Prelude

Role models are important.
-- Officer Alex J. Murphy / RoboCop

One thing has always bothered me as a Ruby developer—Python developers have a great programming style reference (PEP-8) and we never got an official guide, documenting Ruby coding style and best practices. And I do believe that style matters. I also believe that a great hacker community, such as Ruby has, should be quite capable of producing this coveted document.

This guide started its life as our internal company Ruby coding guidelines (written by yours truly). At some point I decided that the work I was doing might be interesting to members of the Ruby community in general and that the world had little need for another internal company guideline. But the world could certainly benefit from a community-driven and community-sanctioned set of practices, idioms and style prescriptions for Ruby programming.

Since the inception of the guide I've received a lot of feedback from members of the exceptional Ruby community around the world. Thanks for all the suggestions and the support! Together we can make a resource beneficial to each and every Ruby developer out there.

By the way, if you're into Rails you might want to check out the complementary Ruby on Rails Style Guide.

The Ruby Style Guide

This Ruby style guide recommends best practices so that real-world Ruby programmers can write code that can be maintained by other real-world Ruby programmers. A style guide that reflects real-world usage gets used, while a style guide that holds to an ideal that has been rejected by the people it is supposed to help risks not getting used at all—no matter how good it is.

The guide is separated into several sections of related rules. I've tried to add the rationale behind the rules (if it's omitted I've assumed it's pretty obvious).

I didn't come up with all the rules out of nowhere—they are mostly based on my extensive career as a professional software engineer, feedback and suggestions from members of the Ruby community and various highly regarded Ruby programming resources, such as "Programming Ruby" and "The Ruby Programming Language".

There are some areas in which there is no clear consensus in the Ruby community regarding a particular style (like string literal quoting, spacing inside hash literals, dot position in multi-line method chaining, etc.). In such scenarios all popular styles are acknowledged and it's up to you to pick one and apply it consistently.

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

Many projects have their own coding style guidelines (often derived from this guide). In the event of any conflicts, such project-specific guides take precedence for that project.

You can generate a PDF or an HTML copy of this guide using Pandoc.

RuboCop is a code analyzer, based on this style guide.

Translations of the guide are available in the following languages:

• Chinese Simplified
• Chinese Traditional
• French
• German
• Japanese
• Korean
• Portuguese
• Russian
• Spanish
• Vietnamese

Table of Contents

• Source Code Layout
• Syntax
• Naming
• Comments
  • Comment Annotations
• Classes & Modules
• Exceptions
• Collections
• Numbers
• Strings
• Date & Time
• Regular Expressions
• Percent Literals
• Metaprogramming
• Misc
• Tools

Source Code Layout

Nearly everybody is convinced that every style but their own is ugly and unreadable. Leave out the "but their own" and they're probably right...
-- Jerry Coffin (on indentation)

• Use UTF-8 as the source file encoding. ^[link]

• Use two spaces per indentation level (aka soft tabs). No hard tabs. ^[link]

  # bad - four spacesdefsome_methoddo_somethingend# gooddefsome_methoddo_somethingend

• Use Unix-style line endings. (*BSD/Solaris/Linux/OS X users are covered by default, Windows users have to be extra careful.) ^[link]

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

    $ git config --global core.autocrlf true

• Don't use ; to separate statements and expressions. As a corollary—use one expression per line. ^[link]

  # badputs'foobar';# superfluous semicolonputs'foo';puts'bar'# two expressions on the same line# goodputs'foobar'puts'foo'puts'bar'puts'foo','bar'# this applies to puts in particular

• Prefer a single-line format for class definitions with no body. ^[link]

  # badclassFooError < StandardErrorend# okishclassFooError < StandardError;end# goodFooError=Class.new(StandardError)

• Avoid single-line methods. Although they are somewhat popular in the wild, there are a few peculiarities about their definition syntax that make their use undesirable. At any rate—there should be no more than one expression in a single-line method. ^[link]

  # baddeftoo_much;something;something_else;end# okish - notice that the first ; is requireddefno_braces_method;bodyend# okish - notice that the second ; is optionaldefno_braces_method;body;end# okish - valid syntax, but no ; makes it kind of hard to readdefsome_method()bodyend# gooddefsome_methodbodyend

  One exception to the rule are empty-body methods.

  # gooddefno_op;end

• Use spaces around operators, after commas, colons and semicolons. Whitespace might be (mostly) irrelevant to the Ruby interpreter, but its proper use is the key to writing easily readable code. ^[link]

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

  The only exception, regarding operators, is the exponent operator:

  # bade=M * c ** 2# goode=M * c**2

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

  # badsome(arg).other[1,2,3].each{|e| putse}# goodsome(arg).other[1,2,3].each{ |e| putse}

  { and } deserve a bit of clarification, since they are used for block and hash literals, as well as string interpolation.

  For hash literals two styles are considered acceptable. The first variant is slightly more readable (and arguably more popular in the Ruby community in general). The second variant has the advantage of adding visual difference between block and hash literals. Whichever one you pick—apply it consistently.

  # good - space after { and before }{one: 1,two: 2}# good - no space after { and before }{one: 1,two: 2}

  With interpolated expressions, there should be no padded-spacing inside the braces.

