= The Ruby Style Guide
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== Introduction

[quote, Officer Alex J. Murphy / RoboCop]

Role models are important.

ifdef::env-github[]
TIP: You can find a beautiful version of this guide with much improved navigation at https://rubystyle.guide.
endif::[]

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 guidelines.
We've tried to add the rationale behind the guidelines (if it's omitted we've assumed it's pretty obvious).

We didn't come up with all the guidelines out of nowhere - they are mostly based on the professional experience of the editors, feedback and suggestions from members of the Ruby community and various highly regarded Ruby programming resources, such as https://pragprog.com/book/ruby4/programming-ruby-1-9-2-0["Programming Ruby"] and http://www.amazon.com/Ruby-Programming-Language-David-Flanagan/dp/0596516177["The Ruby Programming Language"].

This style guide evolves over time as additional conventions are identified and past conventions are rendered obsolete by changes in Ruby itself.

ifdef::env-github[]
You can generate a PDF copy of this guide using https://asciidoctor.org/docs/asciidoctor-pdf/[AsciiDoctor PDF], and an HTML copy https://asciidoctor.org/docs/convert-documents/#converting-a-document-to-html[with] https://asciidoctor.org/#installation[AsciiDoctor] using the following commands:

[source,shell]

Generates README.pdf

asciidoctor-pdf -a allow-uri-read README.adoc

Generates README.html

asciidoctor

[TIP]

Install the rouge gem to get nice syntax highlighting in the generated document.

[source,shell]

gem install rouge

====
endif::[]

[TIP]

If you're into Rails or RSpec you might want to check out the complementary https://github.com/rubocop-hq/rails-style-guide[Ruby on Rails Style Guide] and https://github.com/rubocop-hq/rspec-style-guide[RSpec Style Guide].

TIP: https://github.com/rubocop-hq/rubocop[RuboCop] is a static code analyzer (linter) and formatter, based on this style guide.

=== Guiding Principles

[quote, Harold Abelson, Structure and Interpretation of Computer Programs]

Programs must be written for people to read, and only incidentally for machines to execute.

It's common knowledge that code is read much more often than it is written.
The guidelines provided here are intended to improve the readability of code and make it consistent across the wide spectrum of Ruby code.
They are also meant to reflect real-world usage of Ruby instead of a random ideal. When we had to choose between a very established practice
and a subjectively better alternative we've opted to recommend the established practice.footnote:[Occasionally we might suggest to the reader to consider some alternatives, though.]

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.

Ruby had existed for over 15 years by the time
the guide was created, and the language's flexibility and lack of common standards have contributed to the
creations of numerous styles for just about everything. Rallying people around the cause of community standards
took a lot of time and energy, and we still have a lot of ground to cover.

Ruby is famously optimized for programmer happiness. We'd like to believe that this guide is going to help you optimize for maximum
programmer happiness.

=== A Note about Consistency

[quote, Ralph Waldo Emerson]

A foolish consistency is the hobgoblin of little minds, adored by little statesmen and philosophers and divines.

A style guide is about consistency.
Consistency with this style guide is important.
Consistency within a project is more important.
Consistency within one class or method is the most important.

However, know when to be inconsistent -- sometimes style guide recommendations just aren't applicable. When in doubt, use your best judgment.
Look at other examples and decide what looks best. And don't hesitate to ask!

In particular: do not break backwards compatibility just to comply with this guide!

Some other good reasons to ignore a particular guideline:

• When applying the guideline would make the code less readable, even for someone who is used to reading code that follows this style guide.
• To be consistent with surrounding code that also breaks it (maybe for historic reasons) -- although this is also an opportunity to clean up someone else's mess (in true XP style).
• Because the code in question predates the introduction of the guideline and there is no other reason to be modifying that code.
• When the code needs to remain compatible with older versions of Ruby that don't support the feature recommended by the style guide.

=== Translations

Translations of the guide are available in the following languages:

• https://github.com/JuanitoFatas/ruby-style-guide/blob/master/README-zhCN.md[Chinese Simplified]
• https://github.com/JuanitoFatas/ruby-style-guide/blob/master/README-zhTW.md[Chinese Traditional]
• https://github.com/HassanTC/ruby-style-guide/blob/master/README-EgAr.md[Egyptian Arabic]
• https://github.com/gauthier-delacroix/ruby-style-guide/blob/master/README-frFR.md[French]
• https://github.com/fortissimo1997/ruby-style-guide/blob/japanese/README.ja.md[Japanese]
• https://github.com/dalzony/ruby-style-guide/blob/master/README-koKR.md[Korean]
• https://github.com/rubensmabueno/ruby-style-guide/blob/master/README-PT-BR.md[Portuguese (pt-BR)]
• https://github.com/arbox/ruby-style-guide/blob/master/README-ruRU.md[Russian]
• https://github.com/alemohamad/ruby-style-guide/blob/master/README-esLA.md[Spanish]
• https://github.com/CQBinh/ruby-style-guide/blob/master/README-viVN.md[Vietnamese]

NOTE: These translations are not maintained by our editor team, so their quality
and level of completeness may vary. The translated versions of the guide often
lag behind the upstream English version.

== Source Code Layout

[quote, Jerry Coffin (on indentation)]

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...

=== Source Encoding

Use UTF-8 as the source file encoding.

TIP: UTF-8 has been the default source file encoding since Ruby 2.0.

=== Tabs or Spaces?

Use only spaces for indentation. No hard tabs.

=== Indentation

Use two spaces per indentation level (aka soft tabs).

[source,ruby]

bad - four spaces

def some_method
do_something
end

good

def some_method
do_something
end

[#80-character-limits]
=== Maximum Line Length

Limit lines to 80 characters.

=== No Trailing Whitespace

Avoid trailing whitespace.

=== Line Endings

Use Unix-style line endings.footnote:[*BSD/Solaris/Linux/macOS users are covered by default, Windows users have to be extra careful.]

[TIP]

If you're using Git you might want to add the following configuration setting to protect your project from Windows line endings creeping in:

[source,bash]

$ git config --global core.autocrlf true

====

=== Should I Terminate Files with a Newline?

End each file with a newline.

=== Should I Terminate Expressions with ;?

Don't use ; to terminate statements and expressions.

[source,ruby]

bad

puts 'foobar'; # superfluous semicolon

good

puts 'foobar'

=== One Expression Per Line

Use one expression per line.

[source,ruby]

bad

puts 'foo'; puts 'bar' # two expressions on the same line

good

puts 'foo'
puts 'bar'

puts 'foo', 'bar' # this applies to puts in particular

=== Spaces and Operators

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.

[source,ruby]

sum = 1 + 2
a, b = 1, 2
class FooError < StandardError; end

There are a few exceptions:

• Exponent operator:

[source,ruby]

bad

e = M * c ** 2

good

e = M * c**2

• Slash in rational literals:

[source,ruby]

bad

o_scale = 1 / 48r

good

o_scale = 1/48r

• Safe navigation operator:

[source,ruby]

bad

foo &. bar
foo &.bar
foo&. bar

good

foo&.bar

=== Spaces and Braces

No spaces after (, [ or before ], ).
Use spaces around { and before }.

