (MRI would still complain, but RuboCop won't) if (v = array.grep(/foo/)) do_something(v) # some code end # good v = array.grep(/foo/) if v do_something(v) # some code end
-
Use shorthand self assignment operators whenever applicable. [link]
# bad x = x + y x = x * y x = x**y x = x / y x = x || y x = x && y # good x += y x *= y x **= y x /= y x ||= y x &&= y
-
Use
||=
to initialize variables only if they're not already initialized. [link]# bad name = name ? name : 'Bozhidar' # bad name = 'Bozhidar' unless name # good - 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 befalse
.) [link]# bad - would set enabled to true even if it was false enabled ||= true # good enabled = true if enabled.nil?
-
Use
&&=
to preprocess variables that may or may not exist. Using&&=
will change the value only if it exists, removing the need to check its existence withif
. [link]# bad if something something = something.downcase end # bad something = something ? something.downcase : nil # ok something = something.downcase if something # good something = something && something.downcase # better something &&= something.downcase
-
Avoid explicit use of the case equality operator
===
. As its name implies it is meant to be used implicitly bycase
expressions and outside of them it yields some pretty confusing code. [link]# bad Array === something (1..100) === 7 /something/ === some_string # good something.is_a?(Array) (1..100).include?(7) some_string.match?(/something/)
-
Do not use
eql?
when using==
will do. The stricter comparison semantics provided byeql?
are rarely needed in practice. [link]# bad - eql? is the same as == for strings 'ruby'.eql? some_str # good 'ruby' == some_str 1.0.eql? x # eql? makes sense here if want to differentiate between Integer and Float 1
-
Avoid using Perl-style special variables (like
$:
,$;
, etc. ). They are quite cryptic and their use in anything but one-liner scripts is discouraged. Use the human-friendly aliases provided by theEnglish
library. [link]# bad $:.unshift File.dirname(__FILE__) # good require 'English' $LOAD_PATH.unshift File.dirname(__FILE__)
-
Do not put a space between a method name and the opening parenthesis. [link]
# bad f (3 + 2) + 1 # good f(3 + 2) + 1
-
Always run the Ruby interpreter with the
-w
option so it will warn you if you forget either of the rules above! [link] -
Do not use nested method definitions, use lambda instead. Nested method definitions actually produce methods in the same scope (e.g. class) as the outer method. Furthermore, the "nested method" will be redefined every time the method containing its definition is invoked. [link]
# bad def foo(x) def bar(y) # body omitted end bar(x) end # good - the same as the previous, but no bar redefinition on every foo call def bar(y) # body omitted end def foo(x) bar(x) end # also good def foo(x) bar = ->(y) { ... } bar.call(x) end
-
Use the new lambda literal syntax for single line body blocks. Use the
lambda
method for multi-line blocks. [link]# bad l = lambda { |a, b| a + b } l.call(1, 2) # correct, but looks extremely awkward l = ->(a, b) do tmp = a * 7 tmp * b / 50 end # good l = ->(a, b) { a + b } l.call(1, 2) l = lambda do |a, b| tmp = a * 7 tmp * b / 50 end
-
Don't omit the parameter parentheses when defining a stabby lambda with parameters. [link]
# bad l = ->x, y { something(x, y) } # good l = ->(x, y) { something(x, y) }
-
Omit the parameter parentheses when defining a stabby lambda with no parameters. [link]
# bad l = ->() { something } # good l = -> { something }
-
Prefer
proc
overProc.new
. [link]# bad p = Proc.new { |n| puts n } # good p = proc { |n| puts n }
-
Prefer
proc.call()
overproc[]
orproc.()
for both lambdas and procs. [link]# bad - looks similar to Enumeration access l = ->(v) { puts v } l[1] # also bad - uncommon syntax l = ->(v) { puts v } l.(1) # good l = ->(v) { puts v } l.call(1)
-
Prefix with
_
unused block parameters and local variables. It's also acceptable to use just_
(although it's a bit less descriptive). This convention is recognized by the Ruby interpreter and tools like RuboCop and will suppress their unused variable warnings. [link]# bad result = hash.map { |k, v| v + 1 } def something(x) unused_var, used_var = something_else(x) # some code end # good result = hash.map { |_k, v| v + 1 } def something(x) _unused_var, used_var = something_else(x) # some code end # good result = hash.map { |_, v| v + 1 } def something(x) _, used_var = something_else(x) # some code end
-
Use
$stdout/$stderr/$stdin
instead ofSTDOUT/STDERR/STDIN
.STDOUT/STDERR/STDIN
are constants, and while you can actually reassign (possibly to redirect some stream) constants in Ruby, you'll get an interpreter warning if you do so. [link] -
Use
warn
instead of$stderr.puts
. Apart from being more concise and clear,warn
allows you to suppress warnings if you need to (by setting the warn level to 0 via-W0
). [link] -
Favor the use of
sprintf
and its aliasformat
over the fairly crypticString#%
method. [link]# bad '%d %d' % [20, 10] # => '20 10' # good sprintf('%d %d', 20, 10) # => '20 10' # good sprintf('%<first>d %<second>d', first: 20, second: 10) # => '20 10' format('%d %d', 20, 10) # => '20 10' # good format('%<first>d %<second>d', first: 20, second: 10) # => '20 10'
-
When using named format string tokens, favor
%<name>s
over%{name}
because it encodes information about the type of the value. [link]# bad format('Hello, %{name}', name: 'John') # good format('Hello, %<name>s', name: 'John')
-
Favor the use of
Array#join
over the fairly crypticArray#*
with a string argument. [link]# bad %w[one two three] * ', ' # => 'one, two, three' # good %w[one two three].join(', ') # => 'one, two, three'
-
Use
Array()
instead of explicitArray
check or[*var]
, when dealing with a variable you want to treat as an Array, but you're not certain it's an array. [link]# bad paths = [paths] unless paths.is_a? Array paths.each { |path| do_something(path) } # bad (always creates a new Array instance) [*paths].each { |path| do_something(path) } # good (and a bit more readable) Array(paths).each { |path| do_something(path) }
-
Use ranges or
Comparable#between?