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

• No space after !. ^[link]

  # bad
  ! something# good
  !something

• No space inside range literals. ^[link]

  # bad1 .. 3'a' ... 'z'# good1..3'a'...'z'

• Indent when as deep as case. This is the style established in both "The Ruby Programming Language" and "Programming Ruby". ^[link]

  # badcasewhensong.name == 'Misty'puts'Not again!'whensong.duration > 120puts'Too long!'whenTime.now.hour > 21puts"It's too late"elsesong.playend# goodcasewhensong.name == 'Misty'puts'Not again!'whensong.duration > 120puts'Too long!'whenTime.now.hour > 21puts"It's too late"elsesong.playend

• When assigning the result of a conditional expression to a variable, preserve the usual alignment of its branches. ^[link]

  # bad - pretty convolutedkind=caseyearwhen1850..1889then'Blues'when1890..1909then'Ragtime'when1910..1929then'New Orleans Jazz'when1930..1939then'Swing'when1940..1950then'Bebop'else'Jazz'endresult=ifsome_condcalc_somethingelsecalc_something_elseend# good - it's apparent what's going onkind=caseyearwhen1850..1889then'Blues'when1890..1909then'Ragtime'when1910..1929then'New Orleans Jazz'when1930..1939then'Swing'when1940..1950then'Bebop'else'Jazz'endresult=ifsome_condcalc_somethingelsecalc_something_elseend# good (and a bit more width efficient)kind=caseyearwhen1850..1889then'Blues'when1890..1909then'Ragtime'when1910..1929then'New Orleans Jazz'when1930..1939then'Swing'when1940..1950then'Bebop'else'Jazz'endresult=ifsome_condcalc_somethingelsecalc_something_elseend

• Use empty lines between method definitions and also to break up methods into logical paragraphs internally. ^[link]

  defsome_methoddata=initialize(options)data.manipulate!data.resultenddefsome_methodresultend

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

  # bad - easier to move/add/remove parameters, but still not preferredsome_method(size,count,color,)# badsome_method(size,count,color,)# goodsome_method(size,count,color)

• Use spaces around the = operator when assigning default values to method parameters: ^[link]

  # baddefsome_method(arg1=:default,arg2=nil,arg3=[])# do something...end# gooddefsome_method(arg1=:default,arg2=nil,arg3=[])# do something...end

  While several Ruby books suggest the first style, the second is much more prominent in practice (and arguably a bit more readable).

• Avoid line continuation \ where not required. In practice, avoid using line continuations for anything but string concatenation. ^[link]

  # badresult=1 - \
  2# good (but still ugly as hell)result=1 \
  - 2long_string='First part of the long string' \
  ' and second part of the long string'

• Adopt a consistent multi-line method chaining style. There are two popular styles in the Ruby community, both of which are considered good—leading . (Option A) and trailing . (Option B). ^[link]

  • (Option A) When continuing a chained method invocation on another line keep the . on the second line.

    # bad - need to consult first line to understand second lineone.two.three.four# good - it's immediately clear what's going on the second lineone.two.three.four

  • (Option B) When continuing a chained method invocation on another line, include the . on the first line to indicate that the expression continues.

    # bad - need to read ahead to the second line to know that the chain continuesone.two.three.four# good - it's immediately clear that the expression continues beyond the first lineone.two.three.four

  A discussion on the merits of both alternative styles can be found here.

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

  # starting point (line is too long)defsend_mail(source)Mailer.deliver(to: 'bob@example.com',from: 'us@example.com',subject: 'Important message',body: source.text)end# bad (double indent)defsend_mail(source)Mailer.deliver(to: 'bob@example.com',from: 'us@example.com',subject: 'Important message',body: source.text)end# gooddefsend_mail(source)Mailer.deliver(to: 'bob@example.com',from: 'us@example.com',subject: 'Important message',body: source.text)end# good (normal indent)defsend_mail(source)Mailer.deliver(to: 'bob@example.com',from: 'us@example.com',subject: 'Important message',body: source.text)end

• Align the elements of array literals spanning multiple lines. ^[link]

  # bad - single indentmenu_item=['Spam','Spam','Spam','Spam','Spam','Spam','Spam','Spam','Baked beans','Spam','Spam','Spam','Spam','Spam']# goodmenu_item=['Spam','Spam','Spam','Spam','Spam','Spam','Spam','Spam','Baked beans','Spam','Spam','Spam','Spam','Spam']# goodmenu_item=['Spam','Spam','Spam','Spam','Spam','Spam','Spam','Spam','Baked beans','Spam','Spam','Spam','Spam','Spam']

• Add underscores to large numeric literals to improve their readability. ^[link]

  # bad - how many 0s are there?num=1000000# good - much easier to parse for the human brainnum=1_000_000

• Prefer smallcase letters for numeric literal prefixes. 0o for octal, 0x for hexadecimal and 0b for binary. Do not use 0d prefix for decimal literals. ^[link]

  # badnum=01234num=0O1234num=0X12ABnum=0B10101num=0D1234num=0d1234# good - easier to separate digits from the prefixnum=0o1234num=0x12ABnum=0b10101num=1234

• Use Rdoc and its conventions for API documentation. Don't put an empty line between the comment block and the def. ^[link]

• Limit lines to 80 characters. ^[link]

• Avoid trailing whitespace. ^[link]

• End each file with a newline. ^[link]

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

  # bad=begincomment lineanother comment line=end# good# comment line# another comment line

Syntax

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

  # badSomeClass::some_methodsome_object::some_method# goodSomeClass.some_methodsome_object.some_methodSomeModule::SomeClass::SOME_CONSTSomeModule::SomeClass()

• Use def with parentheses when there are parameters. Omit the parentheses when the method doesn't accept any parameters. ^[link]

  # baddefsome_method()# body omittedend# gooddefsome_method# body omittedend# baddefsome_method_with_parametersparam1,param2# body omittedend# gooddefsome_method_with_parameters(param1,param2)# body omittedend