[source,ruby]

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.

[source,ruby]

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.

[source,ruby]

bad

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

good

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

=== No Space after Bang

No space after !.

[source,ruby]

bad

! something

good

!something

=== No Space inside Range Literals

No space inside range literals.

[source,ruby]

bad

1 .. 3
'a' ... 'z'

good

1..3
'a'...'z'

=== Indent when to case

Indent when as deep as case.

[source,ruby]

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

.A Bit of History

This is the style established in both "The Ruby Programming Language" and "Programming Ruby".
Historically it is derived from the fact that case and switch statements are not blocks, hence should not be indented, and the when and else keywords are labels (compiled in the C language, they are literally labels for JMP calls).

=== Indent Conditional Assignment

When assigning the result of a conditional expression to a variable, preserve the usual alignment of its branches.

[source,ruby]

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

=== Empty Lines between Methods

Use empty lines between method definitions and also to break up methods into logical paragraphs internally.

[source,ruby]

def some_method
data = initialize(options)

data.manipulate!

data.result
end

def some_method
result
end

=== Two or More Empty Lines

Don't use several empty lines in a row.

[source,ruby]

bad - It has two empty lines.

some_method

some_method

good

some_method

some_method

=== Empty Lines around Access Modifier

Use empty lines around access modifiers.

[source,ruby]

bad

class Foo
attr_reader :foo
def foo
# do something...
end
end

good

class Foo
attr_reader :foo

def foo
# do something...
end
end

=== Empty Lines around Bodies

Don't use empty lines around method, class, module, block bodies.

[source,ruby]

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

=== Trailing Comma in Method Arguments

Avoid comma after the last parameter in a method call, especially when the parameters are not on separate lines.

[source,ruby]

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)

=== Spaces around Equals

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

[source,ruby]

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).

=== Line Continuation in Expressions

Avoid line continuation with \ where not required.
In practice, avoid using line continuations for anything but string concatenation.

[source,ruby]

bad (\ is not needed here)

result = 1 -
2

bad (\ is required, but still ugly as hell)

result = 1
- 2

good

result = 1 -
2

long_string = 'First part of the long string'
' and second part of the long string'

=== Multi-line Method Chains

Adopt a consistent multi-line method chaining style.
There are two popular styles in the Ruby community, both of which are considered good - leading . and trailing ..

==== Leading .

When continuing a chained method invocation on another line keep the . on the second line.

[source,ruby]

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

==== Trailing .

When continuing a chained method invocation on another line, include the . on the first line to indicate that the expression continues.

[source,ruby]

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 https://github.com/rubocop-hq/ruby-style-guide/pull/176[here].

=== Method Arguments Alignment

Align the arguments of a method call if they span more than one line.
When aligning arguments is not appropriate due to line-length constraints, single indent for the lines after the first is also acceptable.

[source,ruby]

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

=== Implicit Options Hash

IMPORTANT: As of Ruby 2.7 braces around an options hash are no longer
optional.

Omit the outer braces around an implicit options hash.

[source,ruby]

bad

user.set({ name: 'John', age: 45, permissions: { read: true } })

good

user.set(name: 'John', age: 45, permissions: { read: true })

=== DSL Method Calls

Omit both the outer braces and parentheses for methods that are part of an internal DSL.

[source,ruby]

class Person < ActiveRecord::Base

bad

validates(:name, { presence: true, length: { within: 1..10 } })

good

validates :name, presence: true, length: { within: 1..10 }
end

=== Space in Method Calls

Do not put a space between a method name and the opening parenthesis.

[source,ruby]

bad

f (3 + 2) + 1

good

f(3 + 2) + 1

=== Multi-line Arrays Alignment

Align the elements of array literals spanning multiple lines.

[source,ruby]

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]

== Naming Conventions

[quote, Phil Karlton]

The only real difficulties in programming are cache invalidation and naming things.

=== English for Identifiers

Name identifiers in English.

[source,ruby]

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

=== Snake Case for Symbols, Methods and Variables

Use snake_case for symbols, methods and variables.

[source,ruby]

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

=== Identifiers with a Numeric Suffix

Do not separate numbers from letters on symbols, methods and variables.

[source,ruby]

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

=== CamelCase for Classes

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

[source,ruby]

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

=== Snake Case for Files

Use snake_case for naming files, e.g. hello_world.rb.

=== Snake Case for Directories

Use snake_case for naming directories, e.g. lib/hello_world/hello_world.rb.

=== One Class per File

Aim to have just a single class/module per source file.
Name the file name as the class/module, but replacing CamelCase with snake_case.

=== Screaming Snake Case for Constants

Use SCREAMING_SNAKE_CASE for other constants (those that don't refer to classes and modules).

[source,ruby]

bad

SomeConst = 5

good

SOME_CONST = 5

=== Predicate Methods Suffix

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.

[source,ruby]

bad

def even(value)
end

good

def even?(value)
end

=== Predicate Methods Prefix

Avoid prefixing predicate methods with the auxiliary verbs such as is, does, or can.
These words are redundant and inconsistent with the style of boolean methods in the Ruby core library, such as empty? and include?.

[source,ruby]

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

=== Dangerous Method Suffix

The names of potentially dangerous methods (i.e. methods that modify self or the arguments, exit! (doesn't run the finalizers like exit does), etc) should end with an exclamation mark if there exists a safe version of that dangerous method.

[source,ruby]

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

=== Relationship between Safe and Dangerous Methods

Define the non-bang (safe) method in terms of the bang (dangerous) one if possible.

[source,ruby]

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

=== Unused Variables Prefix

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.

[source,ruby]

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

=== other Parameter

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.

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).

[source,ruby]

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

== Flow of Control

=== for Loops

Do not use for, unless you know exactly why.
Most of the time iterators should be used instead.
for is implemented in terms of each (so you're adding a level of indirection), but with a twist - for doesn't introduce a new scope (unlike each) and variables defined in its block will be visible outside it.

[source,ruby]

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'

=== then in Multi-line Expression

Do not use then for multi-line if/unless/when.

[source,ruby]

bad

if some_condition then

body omitted

end

bad

case foo
when bar then

body omitted

end

good

if some_condition

body omitted

end

good

case foo
when bar

body omitted

end

=== Condition Placement

Always put the condition on the same line as the if/unless in a multi-line conditional.

[source,ruby]

bad

if
some_condition
do_something
do_something_else
end

good

if some_condition
do_something
do_something_else
end

=== Ternary Operator vs if

Prefer the ternary operator(?:) over if/then/else/end constructs.
It's more common and obviously more concise.

[source,ruby]

bad

result = if some_condition then something else something_else end

good

result = some_condition ? something : something_else

=== Nested Ternary Operators

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.

[source,ruby]

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

=== Semicolon in if

Do not use if x; .... Use the ternary operator instead.

[source,ruby]

bad

result = if some_condition; something else something_else end

good

result = some_condition ? something : something_else

=== case vs if-else

Prefer case over if-elsif when compared value is same in each clause.