instead of complex comparison logic when possible. [link]# bad do_something if x >= 1000 && x <= 2000 # good do_something if (1000..2000).include?(x) # good do_something if x.between?(1000, 2000)
-
Favor the use of predicate methods to explicit comparisons with
==
. Numeric comparisons are OK. [link]# bad if x % 2 == 0 end if x % 2 == 1 end if x == nil end # good if x.even? end if x.odd? end if x.nil? end if x.zero? end if x == 0 end
-
Don't do explicit non-
nil
checks unless you're dealing with boolean values. [link]# bad do_something if !something.nil? do_something if something != nil # good do_something if something # good - dealing with a boolean def value_set? !@some_boolean.nil? end
-
Avoid the use of
BEGIN
blocks. [link] -
Do not use
END
blocks. UseKernel#at_exit
instead. [link]# bad END { puts 'Goodbye!' } # good at_exit { puts 'Goodbye!' }
-
Avoid the use of flip-flops. [link]
-
Avoid use of nested conditionals for flow of control. [link]
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[:foo]) 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
next
in loops instead of conditional blocks.# bad [0, 1, 2, 3].each do |item| if item > 1 puts item end end # good [0, 1, 2, 3].each do |item| next unless item > 1 puts item end
-
Prefer
map
overcollect
,find
overdetect
,select
overfind_all
,reduce
overinject
andsize
overlength
. This is not a hard requirement; if the use of the alias enhances readability, it's ok to use it. The rhyming methods are inherited from Smalltalk and are not common in other programming languages. The reason the use ofselect
is encouraged overfind_all
is that it goes together nicely withreject
and its name is pretty self-explanatory. [link] -
Don't use
count
as a substitute forsize
. ForEnumerable
objects other thanArray
it will iterate the entire collection in order to determine its size. [link]# bad some_hash.count # good some_hash.size
-
Use
flat_map
instead ofmap
+flatten
. This does not apply for arrays with a depth greater than 2, i.e. ifusers.first.songs == ['a', ['b','c']]
, then usemap + flatten
rather thanflat_map
.flat_map
flattens the array by 1, whereasflatten
flattens it all the way. [link]# bad all_songs = users.map(&:songs).flatten.uniq # good all_songs = users.flat_map(&:songs).uniq
-
Prefer
reverse_each
toreverse.each
because some classes thatinclude Enumerable
will provide an efficient implementation. Even in the worst case where a class does not provide a specialized implementation, the general implementation inherited fromEnumerable
will be at least as efficient as usingreverse.each
. [link]# bad array.reverse.each { ... } # good array.reverse_each { ... }
Naming
The only real difficulties in programming are cache invalidation and naming things.
-- Phil Karlton
-
Name identifiers in English. [link]
# bad - identifier using non-ascii characters заплата = 1_000 # bad - identifier is a Bulgarian word, written with Latin letters (instead of Cyrillic) zaplata = 1_000 # good salary = 1_000
-
Use
snake_case
for symbols, methods and variables. [link]# bad :'some symbol' :SomeSymbol :someSymbol someVar = 5 def someMethod # some code end def SomeMethod # some code end # good :some_symbol some_var = 5 def some_method # some code end
-
Do not separate numbers from letters on symbols, methods and variables. [link]
# bad :some_sym_1 some_var_1 = 1 var_10 = 10 def some_method_1 # some code end # good :some_sym1 some_var1 = 1 var10 = 10 def some_method1 # some code end
-
Use
CamelCase
for classes and modules. (Keep acronyms like HTTP, RFC, XML uppercase.) [link]# bad class Someclass # some code end class Some_Class # some code end class SomeXml # some code end class XmlSomething # some code end # good class SomeClass # some code end class SomeXML # some code end class XMLSomething # some code end
-
Use
snake_case
for naming files, e.g.hello_world.rb
. [link] -
Use
snake_case
for naming directories, e.g.lib/hello_world/hello_world.rb
. [link] -
Aim to have just a single class/module per source file. Name the file name as the class/module, but replacing
CamelCase
withsnake_case
. [link] -
Use
SCREAMING_SNAKE_CASE
for other constants. [link]# bad SomeConst = 5 # good SOME_CONST = 5
-
The names of predicate methods (methods that return a boolean value) should end in a question mark. (i.e.