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

  # badx=Math.siny# goodx=Math.sin(y)# badarray.deletee# goodarray.delete(e)# badtemperance=Person.new'Temperance',30# goodtemperance=Person.new('Temperance',30)

  Only omit parentheses for

  • Method calls with no arguments:

    # badKernel.exit!()2.even?()fork()'test'.upcase()# goodKernel.exit!2.even?fork'test'.upcase

  • Methods that are part of an internal DSL (e.g., Rake, Rails, RSpec):

    # badexpect(bowling.score).toeq0# goodexpect(bowling.score).toeq(0)

  • Methods that have "keyword" status in Ruby:

    classPerson# badattr_reader(:name,:age)# goodattr_reader:name,:age# body omittedend# badputs(temperance.age)# goodputstemperance.age

• Define optional arguments at the end of the list of arguments. Ruby has some unexpected results when calling methods that have optional arguments at the front of the list. ^[link]

  # baddefsome_method(a=1,b=2,c,d)puts"#{a}, #{b}, #{c}, #{d}"endsome_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'# gooddefsome_method(c,d,a=1,b=2)puts"#{a}, #{b}, #{c}, #{d}"endsome_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'

• Avoid the use of parallel assignment for defining variables. Parallel assignment is allowed when it is the return of a method call, used with the splat operator, or when used to swap variable assignment. Parallel assignment is less readable than separate assignment. ^[link]

  # bada,b,c,d='foo','bar','baz','foobar'# gooda='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,aputsa# => 'bar'putsb# => 'foo'# good - method returndefmulti_return[1,2]endfirst,second=multi_return# good - use with splatfirst, *list=[1,2,3,4]# first => 1, list => [2, 3, 4]hello_array= *'Hello'# => ["Hello"]a= *(1..3)# => [1, 2, 3]

• Avoid the use of unnecessary trailing underscore variables during parallel assignment. Named underscore variables are to be preferred over underscore variables because of the context that they provide. Trailing underscore variables are necessary when there is a splat variable defined on the left side of the assignment, and the splat variable is not an underscore. ^[link]

  # badfoo='one,two,three,four,five'# Unnecessary assignment that does not provide useful informationfirst,second,_=foo.split(',')first,_,_=foo.split(',')first, *_=foo.split(',')# goodfoo='one,two,three,four,five'# The underscores are needed to show that you want all elements# except for the last number of underscore elements
  *beginning,_=foo.split(',')
  *beginning,something,_=foo.split(',')a,=foo.split(',')a,b,=foo.split(',')# Unnecessary assignment to an unused variable, but the assignment# provides us with useful information.first,_second=foo.split(',')first,_second,=foo.split(',')first, *_ending=foo.split(',')

• Do not use for, unless you know exactly why. Most of the time iterators should be used instead. for is implemented in terms 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. ^[link]

  arr=[1,2,3]# badforeleminarrdoputselemend# note that elem is accessible outside of the for loopelem# => 3# goodarr.each{ |elem| putselem}# elem is not accessible outside each's blockelem# => NameError: undefined local variable or method `elem'

• Do not use then for multi-line if/unless. ^[link]

  # badifsome_conditionthen# body omittedend# goodifsome_condition# body omittedend

• Always put the condition on the same line as the if/unless in a multi-line conditional. ^[link]

  # badifsome_conditiondo_somethingdo_something_elseend# goodifsome_conditiondo_somethingdo_something_elseend

• Favor the ternary operator(?:) over if/then/else/end constructs. It's more common and obviously more concise. ^[link]

  # badresult=ifsome_conditionthensomethingelsesomething_elseend# goodresult=some_condition ? something : something_else

• Use one expression per branch in a ternary operator. This also means that ternary operators must not be nested. Prefer if/else constructs in these cases. ^[link]

  # badsome_condition ? (nested_condition ? nested_something : nested_something_else) : something_else# goodifsome_conditionnested_condition ? nested_something : nested_something_elseelsesomething_elseend

• Do not use if x; .... Use the ternary operator instead. ^[link]

  # badresult=ifsome_condition;somethingelsesomething_elseend# goodresult=some_condition ? something : something_else

• Leverage the fact that if and case are expressions which return a result. ^[link]

  # badifconditionresult=xelseresult=yend# goodresult=ifconditionxelseyend

• Use when x then ... for one-line cases. The alternative syntax when x: ... has been removed as of Ruby 1.9. ^[link]

• Do not use when x; .... See the previous rule. ^[link]

• Use ! instead of not. ^[link]

  # bad - parentheses are required because of op precedencex=(not something)# goodx= !something

• Avoid the use of !!. ^[link]

  !! converts a value to boolean, but you don't need this explicit conversion in the condition of a control expression; using it only obscures your intention. If you want to do a nil check, use nil? instead.

  # badx='test'# obscure nil checkif !!x# body omittedend# goodx='test'unlessx.nil?# body omittedend

• The and and or keywords are banned. It's just not worth it. Always use && and || instead. ^[link]

  # bad# boolean expressionifsome_conditionandsome_other_conditiondo_somethingend# control flowdocument.saved?ordocument.save!# good# boolean expressionifsome_condition && some_other_conditiondo_somethingend# control flowdocument.saved? || document.save!