[source,ruby]

bad

if status == :active
perform_action
elsif status == :inactive, status == :hibernating
check_timeout
else
final_action
end

good

case status
when :active
perform_action
when :inactive, :hibernating
check_timeout
else
final_action
end

=== Returning Result from if/case

Leverage the fact that if and case are expressions which return a result.

[source,ruby]

bad

if condition
result = x
else
result = y
end

good

result =
if condition
x
else
y
end

=== One-line Cases

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

NOTE: The alternative syntax when x: ... has been removed as of Ruby 1.9.

=== Semicolon in when

Do not use when x; .... See the previous rule.

=== ! vs not

Use ! instead of not.

[source,ruby]

bad - parentheses are required because of op precedence

x = (not something)

good

x = !something

=== Double Negation

Avoid the use of !!.

!! 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 a nil check, use nil? instead.

[source,ruby]

bad

x = 'test'

obscure nil check

if !!x

body omitted

end

good

x = 'test'
if x

body omitted

end

=== and/or

The and and or 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, use if and unless; && and , are also acceptable but less clear.

[source,ruby]

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!")

=== Multi-line Ternary Operator

Avoid multi-line ?: (the ternary operator); use if/unless instead.

=== if as a Modifier

Prefer modifier if/unless usage when you have a single-line body.
Another good alternative is the usage of control flow &&/, .

[source,ruby]

bad

if some_condition
do_something
end

good

do_something if some_condition

another good option

some_condition && do_something

=== Multi-line if Modifiers

Avoid modifier if/unless usage at the end of a non-trivial multi-line block.

[source,ruby]

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

=== Nested Modifiers

Avoid nested modifier if/unless/while/until usage.
Prefer &&/, if appropriate.

[source,ruby]

bad

do_something if other_condition if some_condition

good

do_something if some_condition && other_condition

=== if vs unless

Prefer unless over if for negative conditions (or control flow , ).

[source,ruby]

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

=== Using else with unless

Do not use unless with else.
Rewrite these with the positive case first.

[source,ruby]

bad

unless success?
puts 'failure'
else
puts 'success'
end

good

if success?
puts 'success'
else
puts 'failure'
end

=== Parentheses around Condition

Don't use parentheses around the condition of a control expression.

[source,ruby]

bad

if (x > 10)

body omitted

end

good

if x > 10

body omitted

end

NOTE: There is an exception to this rule, namely <<safe-assignment-in-condition,safe assignment in condition>>.

=== Multi-line while do

Do not use while/until condition do for multi-line while/until.

[source,ruby]

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

=== while as a Modifier

Prefer modifier while/until usage when you have a single-line body.

[source,ruby]

bad

while some_condition
do_something
end

good

do_something while some_condition

=== while vs until

Prefer until over while for negative conditions.

[source,ruby]

bad

do_something while !some_condition

good

do_something until some_condition

=== Infinite Loop

Use Kernel#loop instead of while/until when you need an infinite loop.

[source,ruby]

bad

while true
do_something
end

until false
do_something
end

good

loop do
do_something
end

=== loop with break

Use Kernel#loop with break rather than begin/end/until or begin/end/while for post-loop tests.

[source,ruby]

bad

begin
puts val
val += 1
end while val < 0

good

loop do
puts val
val += 1
break unless val < 0
end

=== Explicit return

Avoid return where not required for flow of control.

[source,ruby]

bad

def some_method(some_arr)
return some_arr.size
end

good

def some_method(some_arr)
some_arr.size
end

=== Explicit self

Avoid self where not required.
(It is only required when calling a self write accessor, methods named after reserved words, or overloadable operators.)

[source,ruby]

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)
status = :in_progress
end
status == :verified
end

=== Shadowing Methods

As a corollary, avoid shadowing methods with local variables unless they are both equivalent.

[source,ruby]

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

=== Safe Assignment in Condition

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.

[source,ruby]

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

=== BEGIN Blocks

Avoid the use of BEGIN blocks.

=== END Blocks

Do not use END blocks. Use Kernel#at_exit instead.

[source,ruby]

bad

END { puts 'Goodbye!' }

good

at_exit { puts 'Goodbye!' }

=== Nested Conditionals

Avoid use of nested conditionals for flow of control.

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.

[source,ruby]

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.

[source,ruby]

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

== Exceptions

=== raise vs fail

Prefer raise over fail for exceptions.

[source,ruby]

bad

fail SomeException, 'message'

good

raise SomeException, 'message'

=== Raising Explicit RuntimeError

Don't specify RuntimeError explicitly in the two argument version of raise.

[source,ruby]

bad

raise RuntimeError, 'message'

good - signals a RuntimeError by default

raise 'message'

=== Exception Class Messages

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

[source,ruby]

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.

=== return from ensure

Do not return 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.

[source,ruby]

bad

def foo
raise
ensure
return 'very bad idea'
end

=== Implicit begin

Use implicit begin blocks where possible.

[source,ruby]

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

=== Contingency Methods

Mitigate the proliferation of begin blocks by using contingency methods (a term coined by Avdi Grimm).

[source,ruby]

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 }

=== Suppressing Exceptions

Don't suppress exceptions.

[source,ruby]

bad

begin

an exception occurs here

rescue SomeError

the rescue clause does absolutely nothing

end

bad

do_something rescue nil

=== Using rescue as a Modifier

Avoid using rescue in its modifier form.

[source,ruby]

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 => e
handle_error(e)
end

=== Using Exceptions for Flow of Control

Don't use exceptions for flow of control.

[source,ruby]

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

=== Blind Rescues

Avoid rescuing the Exception class.
This will trap signals and calls to exit, requiring you to kill -9 the process.

[source,ruby]

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

=== Exception Rescuing Ordering

Put more specific exceptions higher up the rescue chain, otherwise they'll never be rescued from.

[source,ruby]

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

Release external resources obtained by your program in an ensure block.

[source,ruby]

f = File.open('testfile')
begin

.. process

rescue

.. handle error

ensure
f.close if f
end

=== Auto-release External Resources

Use versions of resource obtaining methods that do automatic resource cleanup when possible.

[source,ruby]

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

=== Standard Exceptions

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

== Assignment & Comparison

=== Parallel Assignment

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.

[source,ruby]

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]

=== Dealing with Trailing Underscore Variables in Destructuring Assignment

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.

[source,ruby]

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(',')

=== Self-assignment

Use shorthand self assignment operators whenever applicable.

[source,ruby]

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

=== Conditional Variable Initialization Shorthand

Use , = to initialize variables only if they're not already initialized.

[source,ruby]

bad

name = name ? name : 'Bozhidar'

bad

name = 'Bozhidar' unless name

good - set name to 'Bozhidar', only if it's nil or false

name, = 'Bozhidar'

[WARNING]

Don't use , = to initialize boolean variables.
(Consider what would happen if the current value happened to be false.)

[source,ruby]

bad - would set enabled to true even if it was false

enabled, = true

good

enabled = true if enabled.nil?

====

=== Existence Check Shorthand

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 with if.