Array#empty?
). Methods that don't return a boolean, shouldn't end in a question mark. [link] -
Avoid prefixing predicate methods with the auxiliary verbs such as
is
,does
, orcan
. These words are redundant and inconsistent with the style of boolean methods in the Ruby core library, such asempty?
andinclude?
. [link]# bad class Person def is_tall? true end def can_play_basketball? false end def does_like_candy? true end end # good class Person def tall? true end def basketball_player? false end def likes_candy? true end end
-
The names of potentially dangerous methods (i.e. methods that modify
self
or the arguments,exit!
(doesn't run the finalizers likeexit
does), etc.) should end with an exclamation mark if there exists a safe version of that dangerous method. [link]# bad - there is no 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
-
Define the non-bang (safe) method in terms of the bang (dangerous) one if possible. [link]
class Array def flatten_once! res = [] each do |e| [*e].each { |f| res << f } end replace(res) end def flatten_once dup.flatten_once! end end
-
When defining binary operators and operator-alike methods, name the parameter
other
for operators with "symmetrical" semantics of operands. Symmetrical semantics means both sides of the operator are typically of same or coercible types. [link]- operators and operator-alike methods with symmetrical semantics (the
parameter should be named
other
):+
,-
,*
,/
,%
,**
,==
,>
,<
,|
,&
,^
,eql?
,equal?
; - operators with non-symmetrical semantics (the parameter should not be
named
other
):<<
,[]
(collection/item relations between operands),===
(pattern/matchable relations);
Note that the rule should be followed only if both sides of the operator have the same semantics. Prominent exception in Ruby core is, for example,
Array#*(int)
.# good def +(other) # body omitted end # bad def <<(other) @internal << other end # good def <<(item) @internal << item end # bad # Returns some string multiplied `other` times def *(other) # body omitted end # good # Returns some string multiplied `num` times def *(num) # body omitted end
- operators and operator-alike methods with symmetrical semantics (the
parameter should be named
Comments
Good code is its own best documentation. As you're about to add a comment, ask yourself, "How can I improve the code so that this comment isn't needed?" Improve the code and then document it to make it even clearer.
-- Steve McConnell
-
Write self-documenting code and ignore the rest of this section. Seriously! [link]
-
Write comments in English. [link]
-
Use one space between the leading
#
character of the comment and the text of the comment. [link] -
Comments longer than a word are capitalized and use punctuation. Use one space after periods. [link]
-
Avoid superfluous comments. [link]
# bad counter += 1 # Increments counter by one.
-
Keep existing comments up-to-date. An outdated comment is worse than no comment at all. [link]
Good code is like a good joke: it needs no explanation.
— old programmers maxim, through 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) [link]
Comment Annotations
-
Annotations should usually be written on the line immediately above the relevant code. [link]
-
The annotation keyword is followed by a colon and a space, then a note describing the problem. [link]
-
If multiple lines are required to describe the problem, subsequent lines should be indented three spaces after the
#
(one general plus two for indentation purpose). [link]def bar # FIXME: This has crashed occasionally since v3.2.1. It may # be related to the BarBazUtil upgrade. baz(:quux) end
-
In cases where the problem is so obvious that any documentation would be redundant, annotations may be left at the end of the offending line with no note. This usage should be the exception and not the rule. [link]
def bar sleep 100 # OPTIMIZE end
-
Use
TODO
to note missing features or functionality that should be added at a later date. [link] -
Use
FIXME
to note broken code that needs to be fixed. [link] -
Use
OPTIMIZE
to note slow or inefficient code that may cause performance problems. [link] -
Use
HACK
to note code smells where questionable coding practices were used and should be refactored away. [link] -
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?
[link] -
Use other custom annotation keywords if it feels appropriate, but be sure to document them in your project's
README
or similar. [link]
Magic Comments
-
Place magic comments above all code and documentation in a file (except shebangs, which are discussed next).