• Avoid multi-line ?: (the ternary operator); use if/unless instead. ^[link]

• Favor modifier if/unless usage when you have a single-line body. Another good alternative is the usage of control flow &&/||. ^[link]

  # badifsome_conditiondo_somethingend# gooddo_somethingifsome_condition# another good optionsome_condition && do_something

• Avoid modifier if/unless usage at the end of a non-trivial multi-line block. ^[link]

  # bad10.timesdo# multi-line body omittedendifsome_condition# goodifsome_condition10.timesdo# multi-line body omittedendend

• Avoid nested modifier if/unless/while/until usage. Favor &&/|| if appropriate. ^[link]

  # baddo_somethingifother_conditionifsome_condition# gooddo_somethingifsome_condition && other_condition

• Favor unless over if for negative conditions (or control flow ||). ^[link]

  # baddo_somethingif !some_condition# baddo_somethingif not some_condition# gooddo_somethingunlesssome_condition# another good optionsome_condition || do_something

• Do not use unless with else. Rewrite these with the positive case first. ^[link]

  # badunlesssuccess?puts'failure'elseputs'success'end# goodifsuccess?puts'success'elseputs'failure'end

• Don't use parentheses around the condition of a control expression. ^[link]

  # badif(x > 10)# body omittedend# goodifx > 10# body omittedend

Note that there is an exception to this rule, namely safe assignment in condition.

• Do not use while/until condition do for multi-line while/until. ^[link]

  # badwhilex > 5do# body omittedenduntilx > 5do# body omittedend# goodwhilex > 5# body omittedenduntilx > 5# body omittedend

• Favor modifier while/until usage when you have a single-line body. ^[link]

  # badwhilesome_conditiondo_somethingend# gooddo_somethingwhilesome_condition

• Favor until over while for negative conditions. ^[link]

  # baddo_somethingwhile !some_condition# gooddo_somethinguntilsome_condition

• Use Kernel#loop instead of while/until when you need an infinite loop. ^[link]

  # badwhiletruedo_somethingenduntilfalsedo_somethingend# goodloopdodo_somethingend

• Use Kernel#loop with break rather than begin/end/until or begin/end/while for post-loop tests. ^[link]

  # badbeginputsvalval += 1endwhileval < 0# goodloopdoputsvalval += 1breakunlessval < 0end

• Omit the outer braces around an implicit options hash. ^[link]

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

• Omit both the outer braces and parentheses for methods that are part of an internal DSL. ^[link]

  classPerson < ActiveRecord::Base# badvalidates(:name,{presence: true,length: {within: 1..10}})# goodvalidates:name,presence: true,length: {within: 1..10}end

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

  # badnames.map{ |name| name.upcase}# goodnames.map(&:upcase)

• 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. ^[link]

  names=%w(BozhidarSteveSarah)# badnames.eachdo |name|
  putsnameend# goodnames.each{ |name| putsname}# badnames.selectdo |name|
  name.start_with?('S')end.map{ |name| name.upcase}# goodnames.select{ |name| name.start_with?('S')}.map(&:upcase)

  Some will argue that multi-line chaining would look OK with the use of {...}, but they should ask themselves—is this code really readable and can the blocks' contents be extracted into nifty methods?

• Consider using explicit block argument to avoid writing block literal that just passes its arguments to another block. Beware of the performance impact, though, as the block gets converted to a Proc. ^[link]

  require'tempfile'# baddefwith_tmp_dirDir.mktmpdirdo |tmp_dir|
  Dir.chdir(tmp_dir){ |dir| yielddir}# block just passes argumentsendend# gooddefwith_tmp_dir(&block)Dir.mktmpdirdo |tmp_dir|
  Dir.chdir(tmp_dir, &block)endendwith_tmp_dirdo |dir|
  puts"dir is accessible as a parameter and pwd is set: #{dir}"end

• Avoid return where not required for flow of control. ^[link]

  # baddefsome_method(some_arr)returnsome_arr.sizeend# gooddefsome_method(some_arr)some_arr.sizeend

• Avoid self where not required. (It is only required when calling a self write accessor.) ^[link]

  # baddefready?ifself.last_reviewed_at > self.last_updated_atself.worker.update(self.content,self.options)self.status=:in_progressendself.status == :verifiedend# gooddefready?iflast_reviewed_at > last_updated_atworker.update(content,options)self.status=:in_progressendstatus == :verifiedend

• As a corollary, avoid shadowing methods with local variables unless they are both equivalent. ^[link]

  classFooattr_accessor:options# okdefinitialize(options)self.options=options# both options and self.options are equivalent hereend# baddefdo_something(options={})unlessoptions[:when] == :lateroutput(self.options[:message])endend# gooddefdo_something(params={})unlessparams[:when] == :lateroutput(options[:message])endendend

• Don't use the return value of = (an assignment) in conditional expressions unless the assignment is wrapped in parentheses. This is a fairly popular idiom among Rubyists that's sometimes referred to as safe assignment in condition. ^[link]

  # bad (+ a warning)ifv=array.grep(/foo/)do_something(v)# some codeend# good (MRI would still complain, but RuboCop won't)if(v=array.grep(/foo/))do_something(v)# some codeend# goodv=array.grep(/foo/)ifvdo_something(v)# some codeend

• Use shorthand self assignment operators whenever applicable. ^[link]

  # badx=x + yx=x * yx=x**yx=x / yx=x || yx=x && y# goodx += yx *= yx **= yx /= yx ||= yx &&= y

• Use ||= to initialize variables only if they're not already initialized. ^[link]

  # badname=name ? name : 'Bozhidar'# badname='Bozhidar'unlessname# good - set name to 'Bozhidar', only if it's nil or falsename ||= 'Bozhidar'

• Don't use ||= to initialize boolean variables. (Consider what would happen if the current value happened to be false.) ^[link]