[source,ruby]

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

=== Explicit Use of the Case Equality Operator

Avoid explicit use of the case equality operator ===.
As its name implies it is meant to be used implicitly by case expressions and outside of them it yields some pretty confusing code.

[source,ruby]

bad

Array === something
(1..100) === 7
/something/ === some_string

good

something.is_a?(Array)
(1..100).include?(7)
some_string.match?(/something/)

=== == vs eql?

Do not use eql? when using == will do.
The stricter comparison semantics provided by eql? are rarely needed in practice.

[source,ruby]

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

== Blocks, Procs & Lambdas

=== Proc Application Shorthand

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

[source,ruby]

bad

names.map {, name, name.upcase }

good

names.map(&:upcase)

=== Single-line Blocks Delimiters

Prefer {...} over do...end for single-line blocks.
Avoid using {...} for multi-line blocks (multi-line chaining is always ugly).
Always use do...end for "control flow" and "method definitions" (e.g. in Rakefiles and certain DSLs).
Avoid do...end when chaining.

[source,ruby]

names = %w[Bozhidar Filipp 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?

=== Explicit Block Argument

Consider using explicit block argument to avoid writing block literal that just passes its arguments to another block.

[source,ruby]

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

=== Trailing Comma in Block Parameters

Avoid comma after the last parameter in a block, except in cases where only a single argument is present and its removal would affect functionality (for instance, array destructuring).

[source,ruby]

bad - easier to move/add/remove parameters, but still not preferred

[[1, 2, 3], [4, 5, 6]].each do, a, b, c,, a + b + c
end

good

[[1, 2, 3], [4, 5, 6]].each do, a, b, c, a + b + c
end

bad

[[1, 2, 3], [4, 5, 6]].each {, a, b, c,, a + b + c }

good

[[1, 2, 3], [4, 5, 6]].each {, a, b, c, a + b + c }

good - this comma is meaningful for array destructuring

[[1, 2, 3], [4, 5, 6]].map {, a,, a }

=== Nested Method Definitions

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.

[source,ruby]

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

=== Multi-line Lambda Definition

Use the new lambda literal syntax for single-line body blocks.
Use the lambda method for multi-line blocks.

[source,ruby]

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

=== Stabby Lambda Definition with Parameters

Don't omit the parameter parentheses when defining a stabby lambda with parameters.

[source,ruby]

bad

l = ->x, y { something(x, y) }

good

l = ->(x, y) { something(x, y) }

=== Stabby Lambda Definition without Parameters

Omit the parameter parentheses when defining a stabby lambda with no parameters.

[source,ruby]

bad

l = ->() { something }

good

l = -> { something }

=== proc vs Proc.new

Prefer proc over Proc.new.

[source,ruby]

bad

p = Proc.new {, n, puts n }

good

p = proc {, n, puts n }

=== Proc Invocation

Prefer proc.call() over proc[] or proc.() for both lambdas and procs.

[source,ruby]

bad - looks similar to Enumeration access

l = ->(v) { puts v }
l[1]

good - most compact form, but might be confusing for newcomers to Ruby

l = ->(v) { puts v }
l.(1)

good - a bit verbose, but crystal clear

l = ->(v) { puts v }
l.call(1)

== Methods

=== Short Methods

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.

=== No Single-line Methods

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.

[source,ruby]

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.

[source,ruby]

good

def no_op; end

=== Double Colons

Use :: only to reference constants (this includes classes and modules) and constructors (like Array() or Nokogiri::HTML()).
Do not use :: for regular method invocation.

[source,ruby]

bad

SomeClass::some_method
some_object::some_method

good

SomeClass.some_method
some_object.some_method
SomeModule::SomeClass::SOME_CONST
SomeModule::SomeClass()

=== Colon Method Definition

Do not use :: to define class methods.

[source,ruby]

bad

class Foo
def self::some_method
end
end

good

class Foo
def self.some_method
end
end

=== Method Definition Parentheses

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

[source,ruby]

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

=== Method Invocation Parentheses

Use parentheses around the arguments of method invocations, especially if the first argument begins with an open parenthesis (, as in f((3 + 2) + 1).

[source,ruby]

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)

==== Method Calls with No Arguments

Always omit parentheses for method calls with no arguments.

[source,ruby]

bad

Kernel.exit!()
2.even?()
fork()
'test'.upcase()

good

Kernel.exit!
2.even?
fork
'test'.upcase

==== Methods That are Part of an Internal DSL

Always omit parentheses for methods that are part of an internal DSL (e.g., Rake, Rails, RSpec):

[source,ruby]

bad

validates(:name, presence: true)

good

validates :name, presence: true

==== Methods That Have "keyword" Status in Ruby

Always omit parentheses for methods that have "keyword" status in Ruby.

[source,ruby]

class Person

bad

attr_reader(:name, :age)

good

attr_reader :name, :age

body omitted

end

==== Non-declarative Methods That Have "keyword" Status in Ruby

Can omit parentheses for methods that have "keyword" status in Ruby, but are not declarative:

[source,ruby]

good

puts(temperance.age)
system('ls')

also good

puts temperance.age
system 'ls'

==== Using super with Arguments

Always use parentheses when calling super with arguments:

[source,ruby]

bad

super name, age

good

super(name, age)

IMPORTANT: When calling super without arguments, super and super() mean different things. Decide what is appropriate for your usage.

=== Too Many Params

Avoid parameter lists longer than three or four parameters.

=== Optional Arguments

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.

[source,ruby]

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'

=== Boolean Keyword Arguments

Use keyword arguments when passing boolean argument to a method.

[source,ruby]

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

=== Keyword Arguments vs Optional Arguments

Prefer keyword arguments over optional arguments.

[source,ruby]

bad

def some_method(a, b = 5, c = 1)

body omitted

end

good

def some_method(a, b: 5, c: 1)

body omitted

end

=== Keyword Arguments vs Option Hashes

Use keyword arguments instead of option hashes.

[source,ruby]

bad

def some_method(options = {})
bar = options.fetch(:bar, false)
puts bar
end

good

def some_method(bar: false)
puts bar
end

=== Private Global Methods

If you really need "global" methods, add them to Kernel and make them private.

== Classes & Modules

=== Consistent Classes

Use a consistent structure in your class definitions.

[source,ruby]

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

=== Mixin Grouping

Split multiple mixins into separate statements.

[source,ruby]

bad

class Person
include Foo, Bar
end

good

class Person

multiple mixins go in separate statements

include Foo
include Bar
end

=== Single-line Classes

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

[source,ruby]

bad

class FooError < StandardError
end

good

class FooError < StandardError; end

okish

FooError = Class.new(StandardError)

NOTE: Many editors/tools will fail to understand properly the usage of Class.new,
so in general it's better to stick to the alternative one-line style.

=== File Classes

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.

[source,ruby]

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

=== Namespace Definition

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 https://cirw.in/blog/constant-lookup.html[lexical scoping], which depends on the module nesting at the point of definition.

[source,ruby]

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

=== Modules vs Classes

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

[source,ruby]

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

=== module_function

Prefer the use of module_function over extend self when you want to turn a module's instance methods into class methods.