# bad # Some documentation about Person # frozen_string_literal: true class Person end # good # frozen_string_literal: true # Some documentation about Person class Person end
-
Place magic comments below shebangs when they are present in a file. [link]
# bad # frozen_string_literal: true #!/usr/bin/env ruby App.parse(ARGV) # good #!/usr/bin/env ruby # frozen_string_literal: true App.parse(ARGV)
-
Use one magic comment per line if you need multiple. [link]
# bad # -*- frozen_string_literal: true; encoding: ascii-8bit -*- # good # frozen_string_literal: true # encoding: ascii-8bit
-
Separate magic comments from code and documentation with a blank line. [link]
# bad # frozen_string_literal: true # Some documentation for Person class Person # Some code end # good # frozen_string_literal: true # Some documentation for Person class Person # Some code end
Classes & Modules
-
Use a consistent structure in your class definitions. [link]
class Person # extend and include go first extend SomeModule include AnotherModule # inner classes CustomError = Class.new(StandardError) # 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 # initialization goes between class methods and other instance methods def initialize end # followed by other 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
-
Split multiple mixins into separate statements. [link]
# bad class Person include Foo, Bar end # good class Person # multiple mixins go in separate statements include Foo include Bar end
-
Don't nest multi-line classes within classes. Try to have such nested classes each in their own file in a folder named like the containing class. [link]
# bad # foo.rb class Foo class Bar # 30 methods inside end class Car # 20 methods inside end # 30 methods inside end # good # foo.rb class Foo # 30 methods inside end # foo/bar.rb class Foo class Bar # 30 methods inside end end # foo/car.rb class Foo class Car # 20 methods inside end end
-
Define (and reopen) namespaced classes and modules using explicit nesting. Using the scope resolution operator can lead to surprising constant lookups due to Ruby's lexical scoping, which depends on the module nesting at the point of definition. [link]
module Utilities class Queue end end # bad class Utilities::Store Module.nesting # => [Utilities::Store] def initialize # Refers to the top level ::Queue class because Utilities isn't in the # current nesting chain. @queue = Queue.new end end # good module Utilities class WaitingList Module.nesting # => [Utilities::WaitingList, Utilities] def initialize @queue = Queue.new # Refers to Utilities::Queue end end end
-
Prefer modules to classes with only class methods. Classes should be used only when it makes sense to create instances out of them. [link]
# bad class SomeClass def self.some_method # body omitted end def self.some_other_method # body omitted end end # good module SomeModule module_function def some_method # body omitted end def some_other_method # body omitted end end
-
Favor the use of
module_function
overextend self
when you want to turn a module's instance methods into class methods. [link]# bad module Utilities extend self def parse_something(string) # do stuff here end def other_utility_method(number, string) # do some more stuff end end # good module Utilities module_function def parse_something(string) # do stuff here end def other_utility_method(number, string) # do some more stuff end end
-
When designing class hierarchies make sure that they conform to the Liskov Substitution Principle. [link]
-
Always supply a proper
to_s
method for classes that represent domain objects. [link]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
-
Use the
attr
family of functions to define trivial accessors or mutators. [link]# 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
-
For accessors and mutators, avoid prefixing method names with
get_
andset_
. It is a Ruby convention to use attribute names for accessors (readers) andattr_name=
for mutators (writers). [link]# bad class Person def get_name "#{@first_name} #{@last_name}" end def set_name(name) @first_name, @last_name = name.split(' ') end end # good class Person def name "#{@first_name} #{@last_name}" end def name=(name) @first_name, @last_name = name.split(' ') end end
-
Avoid the use of
attr
. Useattr_reader
andattr_accessor
instead. [link]# bad - creates a single attribute accessor (deprecated in Ruby 1.9) attr :something, true attr :one, :two, :three # behaves as attr_reader # good attr_accessor :something attr_reader :one, :two, :three
-
Consider using
Struct.new
, which defines the trivial accessors, constructor and comparison operators for you. [link]# good class Person attr_accessor :first_name, :last_name def initialize(first_name, last_name) @first_name = first_name @last_name = last_name end end # better Person = Struct.new(:first_name, :last_name) do end
-
Don't extend an instance initialized by
Struct.new
. Extending it introduces a superfluous class level and may also introduce weird errors if the file is required multiple times. [link]# bad class Person < Struct.new(:first_name, :last_name) end # good Person = Struct.new(:first_name, :last_name)
-
Consider adding factory methods to provide additional sensible ways to create instances of a particular class. [link]
class Person def self.create(options_hash) # body omitted end end
-
Prefer duck-typing over inheritance. [link]
# bad class Animal # abstract method def speak end end # extend superclass class Duck < Animal def speak puts 'Quack! Quack' end end # extend superclass class Dog < Animal def speak puts 'Bau! Bau!' end end # good class Duck def speak puts 'Quack! Quack' end end class Dog def speak puts 'Bau! Bau!' end end
-
Avoid the usage of class (
@@
) variables due to their "nasty" behavior in inheritance. [link]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 everythingpublic
(which is the default). After all we're coding in Ruby now, not in Python. [link] -
Indent the
public
,protected
, andprivate
methods as much as the method definitions they apply to. Leave one blank line above the visibility modifier and one blank line below in order to emphasize that it applies to all methods below it. [link]class SomeClass def public_method # some code end private def private_method # some code end def another_private_method # some code end end
-
Use
def self.method
to define class methods. This makes the code easier to refactor since the class name is not repeated. [link]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 class methods. class << self def first_method # body omitted end def second_method_etc # body omitted end end end
-
Prefer
alias
when aliasing methods in lexical class scope as the resolution ofself
in this context is also lexical, and it communicates clearly to the user that the indirection of your alias will not be altered at runtime or by any subclass unless made explicit. [link]class Westerner def first_name @names.first end alias given_name first_name end
Since
alias
, likedef
, is a keyword, prefer bareword arguments over symbols or strings. In other words, doalias foo bar
, notalias :foo :bar
.Also be aware of how Ruby handles aliases and inheritance: an alias references the method that was resolved at the time the alias was defined; it is not dispatched dynamically.