  # bad - would set enabled to true even if it was falseenabled ||= true# goodenabled=trueifenabled.nil?

• Use &&= to preprocess variables that may or may not exist. Using &&= will change the value only if it exists, removing the need to check its existence with if. ^[link]

  # badifsomethingsomething=something.downcaseend# badsomething=something ? something.downcase : nil# oksomething=something.downcaseifsomething# goodsomething=something && something.downcase# bettersomething &&= something.downcase

• 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. ^[link]

  # badArray === something(1..100) === 7/something/ === some_string# goodsomething.is_a?(Array)(1..100).include?(7)some_string =~ /something/

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

  # bad - eql? is the same as == for strings'ruby'.eql?some_str# good'ruby' == some_str1.0.eql?x# eql? makes sense here if want to differentiate between Integer and Float 1

• Avoid using Perl-style special variables (like $:, $;, etc. ). They are quite cryptic and their use in anything but one-liner scripts is discouraged. Use the human-friendly aliases provided by the English library. ^[link]

  # bad
  $:.unshiftFile.dirname(__FILE__)# goodrequire'English'
  $LOAD_PATH.unshiftFile.dirname(__FILE__)

• Do not put a space between a method name and the opening parenthesis. ^[link]

  # badf(3 + 2) + 1# goodf(3 + 2) + 1

• Always run the Ruby interpreter with the -w option so it will warn you if you forget either of the rules above! ^[link]

• Do not use nested method definitions, use lambda instead. Nested method definitions actually produce methods in the same scope (e.g. class) as the outer method. Furthermore, the "nested method" will be redefined every time the method containing its definition is invoked. ^[link]

  # baddeffoo(x)defbar(y)# body omittedendbar(x)end# good - the same as the previous, but no bar redefinition on every foo calldefbar(y)# body omittedenddeffoo(x)bar(x)end# also gooddeffoo(x)bar=->(y){ ... }bar.call(x)end

• Use the new lambda literal syntax for single line body blocks. Use the lambda method for multi-line blocks. ^[link]

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

• Don't omit the parameter parentheses when defining a stabby lambda with parameters. ^[link]

  # badl=->x,y{something(x,y)}# goodl=->(x,y){something(x,y)}

• Omit the parameter parentheses when defining a stabby lambda with no parameters. ^[link]

  # badl=->(){something}# goodl=->{something}

• Prefer proc over Proc.new. ^[link]

  # badp=Proc.new{ |n| putsn}# goodp=proc{ |n| putsn}

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

  # bad - looks similar to Enumeration accessl=->(v){putsv}l[1]# also bad - uncommon syntaxl=->(v){putsv}l.(1)# goodl=->(v){putsv}l.call(1)

• Prefix with _ unused block parameters and local variables. It's also acceptable to use just _ (although it's a bit less descriptive). This convention is recognized by the Ruby interpreter and tools like RuboCop and will suppress their unused variable warnings. ^[link]

  # badresult=hash.map{ |k,v| v + 1}defsomething(x)unused_var,used_var=something_else(x)# some codeend# goodresult=hash.map{ |_k,v| v + 1}defsomething(x)_unused_var,used_var=something_else(x)# some codeend# goodresult=hash.map{ |_,v| v + 1}defsomething(x)_,used_var=something_else(x)# some codeend

• 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. ^[link]

• Use warn instead of $stderr.puts. Apart from being more concise and clear, warn allows you to suppress warnings if you need to (by setting the warn level to 0 via -W0). ^[link]

• Favor the use of sprintf and its alias format over the fairly cryptic String#% method. ^[link]

  # bad'%d %d' % [20,10]# => '20 10'# goodsprintf('%d %d',20,10)# => '20 10'# goodsprintf('%{first} %{second}',first: 20,second: 10)# => '20 10'format('%d %d',20,10)# => '20 10'# goodformat('%{first} %{second}',first: 20,second: 10)# => '20 10'

• Favor the use of Array#join over the fairly cryptic Array#* with a string argument. ^[link]

  # bad%w(onetwothree) * ', '# => 'one, two, three'# good%w(onetwothree).join(', ')# => 'one, two, three'

• 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. ^[link]

  # badpaths=[paths]unlesspaths.is_a?Arraypaths.each{ |path| do_something(path)}# bad (always creates a new Array instance)[*paths].each{ |path| do_something(path)}# good (and a bit more readable)Array(paths).each{ |path| do_something(path)}

• Use ranges or Comparable#between? instead of complex comparison logic when possible. ^[link]

  # baddo_somethingifx >= 1000 && x <= 2000# gooddo_somethingif(1000..2000).include?(x)# gooddo_somethingifx.between?(1000,2000)

• Favor the use of predicate methods to explicit comparisons with ==. Numeric comparisons are OK. ^[link]

  # badifx % 2 == 0endifx % 2 == 1endifx == nilend# goodifx.even?endifx.odd?endifx.nil?endifx.zero?endifx == 0end

• Don't do explicit non-nil checks unless you're dealing with boolean values. ^[link]

  # baddo_somethingif !something.nil?do_somethingifsomething != nil# gooddo_somethingifsomething# good - dealing with a booleandefvalue_set?
  !@some_boolean.nil?end

• Avoid the use of BEGIN blocks. ^[link]

• Do not use END blocks. Use Kernel#at_exit instead. ^[link]

  # bad
  END {puts'Goodbye!'}# goodat_exit{puts'Goodbye!'}

• Avoid the use of flip-flops. ^[link]

• Avoid use of nested conditionals for flow of control. ^[link]

  Prefer a guard clause when you can assert invalid data. A guard clause is a conditional statement at the top of a function that bails out as soon as it can.