[source,ruby]

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

=== Liskov

When designing class hierarchies make sure that they conform to the https://en.wikipedia.org/wiki/Liskov_substitution_principle[Liskov Substitution Principle].

=== SOLID design

Try to make your classes as https://en.wikipedia.org/wiki/SOLID[SOLID] as possible.

=== Define to_s

Always supply a proper to_s method for classes that represent domain objects.

[source,ruby]

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

=== attr Family

Use the attr family of functions to define trivial accessors or mutators.

[source,ruby]

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

=== Accessor/Mutator Method Names

For accessors and mutators, avoid prefixing method names with get_ and set_.
It is a Ruby convention to use attribute names for accessors (readers) and attr_name= for mutators (writers).

[source,ruby]

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

=== attr

Avoid the use of attr.
Use attr_reader and attr_accessor instead.

[source,ruby]

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

=== Struct.new

Consider using Struct.new, which defines the trivial accessors, constructor and comparison operators for you.

[source,ruby]

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

=== No Extend Struct.new

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.

[source,ruby]

bad

class Person < Struct.new(:first_name, :last_name)
end

good

Person = Struct.new(:first_name, :last_name)

=== Duck Typing

Prefer https://en.wikipedia.org/wiki/Duck_typing[duck-typing] over inheritance.

[source,ruby]

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

=== No Class Vars

Avoid the usage of class (@@) variables due to their "nasty" behavior in inheritance.

[source,ruby]

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.

=== Visibility

Assign proper visibility levels to methods (private, protected) in accordance with their intended usage.
Don't go off leaving everything public (which is the default).
After all we're coding in Ruby now, not in Python.

=== Indent public/private/protected

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 in order to emphasize that it applies to all methods below it.

[source,ruby]

good

class SomeClass
def public_method
# some code
end

private

def private_method
# some code
end

def another_private_method
# some code
end
end

=== def self Class Methods

Use def self.method to define class methods.
This makes the code easier to refactor since the class name is not repeated.

[source,ruby]

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

=== Alias Method Lexically

Prefer alias when aliasing methods in lexical class scope as the resolution of self 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.

[source,ruby]

class Westerner
def first_name
@names.first
end

alias given_name first_name
end

Since alias, like def, is a keyword, prefer bareword arguments over symbols or strings.
In other words, do alias foo bar, not alias :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.

[source,ruby]

class Fugitive < Westerner
def first_name
'Nobody'
end
end

In this example, Fugitive#given_name would still call the original Westerner#first_name method, not Fugitive#first_name.
To override the behavior of Fugitive#given_name as well, you'd have to redefine it in the derived class.

[source,ruby]

class Fugitive < Westerner
def first_name
'Nobody'
end

alias given_name first_name
end

=== alias_method

Always use alias_method when aliasing methods of modules, classes, or singleton classes at runtime, as the lexical scope of alias leads to unpredictability in these cases.

[source,ruby]

module Mononymous
def self.included(other)
other.class_eval { alias_method :full_name, :given_name }
end
end

class Sting < Westerner
include Mononymous
end

=== Class and self

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.

[source,ruby]

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

== Classes: Constructors

=== Factory Methods

Consider adding factory methods to provide additional sensible ways to create instances of a particular class.

[source,ruby]

class Person
def self.create(options_hash)
# body omitted
end
end

=== Disjunctive Assignment in Constructor

In constructors, avoid unnecessary disjunctive assignment (, =) of instance variables.
Prefer plain assignment.
In ruby, instance variables (beginning with an @) are nil until assigned a value, so in most cases the disjunction is unnecessary.

[source,ruby]

bad

def initialize
@x, = 1
end

good

def initialize
@x = 1
end

== Comments

[quote, Steve McConnell]

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.

=== No Comments

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

=== Rationale Comments

If the how can be made self-documenting, but not the why (e.g. the code works around non-obvious library behavior, or implements an algorithm from an academic paper), add a comment explaining the rationale behind the code.

[source,ruby]

bad

x = BuggyClass.something.dup

def compute_dependency_graph
...30 lines of recursive graph merging...
end

good

BuggyClass returns an internal object, so we have to dup it to modify it.

x = BuggyClass.something.dup

This is algorithm 6.4(a) from Worf & Yar's Amazing Graph Algorithms (2243).

def compute_dependency_graph
...30 lines of recursive graph merging...
end

=== English Comments

Write comments in English.

=== Hash Space

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

=== English Syntax

Comments longer than a word are capitalized and use punctuation.
Use https://en.wikipedia.org/wiki/Sentence_spacing[one space] after periods.

=== No Superfluous Comments

Avoid superfluous comments.

[source,ruby]

bad

counter += 1 # Increments counter by one.

=== Comment Upkeep

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

=== Refactor, Don't Comment

[quote, old programmers maxim, through http://eloquentruby.com/blog/2011/03/07/good-code-and-good-jokes/[Russ Olsen]]

Good code is like a good joke: it needs no explanation.

Avoid writing comments to explain bad code.
Refactor the code to make it self-explanatory.
("Do or do not - there is no try." Yoda)

== Comment Annotations

=== Annotations Placement

Annotations should usually be written on the line immediately above the relevant code.

[source,ruby]

bad

def bar
baz(:quux) # FIXME: This has crashed occasionally since v3.2.1.
end

good

def bar

FIXME: This has crashed occasionally since v3.2.1.

baz(:quux)
end

=== Annotations Keyword Format

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

[source,ruby]

bad

def bar

FIXME This has crashed occasionally since v3.2.1.

baz(:quux)
end

good

def bar

FIXME: This has crashed occasionally since v3.2.1.

baz(:quux)
end

=== Multi-line Annotations Indentation

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).

[source,ruby]

def bar

FIXME: This has crashed occasionally since v3.2.1. It may

be related to the BarBazUtil upgrade.

baz(:quux)
end

=== Inline Annotations

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.

[source,ruby]

def bar
sleep 100 # OPTIMIZE
end

=== TODO

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

=== FIXME

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

=== OPTIMIZE

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

=== HACK

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

=== REVIEW

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?

=== Document Annotations

Use other custom annotation keywords if it feels appropriate, but be sure to document them in your project's README or similar.

== Magic Comments

=== Magic Comments First

Place magic comments above all code and documentation in a file (except shebangs, which are discussed next).

[source,ruby]

bad

Some documentation about Person

frozen_string_literal: true

class Person
end

good

frozen_string_literal: true

Some documentation about Person

class Person
end

=== Below Shebang

Place magic comments below shebangs when they are present in a file.

[source,ruby]

bad

frozen_string_literal: true

#!/usr/bin/env ruby

App.parse(ARGV)

good

#!/usr/bin/env ruby

frozen_string_literal: true

App.parse(ARGV)

=== One Magic Comment per Line

Use one magic comment per line if you need multiple.

[source,ruby]

bad

-- frozen_string_literal: true; encoding: ascii-8bit --

good

frozen_string_literal: true

encoding: ascii-8bit

=== Separate Magic Comments from Code

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

[source,ruby]

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

== Collections

=== Literal Array and Hash

Prefer literal array and hash creation notation (unless you need to pass parameters to their constructors, that is).