class Fugitive < Westerner def first_name 'Nobody' end end
In this example,
Fugitive#given_name
would still call the originalWesterner#first_name
method, notFugitive#first_name
. To override the behavior ofFugitive#given_name
as well, you'd have to redefine it in the derived class.class Fugitive < Westerner def first_name 'Nobody' end alias given_name first_name end
-
Always use
alias_method
when aliasing methods of modules, classes, or singleton classes at runtime, as the lexical scope ofalias
leads to unpredictability in these cases. [link]module Mononymous def self.included(other) other.class_eval { alias_method :full_name, :given_name } end end class Sting < Westerner include Mononymous end
-
When class (or module) methods call other such methods, omit the use of a leading
self
or own name followed by a.
when calling other such methods. This is often seen in "service classes" or other similar concepts where a class is treated as though it were a function. This convention tends to reduce repetitive boilerplate in such classes. [link]class TestClass # bad -- more work when class renamed/method moved def self.call(param1, param2) TestClass.new(param1).call(param2) end # bad -- more verbose than necessary def self.call(param1, param2) self.new(param1).call(param2) end # good def self.call(param1, param2) new(param1).call(param2) end # ...other methods... end
Exceptions
-
Prefer
raise
overfail
for exceptions. [link]# bad fail SomeException, 'message' # good raise SomeException, 'message'
-
Don't specify
RuntimeError
explicitly in the two argument version ofraise
. [link]# 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. [link]# 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`.
-
Do not return from an
ensure
block. If you explicitly return from a method inside anensure
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. [link]# bad def foo raise ensure return 'very bad idea' end
-
Use implicit begin blocks where possible. [link]
# 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
-
Mitigate the proliferation of
begin
blocks by using contingency methods (a term coined by Avdi Grimm). [link]# bad begin something_that_might_fail rescue IOError # handle IOError end begin something_else_that_might_fail rescue IOError # handle IOError end # good def with_io_error_handling yield rescue IOError # handle IOError end with_io_error_handling { something_that_might_fail } with_io_error_handling { something_else_that_might_fail }
-
Don't suppress exceptions. [link]
# 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. [link]# 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 => ex handle_error(ex) end
-
Don't use exceptions for flow of control. [link]
# 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 toexit
, requiring you tokill -9
the process. [link]# 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
-
Put more specific exceptions higher up the rescue chain, otherwise they'll never be rescued from. [link]
# bad begin # some code rescue StandardError => e # some handling rescue IOError => e # some handling that will never be executed end # good begin # some code rescue IOError => e # some handling rescue StandardError => e # some handling end
-
Release external resources obtained by your program in an
ensure
block. [link]f = File.open('testfile') begin # .. process rescue # .. handle error ensure f.close if f end
-
Use versions of resource obtaining methods that do automatic resource cleanup when possible. [link]
# bad - you need to close the file descriptor explicitly f = File.open('testfile') # some action on the file f.close # good - the file descriptor is closed automatically File.open('testfile') do |f| # some action on the file end
-
Favor the use of exceptions from the standard library over introducing new exception classes. [link]
Collections
-
Prefer literal array and hash creation notation (unless you need to pass parameters to their constructors, that is). [link]
# bad arr = Array.new hash = Hash.new # good arr = [] arr = Array.new(10) hash = {} hash = Hash.new(0)
-
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. [link]# bad STATES = ['draft', 'open', 'closed'] # good STATES = %w[draft open closed]
-
Prefer
%i
to the literal array syntax when you need an array of symbols (and you don't need to maintain Ruby 1.9 compatibility). Apply this rule only to arrays with two or more elements. [link]# bad STATES = [:draft, :open, :closed] # good STATES = %i[draft open closed]
-
Avoid comma after the last item of an
Array
orHash
literal, especially when the items are not on separate lines. [link]# bad - easier to move/add/remove items, but still not preferred VALUES = [ 1001, 2020, 3333, ] # bad VALUES = [1001, 2020, 3333, ] # good VALUES = [1001, 2020, 3333]
-
Avoid the creation of huge gaps in arrays. [link]
arr = [] arr[100] = 1 # now you have an array with lots of nils
-
When accessing the first or last element from an array, prefer
first
orlast
over[0]
or[-1]
. [link] -
Use
Set
instead ofArray
when dealing with unique elements.Set
implements a collection of unordered values with no duplicates. This is a hybrid ofArray
's intuitive inter-operation facilities andHash
's fast lookup. [link] -
Prefer symbols instead of strings as hash keys. [link]
# 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. [link]
-
Use the Ruby 1.9 hash literal syntax when your hash keys are symbols. [link]
# bad hash = { :one => 1, :two => 2, :three => 3 } # good hash = { one: 1, two: 2, three: 3 }
-
Don't mix the Ruby 1.9 hash syntax with hash rockets in the same hash literal. When you've got keys that are not symbols stick to the hash rockets syntax. [link]
# bad { a: 1, 'b' => 2 } # good { :a => 1, 'b' => 2 }
-
Use
Hash#key?
instead ofHash#has_key?
andHash#value?
instead ofHash#has_value?