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

  Prefer next in loops instead of conditional blocks.

  # bad[0,1,2,3].eachdo |item|
  ifitem > 1putsitemendend# good[0,1,2,3].eachdo |item|
  nextunlessitem > 1putsitemend

• 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. ^[link]

• 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. ^[link]

  # badsome_hash.count# goodsome_hash.size

• 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. ^[link]

  # badall_songs=users.map(&:songs).flatten.uniq# goodall_songs=users.flat_map(&:songs).uniq

• 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. ^[link]

  # badarray.reverse.each{ ... }# goodarray.reverse_each{ ... }

Naming

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

• Name identifiers in English. ^[link]

  # bad - identifier using non-ascii charactersзаплата=1_000# bad - identifier is a Bulgarian word, written with Latin letters (instead of Cyrillic)zaplata=1_000# goodsalary=1_000

• Use snake_case for symbols, methods and variables. ^[link]

  # bad:'some symbol':SomeSymbol:someSymbolsomeVar=5var_10=10defsomeMethod# some codeenddefSomeMethod# some codeend# good:some_symbolsome_var=5var10=10defsome_method# some codeend

• Do not separate numbers from letters on symbols, methods and variables. ^[link]

  # bad:some_sym_1some_var_1=1defsome_method_1# some codeend# good:some_sym1some_var1=1defsome_method1# some codeend

• Use CamelCase for classes and modules. (Keep acronyms like HTTP, RFC, XML uppercase.) ^[link]

  # badclassSomeclass# some codeendclassSome_Class# some codeendclassSomeXml# some codeendclassXmlSomething# some codeend# goodclassSomeClass# some codeendclassSomeXML# some codeendclassXMLSomething# some codeend

• Use snake_case for naming files, e.g. hello_world.rb. ^[link]

• Use snake_case for naming directories, e.g. lib/hello_world/hello_world.rb. ^[link]

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

• Use SCREAMING_SNAKE_CASE for other constants. ^[link]

  # badSomeConst=5# goodSOME_CONST=5

• The names of predicate methods (methods that return a boolean value) should end in a question mark. (i.e. Array#empty?). Methods that don't return a boolean, shouldn't end in a question mark. ^[link]

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

  # badclassPersondefis_tall?trueenddefcan_play_basketball?falseenddefdoes_like_candy?trueendend# goodclassPersondeftall?trueenddefbasketball_player?falseenddeflikes_candy?trueendend

• 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. ^[link]

  # bad - there is no matching 'safe' methodclassPersondefupdate!endend# goodclassPersondefupdateendend# goodclassPersondefupdate!enddefupdateendend

• Define the non-bang (safe) method in terms of the bang (dangerous) one if possible. ^[link]

  classArraydefflatten_once!res=[]eachdo |e|
  [*e].each{ |f| res << f}endreplace(res)enddefflatten_oncedup.flatten_once!endend

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

  def +(other)# body omittedend

Comments

Good code is its own best documentation. As you're about to add a comment, ask yourself, "How can I improve the code so that this comment isn't needed?" Improve the code and then document it to make it even clearer.
-- Steve McConnell

• Write self-documenting code and ignore the rest of this section. Seriously! ^[link]

• Write comments in English. ^[link]

• Use one space between the leading # character of the comment and the text of the comment. ^[link]

• Comments longer than a word are capitalized and use punctuation. Use one space after periods. ^[link]

• Avoid superfluous comments. ^[link]

  # badcounter += 1# Increments counter by one.

• Keep existing comments up-to-date. An outdated comment is worse than no comment at all. ^[link]

Good code is like a good joke: it needs no explanation.
— old programmers maxim, through Russ Olsen

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

Comment Annotations

• Annotations should usually be written on the line immediately above the relevant code. ^[link]

• The annotation keyword is followed by a colon and a space, then a note describing the problem. ^[link]

• If multiple lines are required to describe the problem, subsequent lines should be indented three spaces after the # (one general plus two for indentation purpose). ^[link]

  defbar# FIXME: This has crashed occasionally since v3.2.1. It may# be related to the BarBazUtil upgrade.baz(:quux)end

• In cases where the problem is so obvious that any documentation would be redundant, annotations may be left at the end of the offending line with no note. This usage should be the exception and not the rule. ^[link]

  defbarsleep100# OPTIMIZEend

• Use TODO to note missing features or functionality that should be added at a later date. ^[link]

• Use FIXME to note broken code that needs to be fixed. ^[link]

• Use OPTIMIZE to note slow or inefficient code that may cause performance problems. ^[link]

• Use HACK to note code smells where questionable coding practices were used and should be refactored away. ^[link]

• Use REVIEW to note anything that should be looked at to confirm it is working as intended. For example: REVIEW: Are we sure this is how the client does X currently?^[link]

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

Classes & Modules

• Use a consistent structure in your class definitions. ^[link]

  classPerson# extend and include go firstextendSomeModuleincludeAnotherModule# inner classesCustomError=Class.new(StandardError)# constants are nextSOME_CONSTANT=20# afterwards we have attribute macrosattr_reader:name# followed by other macros (if any)validates:name# public class methods are next in linedefself.some_methodend# initialization goes between class methods and other instance methodsdefinitializeend# followed by other public instance methodsdefsome_methodend# protected and private methods are grouped near the endprotecteddefsome_protected_methodendprivatedefsome_private_methodendend

• Don't nest multi-line classes within classes. Try to have such nested classes each in their own file in a folder named like the containing class. ^[link]