[source,ruby]

bad

arr = Array.new
hash = Hash.new

good

arr = []
arr = Array.new(10)
hash = {}
hash = Hash.new(0)

=== %w

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.

[source,ruby]

bad

STATES = ['draft', 'open', 'closed']

good

STATES = %w[draft open closed]

=== %i

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.

[source,ruby]

bad

STATES = [:draft, :open, :closed]

good

STATES = %i[draft open closed]

=== No Trailing Array Commas

Avoid comma after the last item of an Array or Hash literal, especially when the items are not on separate lines.

[source,ruby]

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]

=== No Gappy Arrays

Avoid the creation of huge gaps in arrays.

[source,ruby]

arr = []
arr[100] = 1 # now you have an array with lots of nils

=== first and last

When accessing the first or last element from an array, prefer first or last over [0] or [-1].

=== Set vs Array

Use Set instead of Array when dealing with unique elements.
Set implements a collection of unordered values with no duplicates.
This is a hybrid of Array's intuitive inter-operation facilities and Hash's fast lookup.

=== Symbols as Keys

Prefer symbols instead of strings as hash keys.

[source,ruby]

bad

hash = { 'one' => 1, 'two' => 2, 'three' => 3 }

good

hash = { one: 1, two: 2, three: 3 }

=== No Mutable Keys

Avoid the use of mutable objects as hash keys.

=== Hash Literals

Use the Ruby 1.9 hash literal syntax when your hash keys are symbols.

[source,ruby]

bad

hash = { :one => 1, :two => 2, :three => 3 }

good

hash = { one: 1, two: 2, three: 3 }

=== No Mixed Hash Syntaces

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.

[source,ruby]

bad

{ a: 1, 'b' => 2 }

good

{ :a => 1, 'b' => 2 }

=== Hash#key?

Use Hash#key? instead of Hash#has_key? and Hash#value? instead of Hash#has_value?.

[source,ruby]

bad

hash.has_key?(:test)
hash.has_value?(value)

good

hash.key?(:test)
hash.value?(value)

=== Hash#each

Use Hash#each_key instead of Hash#keys.each and Hash#each_value instead of Hash#values.each.

[source,ruby]

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 }

=== Hash#fetch

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

[source,ruby]

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)

=== Hash#fetch defaults

Introduce default values for hash keys via Hash#fetch as opposed to using custom logic.

[source,ruby]

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

=== Use Hash Blocks

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.

[source,ruby]

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 }

=== Hash#values_at

Use Hash#values_at when you need to retrieve several values consecutively from a hash.

[source,ruby]

bad

email = data['email']
username = data['nickname']

good

email, username = data.values_at('email', 'nickname')

=== Hash#transform_keys and Hash#transform_values

Prefer transform_keys or transform_values over each_with_object or map when transforming just the keys or just the values of a hash.

[source,ruby]

bad

{a: 1, b: 2}.each_with_object({}) {, (k, v), h, h[k] = v * v }
{a: 1, b: 2}.map {, k, v, [k.to_s, v] }.to_h

good

{a: 1, b: 2}.transform_values {, v, v * v }
{a: 1, b: 2}.transform_keys {, k, k.to_s }

=== Ordered Hashes

Rely on the fact that as of Ruby 1.9 hashes are ordered.

=== No Modifying Collections

Do not modify a collection while traversing it.

=== Accessing Elements Directly

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 [] on nil.

[source,ruby]

bad

Regexp.last_match[1]

good

Regexp.last_match(1)

=== Provide Alternate Accessor to Collections

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.

[source,ruby]

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

=== map/find/select/reduce/size

Prefer map over collect, find over detect, select over find_all, reduce over inject and size over length.
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 of select is encouraged over find_all is that it goes together nicely with reject and its name is pretty self-explanatory.

=== count vs size

Don't use count as a substitute for size.
For Enumerable objects other than Array it will iterate the entire collection in order to determine its size.

[source,ruby]

bad

some_hash.count

good

some_hash.size

=== flat_map

Use flat_map instead of map + flatten.
This does not apply for arrays with a depth greater than 2, i.e. if users.first.songs == ['a', ['b','c']], then use map + flatten rather than flat_map.
flat_map flattens the array by 1, whereas flatten flattens it all the way.

[source,ruby]

bad

all_songs = users.map(&:songs).flatten.uniq

good

all_songs = users.flat_map(&:songs).uniq

=== reverse_each

Prefer reverse_each to reverse.each because some classes that include Enumerable will provide an efficient implementation.
Even in the worst case where a class does not provide a specialized implementation, the general implementation inherited from Enumerable will be at least as efficient as using reverse.each.

[source,ruby]

bad

array.reverse.each { ... }

good

array.reverse_each { ... }

== Numbers

=== Underscores in Numerics

Add underscores to large numeric literals to improve their readability.

[source,ruby]

bad - how many 0s are there?

num = 1000000

good - much easier to parse for the human brain

num = 1_000_000

=== Numeric Literal Prefixes

Prefer lowercase letters for numeric literal prefixes.
0o for octal, 0x for hexadecimal and 0b for binary.
Do not use 0d prefix for decimal literals.

[source,ruby]

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

=== Integer Type Checking

Use Integer to check type of an integer number.
Since Fixnum is platform-dependent, checking against it will return different results on 32-bit and 64-bit machines.

[source,ruby]

timestamp = Time.now.to_i

bad

timestamp.is_a?(Fixnum)
timestamp.is_a?(Bignum)

good

timestamp.is_a?(Integer)

=== Random Numbers

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:

[source,ruby]

bad

rand(6) + 1

good

rand(1..6)

=== Float Division

When performing float-division on two integers, either use fdiv or convert one-side integer to float.

[source,ruby]

bad

a.to_f / b.to_f

good

a.to_f / b
a / b.to_f
a.fdiv(b)

== Strings

=== String Interpolation

Prefer string interpolation and string formatting instead of string concatenation:

[source,ruby]

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)

=== Consistent String Literals

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 and double quotes by default.

NOTE: The string literals in this guide are using single quotes by default.

==== Single Quote

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

[source,ruby]

bad

name = "Bozhidar"

name = 'De'Andre'

good

name = 'Bozhidar'

name = "De'Andre"

==== Double Quote

Prefer double-quotes unless your string literal contains " or escape characters you want to suppress.

[source,ruby]

bad

name = 'Bozhidar'

sarcasm = "I "like" it."

good

name = "Bozhidar"

sarcasm = 'I "like" it.'

=== No Character Literals

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).

[source,ruby]

bad

char = ?c

good

char = 'c'

=== Curlies Interpolate

Don't leave out {} around instance and global variables being interpolated into a string.

[source,ruby]

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}"

=== No to_s

Don't use Object#to_s on interpolated objects.
It's invoked on them automatically.

[source,ruby]

bad

message = "This is the #{result.to_s}."

good

message = "This is the #{result}."