. [link]# bad hash.has_key?(:test) hash.has_value?(value) # good hash.key?(:test) hash.value?(value)
-
Use
Hash#each_key
instead ofHash#keys.each
andHash#each_value
instead ofHash#values.each
. [link]# bad hash.keys.each { |k| p k } hash.values.each { |v| p v } hash.each { |k, _v| p k } hash.each { |_k, v| p v } # good hash.each_key { |k| p k } hash.each_value { |v| p v }
-
Use
Hash#fetch
when dealing with hash keys that should be present. [link]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. [link]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 works correctly with falsy values batman.fetch(:is_evil, true) # => false
-
Prefer the use of the block instead of the default value in
Hash#fetch
if the code that has to be evaluated may have side effects or be expensive. [link]batman = { name: 'Bruce Wayne' } # bad - if we use the default value, we eager evaluate it # so it can slow the program down if done multiple times batman.fetch(:powers, obtain_batman_powers) # obtain_batman_powers is an expensive call # good - blocks are lazy evaluated, so only triggered in case of KeyError exception batman.fetch(:powers) { obtain_batman_powers }
-
Use
Hash#values_at
when you need to retrieve several values consecutively from a hash. [link]# bad email = data['email'] username = data['nickname'] # good email, username = data.values_at('email', 'nickname')
-
Rely on the fact that as of Ruby 1.9 hashes are ordered. [link]
-
Do not modify a collection while traversing it. [link]
-
When accessing elements of a collection, avoid direct access via
[n]
by using an alternate form of the reader method if it is supplied. This guards you from calling[]
onnil
. [link]# bad Regexp.last_match[1] # good Regexp.last_match(1)
-
When providing an accessor for a collection, provide an alternate form to save users from checking for
nil
before accessing an element in the collection. [link]# bad def awesome_things @awesome_things end # good def awesome_things(index = nil) if index && @awesome_things @awesome_things[index] else @awesome_things end end
Numbers
-
Use
Integer
to check type of an integer number. SinceFixnum
is platform-dependent, checking against it will return different results on 32-bit and 64-bit machines. [link]timestamp = Time.now.to_i # bad timestamp.is_a? Fixnum timestamp.is_a? Bignum # good timestamp.is_a? Integer
-
Prefer to use ranges when generating random numbers instead of integers with offsets, since it clearly states your intentions. Imagine simulating a roll of a dice: [link]
# bad rand(6) + 1 # good rand(1..6)
Strings
-
Prefer string interpolation and string formatting instead of string concatenation: [link]
# 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)
-
Adopt a consistent string literal quoting style. There are two popular styles in the Ruby community, both of which are considered good—single quotes by default (Option A) and double quotes by default (Option B). [link]
-
(Option A) Prefer single-quoted strings when you don't need string interpolation or special symbols such as
\t
,\n
,'
, etc.# bad name = "Bozhidar" name = 'De\'Andre' # good name = 'Bozhidar' name = "De'Andre"
-
(Option B) Prefer double-quotes unless your string literal contains
"
or escape characters you want to suppress.# bad name = 'Bozhidar' sarcasm = "I \"like\" it." # good name = "Bozhidar" sarcasm = 'I "like" it.'
The string literals in this guide are aligned with the first style.
-
-
Don't use the character literal syntax
?x
. Since Ruby 1.9 it's basically redundant—?x
would be interpreted as'x'
(a string with a single character in it). [link]# bad char = ?c # good char = 'c'
-
Don't leave out
{}
around instance and global variables being interpolated into a string. [link]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 = #$global" # good puts "$global = #{$global}"
-
Don't use
Object#to_s
on interpolated objects. It's invoked on them automatically. [link]# bad message = "This is the #{result.to_s}." # good message = "This is the #{result}."
-
Avoid using
String#+
when you need to construct large data chunks. Instead, useString#<<
. Concatenation mutates the string instance in-place and is always faster thanString#+
, which creates a bunch of new string objects. [link]# bad html = '' html += '<h1>Page title</h1>' paragraphs.each do |paragraph| html += "<p>#{paragraph}</p>" end # good and also fast html = '' html << '<h1>Page title</h1>' paragraphs.each do |paragraph| html << "<p>#{paragraph}</p>" end
-
Don't use
String#gsub
in scenarios in which you can use a faster and more specialized alternative. [link]url = 'http://example.com' str = 'lisp-case-rules' # bad url.gsub('http://', 'https://') str.gsub('-', '_') # good url.sub('http://', 'https://') str.tr('-', '_')
-
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. [link]
code = <<-END.gsub(/^\s+\|/, '') |def test | some_method | other_method |end END # => "def test\n some_method\n other_method\nend\n"
-
Use Ruby 2.3's squiggly heredocs for nicely indented multi-line strings. [link]
# bad - using Powerpack String#strip_margin code = <<-RUBY.strip_margin('|') |def test | some_method | other_method |end RUBY # also bad code = <<-RUBY def test some_method other_method end RUBY # good code = <<~RUBY def test some_method other_method end RUBY
-
Use descriptive delimiters for heredocs. Delimiters add valuable information about the heredoc content, and as an added bonus some editors can highlight code within heredocs if the correct delimiter is used. [link]
# bad code = <<~END def foo bar end END # good code = <<~RUBY def foo bar end RUBY # good code = <<~SUMMARY An imposing black structure provides a connection between the past and the future in this enigmatic adaptation of a short story by revered sci-fi author Arthur C. Clarke. SUMMARY
Date & Time
-
Prefer
Time.now
overTime.new
when retrieving the current system time. [link] -
Don't use
DateTime
unless you need to account for historical calendar reform—and if you do, explicitly specify thestart
argument to clearly state your intentions. [link]# bad - uses DateTime for current time DateTime.now # good - uses Time for current time Time.now # bad - uses DateTime for modern date DateTime.iso8601('2016-06-29') # good - uses Date for modern date Date.iso8601('2016-06-29') # good - uses DateTime with start argument for historical date DateTime.iso8601('1751-04-23', Date::ENGLAND)
Regular Expressions
Some people, when confronted with a problem, think "I know, I'll use regular expressions." Now they have two problems.