  # bad# foo.rbclassFooclassBar# 30 methods insideendclassCar# 20 methods insideend# 30 methods insideend# good# foo.rbclassFoo# 30 methods insideend# foo/bar.rbclassFooclassBar# 30 methods insideendend# foo/car.rbclassFooclassCar# 20 methods insideendend

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

  # badclassSomeClassdefself.some_method# body omittedenddefself.some_other_method# body omittedendend# goodmoduleSomeModulemodule_functiondefsome_method# body omittedenddefsome_other_method# body omittedendend

• Favor the use of module_function over extend self when you want to turn a module's instance methods into class methods. ^[link]

  # badmoduleUtilitiesextendselfdefparse_something(string)# do stuff hereenddefother_utility_method(number,string)# do some more stuffendend# goodmoduleUtilitiesmodule_functiondefparse_something(string)# do stuff hereenddefother_utility_method(number,string)# do some more stuffendend

• When designing class hierarchies make sure that they conform to the Liskov Substitution Principle. ^[link]

• Try to make your classes as SOLID as possible. ^[link]

• Always supply a proper to_s method for classes that represent domain objects. ^[link]

  classPersonattr_reader:first_name,:last_namedefinitialize(first_name,last_name)@first_name=first_name@last_name=last_nameenddefto_s"#{@first_name}#{@last_name}"endend

• Use the attr family of functions to define trivial accessors or mutators. ^[link]

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

• 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). ^[link]

  # badclassPersondefget_name"#{@first_name}#{@last_name}"enddefset_name(name)@first_name,@last_name=name.split(' ')endend# goodclassPersondefname"#{@first_name}#{@last_name}"enddefname=(name)@first_name,@last_name=name.split(' ')endend

• Avoid the use of attr. Use attr_reader and attr_accessor instead. ^[link]

  # bad - creates a single attribute accessor (deprecated in Ruby 1.9)attr:something,trueattr:one,:two,:three# behaves as attr_reader# goodattr_accessor:somethingattr_reader:one,:two,:three

• Consider using Struct.new, which defines the trivial accessors, constructor and comparison operators for you. ^[link]

  # goodclassPersonattr_accessor:first_name,:last_namedefinitialize(first_name,last_name)@first_name=first_name@last_name=last_nameendend# betterPerson=Struct.new(:first_name,:last_name)doend

• Don't extend an instance initialized by Struct.new. Extending it introduces a superfluous class level and may also introduce weird errors if the file is required multiple times. ^[link]

  # badclassPerson < Struct.new(:first_name,:last_name)end# goodPerson=Struct.new(:first_name,:last_name)

• Consider adding factory methods to provide additional sensible ways to create instances of a particular class. ^[link]

  classPersondefself.create(options_hash)# body omittedendend

• Prefer duck-typing over inheritance. ^[link]

  # badclassAnimal# abstract methoddefspeakendend# extend superclassclassDuck < Animaldefspeakputs'Quack! Quack'endend# extend superclassclassDog < Animaldefspeakputs'Bau! Bau!'endend# goodclassDuckdefspeakputs'Quack! Quack'endendclassDogdefspeakputs'Bau! Bau!'endend

• Avoid the usage of class (@@) variables due to their "nasty" behavior in inheritance. ^[link]

  classParent@@class_var='parent'defself.print_class_varputs@@class_varendendclassChild < Parent@@class_var='child'endParent.print_class_var# => will print 'child'

  As you can see all the classes in a class hierarchy actually share one class variable. Class instance variables should usually be preferred over class variables.

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

• 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. ^[link]

  classSomeClassdefpublic_method# some codeendprivatedefprivate_method# some codeenddefanother_private_method# some codeendend

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

  classTestClass# baddefTestClass.some_method# body omittedend# gooddefself.some_other_method# body omittedend# Also possible and convenient when you# have to define many class methods.class << selfdeffirst_method# body omittedenddefsecond_method_etc# body omittedendendend

• 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. ^[link]

  classWesternerdeffirst_name@names.firstendaliasgiven_namefirst_nameend

  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.

  classFugitive < Westernerdeffirst_name'Nobody'endend

  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.

  classFugitive < Westernerdeffirst_name'Nobody'endaliasgiven_namefirst_nameend

• 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. ^[link]

  moduleMononymousdefself.included(other)other.class_eval{alias_method:full_name,:given_name}endendclassSting < WesternerincludeMononymousend

• 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 boilerpate in such classes. ^[link]

  classTestClass# bad -- more work when class renamed/method moveddefself.call(param1,param2)TestClass.new(param1).call(param2)end# bad -- more verbose than necessarydefself.call(param1,param2)self.new(param1).call(param2)end# gooddefself.call(param1,param2)new(param1).call(param2)end# ...other methods...end

Exceptions

• Prefer raise over fail for exceptions. ^[link]

  # badfailSomeException,'message'# goodraiseSomeException,'message'

• Don't specify RuntimeError explicitly in the two argument version of raise. ^[link]

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

• Prefer supplying an exception class and a message as two separate arguments to raise, instead of an exception instance. ^[link]

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

• 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. ^[link]

  # baddeffooraiseensurereturn'very bad idea'end

• Use implicit begin blocks where possible. ^[link]

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

• Mitigate the proliferation of begin blocks by using contingency methods (a term coined by Avdi Grimm). ^[link]

  # badbeginsomething_that_might_failrescueIOError# handle IOErrorendbeginsomething_else_that_might_failrescueIOError# handle IOErrorend# gooddefwith_io_error_handlingyieldrescueIOError# handle IOErrorendwith_io_error_handling{something_that_might_fail}with_io_error_handling{something_else_that_might_fail}