=== String Concatenation

Avoid using String#+ when you need to construct large data chunks.
Instead, use String#<<.
Concatenation mutates the string instance in-place and is always faster than String#+, which creates a bunch of new string objects.

[source,ruby]

bad

html = ''
html += 'Page title'

paragraphs.each do, paragraph, html += "#{paragraph}"
end

good and also fast

html = ''
html << 'Page title'

paragraphs.each do, paragraph, html << "#{paragraph}"
end

=== Don't Abuse gsub

Don't use String#gsub in scenarios in which you can use a faster and more specialized alternative.

[source,ruby]

url = 'http://example.com'
str = 'lisp-case-rules'

bad

url.gsub('http://', 'https://')
str.gsub('-', '_')

good

url.sub('http://', 'https://')
str.tr('-', '_')

=== sprintf

Prefer the use of sprintf and its alias format over the fairly cryptic String#% method.

[source,ruby]

bad

'%d %d' % [20, 10]

=> '20 10'

good

sprintf('%d %d', 20, 10)

=> '20 10'

good

sprintf('%d %d', first: 20, second: 10)

=> '20 10'

format('%d %d', 20, 10)

=> '20 10'

good

format('%d %d', first: 20, second: 10)

=> '20 10'

=== Named Format Tokens

When using named format string tokens, favor %<name>s over %{name} because it encodes information about the type of the value.

[source,ruby]

bad

format('Hello, %{name}', name: 'John')

good

format('Hello, %s', name: 'John')

=== Long Strings

Break long strings into multiple lines but don't concatenate them with +.
If you want to add newlines, use heredoc. Otherwise use \:

[source,ruby]

bad

"Lorem Ipsum is simply dummy text of the printing and typesetting industry. " +
"Lorem Ipsum has been the industry's standard dummy text ever since the 1500s, " +
"when an unknown printer took a galley of type and scrambled it to make a type specimen book."

good

<<~LOREM
Lorem Ipsum is simply dummy text of the printing and typesetting industry.
Lorem Ipsum has been the industry's standard dummy text ever since the 1500s,
when an unknown printer took a galley of type and scrambled it to make a type specimen book.
LOREM

good

"Lorem Ipsum is simply dummy text of the printing and typesetting industry. "
"Lorem Ipsum has been the industry's standard dummy text ever since the 1500s, "
"when an unknown printer took a galley of type and scrambled it to make a type specimen book."

== Heredocs

=== Squiggly Heredocs

Use Ruby 2.3's squiggly heredocs for nicely indented multi-line strings.

[source,ruby]

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

=== Heredoc Delimiters

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.

[source,ruby]

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

=== Heredoc Method Calls

Place method calls with heredoc receivers on the first line of the heredoc definition.
The bad form has significant potential for error if a new line is added or removed.

[source,ruby]

bad

query = <<~SQL
select foo from bar
SQL
.strip_indent

good

query = <<~SQL.strip_indent
select foo from bar
SQL

=== Heredoc Argument Closing Parentheses

Place the closing parenthesis for method calls with heredoc arguments on the first line of the heredoc definition.
The bad form has potential for error if the new line before the closing parenthesis is removed.

[source,ruby]

bad

foo(<<~SQL
select foo from bar
SQL
)

good

foo(<<~SQL)
select foo from bar
SQL

== Date & Time

=== Time.now

Prefer Time.now over Time.new when retrieving the current system time.

=== No DateTime

Don't use DateTime unless you need to account for historical calendar reform - and if you do, explicitly specify the start argument to clearly state your intentions.

[source,ruby]

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

[quote, Jamie Zawinski]

Some people, when confronted with a problem, think
"I know, I'll use regular expressions." Now they have two problems.

=== No Regexp for Plaintext

Don't use regular expressions if you just need plain text search in string: string['text']

=== Regexp String Index

For simple constructions you can use regexp directly through string index.

[source,ruby]

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'

=== Non Capturing Regexp

Use non-capturing groups when you don't use the captured result.

[source,ruby]

bad

/(first, second)/

good

/(?:first, second)/

=== No Perl Regexp Last Matchers

Don't use the cryptic Perl-legacy variables denoting last regexp group matches ($1, $2, etc).
Use Regexp.last_match(n) instead.

[source,ruby]

/(regexp)/ =~ string
...

bad

process $1

good

process Regexp.last_match(1)

=== No Numbered Regexes

Avoid using numbered groups as it can be hard to track what they contain.
Named groups can be used instead.

[source,ruby]

bad

/(regexp)/ =~ string

some code

process Regexp.last_match(1)

good

/(?<meaningful_var>regexp)/ =~ string

some code

process meaningful_var

=== Limit Escapes

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

=== Caret and Dollar Regexp

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/).

[source,ruby]

string = "some injection\nusername"
string[/^username$/] # matches
string[/\Ausername\z/] # doesn't match

=== Comment Regexes

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.

[source,ruby]

regexp = /
start # some text
\s # white space char
(group) # first group
(?:alt1, alt2) # some alternation
end
/x

=== gsub Blocks

For complex replacements sub/gsub can be used with a block or a hash.

[source,ruby]

words = 'foo bar'
words.sub(/f/, 'f' => 'F') # => 'Foo bar'
words.gsub(/\w+/) {, word, word.capitalize } # => 'Foo Bar'

== Percent Literals

=== %q shorthand

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.

[source,ruby]

bad (no interpolation needed)

%(Some text)

should be 'Some text'

bad (no double-quotes)

%(This is #{quality} style)

should be "This is #{quality} style"

bad (multiple lines)

%(\n#{exclamation}\n)

should be a heredoc.

good (requires interpolation, has quotes, single line)

%(#{name})

=== %q

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.

[source,ruby]

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("What did you say?")

=== %r

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

[source,ruby]

bad

%r{\s+}

good

%r{^/(.)$}
%r{^/blog/2011/(.)$}

=== %x

Avoid the use of %x unless you're going to invoke a command with backquotes in it (which is rather unlikely).

[source,ruby]

bad

date = %x(date)

good

date = date
echo = %x(echo date)

=== %s

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.

=== Percent Literal Braces

Use the braces that are the most appropriate for the various kinds of percent literals.

• () 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)

[source,ruby]

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

=== No Needless Metaprogramming

Avoid needless metaprogramming.

=== No Monkey Patching

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

=== Block class_eval

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

==== Supply Location

When you use the string-interpolated form, always supply +__FILE__+ and +__LINE__+, so that your backtraces make sense:

[source,ruby]

class_eval 'def use_relative_model_naming?; true; end', FILE, LINE

==== define_method

define_method is preferable to class_eval { def ... }

=== eval Comment Docs

When using class_eval (or other eval) with string interpolation, add a comment block showing its appearance if interpolated (a practice used in Rails code):

[source,ruby]

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

=== No method_missing

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.luanch_state = false.
Consider using delegation, proxy, or define_method instead.
If you must use method_missing:

• Be sure to http://blog.marc-andre.ca/2010/11/methodmissing-politely.html[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:

[source,ruby]

bad

def method_missing(meth, params, &block)
if /^find_by_(?.)/ =~ meth
# ... lots of code to do a find_by
else
super
end
end

good

def method_missing(meth, params, &block)
if /^find_by_(?.)/ =~ 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

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

[source,ruby]

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__+

Prefer +__send__+ over send, as send may overlap with existing methods.