-- Jamie Zawinski
-
Don't use regular expressions if you just need plain text search in string:
string['text']
[link] -
For simple constructions you can use regexp directly through string index. [link]
match = string[/regexp/] # get content of matched regexp first_group = string[/text(grp)/, 1] # get content of captured group string[/text (grp)/, 1] = 'replace' # string => 'text replace'
-
Use non-capturing groups when you don't use the captured result. [link]
# bad /(first|second)/ # good /(?:first|second)/
-
Don't use the cryptic Perl-legacy variables denoting last regexp group matches (
$1
,$2
, etc). UseRegexp.last_match(n)
instead. [link]/(regexp)/ =~ string ... # bad process $1 # good process Regexp.last_match(1)
-
Avoid using numbered groups as it can be hard to track what they contain. Named groups can be used instead. [link]
# bad /(regexp)/ =~ string # some code process Regexp.last_match(1) # good /(?<meaningful_var>regexp)/ =~ string # some code process meaningful_var
-
Character classes have only a few special characters you should care about:
^
,-
,\
,]
, so don't escape.
or brackets in[]
. [link] -
Be careful with
^
and$
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/
). [link]string = "some injection\nusername" string[/^username$/] # matches string[/\Ausername\z/] # doesn't match
-
Use
x
modifier for complex regexps. This makes them more readable and you can add some useful comments. Just be careful as spaces are ignored. [link]regexp = / start # some text \s # white space char (group) # first group (?:alt1|alt2) # some alternation end /x
-
For complex replacements
sub
/gsub
can be used with a block or a hash. [link]words = 'foo bar' words.sub(/f/, 'f' => 'F') # => 'Foo bar' words.gsub(/\w+/) { |word| word.capitalize } # => 'Foo Bar'
Percent Literals
-
Use
%()
(it's a shorthand for%Q
) for single-line strings which require both interpolation and embedded double-quotes. For multi-line strings, prefer heredocs. [link]# bad (no interpolation needed) %(<div class="text">Some text</div>) # should be '<div class="text">Some text</div>' # bad (no double-quotes) %(This is #{quality} style) # should be "This is #{quality} style" # bad (multiple lines) %(<div>\n<span class="big">#{exclamation}</span>\n</div>) # should be a heredoc. # good (requires interpolation, has quotes, single line) %(<tr><td class="name">#{name}</td>)
-
Avoid
%()
or the equivalent%q()
unless you have a string with both'
and"
in it. Regular string literals are more readable and should be preferred unless a lot of characters would have to be escaped in them. [link]# bad name = %q(Bruce Wayne) time = %q(8 o'clock) question = %q("What did you say?") # good name = 'Bruce Wayne' time = "8 o'clock" question = '"What did you say?"' quote = %q(<p class='quote'>"What did you say?"</p>)
-
Use
%r
only for regular expressions matching at least one'/'
character. [link]# bad %r{\s+} # good %r{^/(.*)$} %r{^/blog/2011/(.*)$}
-
Avoid the use of
%x
unless you're going to invoke a command with backquotes in it (which is rather unlikely). [link]# bad date = %x(date) # good date = `date` echo = %x(echo `date`)
-
Avoid the use of
%s
. It seems that the community has decided:"some string"
is the preferred way to create a symbol with spaces in it. [link] -
Use the braces that are the most appropriate for the various kinds of percent literals. [link]
()
for string literals(%q
,%Q
).[]
for array literals(%w
,%i
,%W
,%I
) as it is aligned with the standard array literals.{}
for regexp literals(%r
) since parentheses often appear inside regular expressions. That's why a less common character with{
is usually the best delimiter for%r
literals.()
for all other literals (e.g.%s
,%x
)
# bad %q{"Test's king!", John said.} # good %q("Test's king!", John said.) # bad %w(one two three) %i(one two three) # good %w[one two three] %i[one two three] # bad %r((\w+)-(\d+)) %r{\w{1,2}\d{2,5}} # good %r{(\w+)-(\d+)} %r|\w{1,2}\d{2,5}|
Metaprogramming
-
Avoid needless metaprogramming. [link]
-
Do not mess around in core classes when writing libraries. (Do not monkey-patch them.) [link]
-
The block form of
class_eval
is preferable to the string-interpolated form. [link]-
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 toclass_eval{ def ... }
-
-
When using
class_eval
(or othereval
) with string interpolation, add a comment block showing its appearance if interpolated (a practice used in Rails code): [link]# 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}(*params, &block) # def capitalize(*params, &block) to_str.#{unsafe_method}(*params, &block) # to_str.capitalize(*params, &block) end # end def #{unsafe_method}!(*params) # def capitalize!(*params) @dirty = true # @dirty = true super # super end # end EOT end end
-
Avoid using
method_missing
for metaprogramming because backtraces become messy, the behavior is not listed in#methods
, and misspelled method calls might silently work, e.g.nukes.launch_state = false
. Consider using delegation, proxy, ordefine_method
instead. If you must usemethod_missing
: [link]-
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, *params, &block) if /^find_by_(?<prop>.*)/ =~ meth # ... lots of code to do a find_by else super end end # good def method_missing(meth, *params, &block) if /^find_by_(?<prop>.*)/ =~ meth find_by(prop, *params, &block) else super end end # best of all, though, would to define_method as each findable attribute is declared
-
-
Prefer
public_send
oversend
so as not to circumventprivate
/protected
visibility. [link]# We have an ActiveModel Organization that includes concern Activatable module Activatable extend ActiveSupport::Concern included do before_create :create_token end private def reset_token # some code end def create_token # some code end def activate! # some code end end class Organization < ActiveRecord::Base include Activatable end linux_organization = Organization.find(...) # BAD - violates privacy linux_organization.send(:reset_token) # GOOD - should throw an exception linux_organization.public_send(:reset_token)
-
Prefer
__send__
oversend
, assend
may overlap with existing methods. [link]require 'socket' u1 = UDPSocket.new u1.bind('127.0.0.1', 4913) u2 = UDPSocket.new u2.connect('127.0.0.1', 4913) # Won't send a message to the receiver obj. # Instead it will send a message via UDP socket. u2.send :sleep, 0 # Will actually send a message to the receiver obj. u2.__send__ ...
Misc
-
Write
ruby -w
safe code. [link] -
Avoid hashes as optional parameters. Does the method do too much? (Object initializers are exceptions for this rule). [link]
-
Avoid methods longer than 10 LOC (lines of code). Ideally, most methods will be shorter than 5 LOC. Empty lines do not contribute to the relevant LOC. [link]
-
Avoid parameter lists longer than three or four parameters. [link]
-
If you really need "global" methods, add them to Kernel and make them private. [link]
-
Use module instance variables instead of global variables. [link]
# bad $foo_bar = 1 # good module Foo class << self attr_accessor :bar end end Foo.bar = 1
-
Use
OptionParser
for parsing complex command line options andruby -s
for trivial command line options. [link] -
Code in a functional way, avoiding mutation when that makes sense. [link]
a = []; [1, 2, 3].each { |i| a << i * 2 } # bad a = [1, 2, 3].map { |i| i * 2 } # good a = {}; [1, 2, 3].each { |i| a[i] = i * 17 } # bad a = [1, 2, 3].reduce({}) { |h, i| h[i] = i * 17; h } # good a = [1, 2, 3].each_with_object({}) { |i, h| h[i] = i * 17 } # good
-
Do not mutate parameters unless that is the purpose of the method. [link]
-
Avoid more than three levels of block nesting. [link]
-
Be consistent. In an ideal world, be consistent with these guidelines. [link]
-
Use common sense. [link]
Tools
Here are some tools to help you automatically check Ruby code against this guide.
RuboCop
RuboCop is a Ruby static code analyzer and formatter, based on this style guide. RuboCop already covers a significant portion of the Guide and plugins for most popular Ruby editors and IDEs.
RubyMine
RubyMine's code inspections are partially based on this guide.
Contributing
The guide is still a work in progress—some rules are lacking examples, some rules don't have examples that illustrate them clearly enough. Improving such rules is a great (and simple way) to help the Ruby community!
In due time these issues will (hopefully) be addressed—just keep them in mind for now.
Nothing written in this guide is set in stone. It's my desire to work together with everyone interested in Ruby coding style, so that we could ultimately create a resource that will be beneficial to the entire Ruby community.
Feel free to open tickets or send pull requests with improvements. Thanks in advance for your help!
You can also support the project (and RuboCop) with financial contributions via Patreon.
How to Contribute?
It's easy, just follow the contribution guidelines.
License
This work is licensed under a Creative Commons Attribution 3.0 Unported License
Spread the Word
A community-driven style guide is of little use to a community that doesn't know about its existence. Tweet about the guide, share it with your friends and colleagues. Every comment, suggestion or opinion we get makes the guide just a little bit better. And we want to have the best possible guide, don't we?
Cheers,
Bozhidar