[source,ruby]

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 ...

== API Documentation

=== YARD

Use https://yardoc.org/[YARD] and its conventions for API documentation.

=== RD (Block) Comments

Don't use block comments.
They cannot be preceded by whitespace and are not as easy to spot as regular comments.

[source,ruby]

bad

=begin
comment line
another comment line
=end

good

comment line

another comment line

.From Perl's POD to RD

This is not a really a block comment syntax, but more of
an attempt to emulate Perl's https://perldoc.perl.org/perlpod.html[POD] documentation system.

There's an https://github.com/uwabami/rdtool[rdtool] for Ruby that's pretty similar to POD.
Basically rdtool scans a file for =begin and =end{=begin...=end@{=begin pairs, and
extracts the text between them all. This text is assumed to be documentation in
https://github.com/uwabami/rdtool/blob/master/doc/rd-draft.rd[RD format].
You can read more about it
https://ruby-doc.com/docs/ProgrammingRuby/html/rdtool.html[here].

RD predated the rise of RDoc and YARD and was effectively obsoleted by them.footnote:[According to this https://en.wikipedia.org/wiki/Ruby_Document_format[Wikipedia article] the format used to be popular until the early 2000s when it was superseded by RDoc.]

== Gemfile and Gemspec

=== No RUBY_VERSION in the gemspec

The gemspec should not contain RUBY_VERSION as a condition to switch dependencies.
RUBY_VERSION is determined by rake release, so users may end up with wrong dependency.

[source,ruby]

bad

Gem::Specification.new do, s, if RUBY_VERSION >= '2.5'
s.add_runtime_dependency 'gem_a'
else
s.add_runtime_dependency 'gem_b'
end
end

Fix by either:

• Post-install messages.
• Add both gems as dependency (if permissible).
• If development dependencies, move to Gemfile.

== Misc

=== No Flip-flops

Avoid the use of flip-flops.

NOTE: They are deprecated as of Ruby 2.6.

=== No non-nil Checks

Don't do explicit non-nil checks unless you're dealing with boolean values.

[source,ruby]

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

=== Global stdout

Use $stdout/$stderr/$stdin instead of STDOUT/STDERR/STDIN.
STDOUT/STDERR/STDIN are constants, and while you can actually reassign (possibly to redirect some stream) constants in Ruby, you'll get an interpreter warning if you do so.

=== Warn

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).

=== Array#join

Prefer the use of Array#join over the fairly cryptic Array#* with a string argument.

[source,ruby]

bad

%w[one two three] * ', '

=> 'one, two, three'

good

%w[one two three].join(', ')

=> 'one, two, three'

=== Array Coercion

Use Array() instead of explicit Array check or [*var], when dealing with a variable you want to treat as an Array, but you're not certain it's an array.

[source,ruby]

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) }

=== Ranges or between

Use ranges or Comparable#between? instead of complex comparison logic when possible.

[source,ruby]

bad

do_something if x >= 1000 && x <= 2000

good

do_something if (1000..2000).include?(x)

good

do_something if x.between?(1000, 2000)

=== Predicate Methods

Prefer the use of predicate methods to explicit comparisons with ==.
Numeric comparisons are OK.

[source,ruby]

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

=== No Cryptic Perlisms

Avoid using Perl-style special variables (like $:, $;, etc).
They are quite cryptic and their use in anything but one-liner scripts is discouraged.

[source,ruby]

bad

$:.unshift File.dirname(FILE)

good

$LOAD_PATH.unshift File.dirname(FILE)

Use the human-friendly aliases provided by the English library if required.

[source,ruby]

bad

print $', $$

good

require 'English'
print $POSTMATCH, $PID

=== Always Warn

Write ruby -w safe code.

=== No Optional Hash Params

Avoid hashes as optional parameters.
Does the method do too much? (Object initializers are exceptions for this rule).

=== Instance Vars

Use module instance variables instead of global variables.

[source,ruby]

bad

$foo_bar = 1

good

module Foo
class << self
attr_accessor :bar
end
end

Foo.bar = 1

=== OptionParser

Use OptionParser for parsing complex command line options and ruby -s for trivial command line options.

=== No Param Mutations

Do not mutate parameters unless that is the purpose of the method.

=== Three is the Number Thou Shalt Count

Avoid more than three levels of block nesting.

=== Functional Code

Code in a functional way, avoiding mutation when that makes sense.

[source,ruby]

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

== Tools

Here are some tools to help you automatically check Ruby code against this guide.

=== RuboCop

https://github.com/rubocop-hq/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 has https://docs.rubocop.org/en/stable/integration_with_other_tools/[plugins] for most popular Ruby editors and IDEs.

TIP: RuboCop's cops (code checks) have links to the guidelines that they are based on, as part of their metadata.

=== RubyMine

http://www.jetbrains.com/ruby/[RubyMine]'s code inspections are http://confluence.jetbrains.com/display/RUBYDEV/RubyMine+Inspections[partially based] on this guide.

== History

This guide started its life as an internal company Ruby coding guidelines (written by https://github.com/bbatsov[Bozhidar Batsov]).
Bozhidar had always been bothered as a Ruby developer about one thing - Python developers had a great programming style reference (https://www.python.org/dev/peps/pep-0008/[PEP-8]) and Rubyists never got an official guide, documenting Ruby coding style and best practices.
Bozhidar firmly believed that style matters.
He also believed that a great hacker community, such as Ruby has, should be quite capable of producing this coveted document.
The rest is history...

At some point Bozhidar decided that the work he 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.

Bozhidar served as the guide's only editor for a few years, before a team of editors was formed once the project transitioned to RuboCop HQ.

Since the inception of the guide we'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.

== Contributing

The guide is still a work in progress - some guidelines are lacking examples, some guidelines don't have examples that illustrate them clearly enough.
Improving such guidelines 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 our 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 https://www.patreon.com/bbatsov[Patreon].

=== How to Contribute?

It's easy, just follow the contribution guidelines below:

• https://help.github.com/articles/fork-a-repo[Fork] https://github.com/rubocop-hq/ruby-style-guide[rubocop-hq/ruby-style-guide] on GitHub
• Make your feature addition or bug fix in a feature branch.
• Include a http://tbaggery.com/2008/04/19/a-note-about-git-commit-messages.html[good description] of your changes
• Push your feature branch to GitHub
• Send a https://help.github.com/articles/using-pull-requests[Pull Request]

== Colophon

This guide is written in http://asciidoc.org/[AsciiDoc] and is published as HTML using https://asciidoctor.org/[AsciiDoctor].
The HTML version of the guide is hosted on GitHub Pages.

Originally the guide was written in Markdown, but was converted to AsciiDoc in 2019.

== License

image:https://i.creativecommons.org/l/by/3.0/88x31.png[Creative Commons License] This work is licensed under a http://creativecommons.org/licenses/by/3.0/deed.en_US[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?