• Don't suppress exceptions. ^[link]

  # badbegin# an exception occurs hererescueSomeError# the rescue clause does absolutely nothingend# baddo_somethingrescuenil

• Avoid using rescue in its modifier form. ^[link]

  # bad - this catches exceptions of StandardError class and its descendant classesread_filerescuehandle_error($!)# good - this catches only the exceptions of Errno::ENOENT class and its descendant classesdeffooread_filerescueErrno::ENOENT=>exhandle_error(ex)end

• Don't use exceptions for flow of control. ^[link]

  # badbeginn / drescueZeroDivisionErrorputs'Cannot divide by 0!'end# goodifd.zero?puts'Cannot divide by 0!'elsen / dend

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

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

• Put more specific exceptions higher up the rescue chain, otherwise they'll never be rescued from. ^[link]

  # badbegin# some coderescueStandardError=>e# some handlingrescueIOError=>e# some handling that will never be executedend# goodbegin# some coderescueIOError=>e# some handlingrescueStandardError=>e# some handlingend

• Release external resources obtained by your program in an ensure block. ^[link]

  f=File.open('testfile')begin# .. processrescue# .. handle errorensuref.closeiffend

• Use versions of resource obtaining methods that do automatic resource cleanup when possible. ^[link]

  # bad - you need to close the file descriptor explicitlyf=File.open('testfile')# some action on the filef.close# good - the file descriptor is closed automaticallyFile.open('testfile')do |f|
  # some action on the fileend

• Favor the use of exceptions from the standard library over introducing new exception classes. ^[link]

Collections

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

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

• Prefer %w to the literal array syntax when you need an array of words (non-empty strings without spaces and special characters in them). Apply this rule only to arrays with two or more elements. ^[link]

  # badSTATES=['draft','open','closed']# goodSTATES=%w(draftopenclosed)

• Prefer %i to the literal array syntax when you need an array of symbols (and you don't need to maintain Ruby 1.9 compatibility). Apply this rule only to arrays with two or more elements. ^[link]

  # badSTATES=[:draft,:open,:closed]# goodSTATES=%i(draftopenclosed)

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

  # bad - easier to move/add/remove items, but still not preferredVALUES=[1001,2020,3333,]# badVALUES=[1001,2020,3333,]# goodVALUES=[1001,2020,3333]

• Avoid the creation of huge gaps in arrays. ^[link]

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

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

• 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. ^[link]

• Prefer symbols instead of strings as hash keys. ^[link]

  # badhash={'one'=>1,'two'=>2,'three'=>3}# goodhash={one: 1,two: 2,three: 3}

• Avoid the use of mutable objects as hash keys. ^[link]

• Use the Ruby 1.9 hash literal syntax when your hash keys are symbols. ^[link]

  # badhash={:one=>1,:two=>2,:three=>3}# goodhash={one: 1,two: 2,three: 3}

• Don't mix the Ruby 1.9 hash syntax with hash rockets in the same hash literal. When you've got keys that are not symbols stick to the hash rockets syntax. ^[link]

  # bad{a: 1,'b'=>2}# good{:a=>1,'b'=>2}

• Use Hash#key? instead of Hash#has_key? and Hash#value? instead of Hash#has_value?. As noted here by Matz, the longer forms are considered deprecated. ^[link]

  # badhash.has_key?(:test)hash.has_value?(value)# goodhash.key?(:test)hash.value?(value)

• Use Hash#each_key instead of Hash#keys.each and Hash#each_value instead of Hash#values.each. ^[link]

  # badhash.keys.each{ |k| pk}hash.values.each{ |v| pv}hash.each{ |k,_v| pk}hash.each{ |_k,v| pv}# goodhash.each_key{ |k| pk}hash.each_value{ |v| pv}

• Use Hash#fetch when dealing with hash keys that should be present. ^[link]

  heroes={batman: 'Bruce Wayne',superman: 'Clark Kent'}# bad - if we make a mistake we might not spot it right awayheroes[:batman]# => 'Bruce Wayne'heroes[:supermann]# => nil# good - fetch raises a KeyError making the problem obviousheroes.fetch(:supermann)

• Introduce default values for hash keys via Hash#fetch as opposed to using custom logic. ^[link]

  batman={name: 'Bruce Wayne',is_evil: false}# bad - if we just use || operator with falsy value we won't get the expected resultbatman[:is_evil] || true# => true# good - fetch work correctly with falsy valuesbatman.fetch(:is_evil,true)# => false

• Prefer the use of the block instead of the default value in Hash#fetch if the code that has to be evaluated may have side effects or be expensive. ^[link]

  batman={name: 'Bruce Wayne'}# bad - if we use the default value, we eager evaluate it# so it can slow the program down if done multiple timesbatman.fetch(:powers,obtain_batman_powers)# obtain_batman_powers is an expensive call# good - blocks are lazy evaluated, so only triggered in case of KeyError exceptionbatman.fetch(:powers){obtain_batman_powers}

• Use Hash#values_at when you need to retrieve several values consecutively from a hash. ^[link]

  # bademail=data['email']username=data['nickname']# goodemail,username=data.values_at('email','nickname')

• Rely on the fact that as of Ruby 1.9 hashes are ordered. ^[link]

• Do not modify a collection while traversing it. ^[link]

• When accessing elements of a collection, avoid direct access via [n] by using an alternate form of the reader method if it is supplied. This guards you from calling [] on nil. ^[link]

  # badRegexp.last_match[1]# goodRegexp.last_match(1)

• When providing an accessor for a collection, provide an alternate form to save users from checking for nil before accessing an element in the collection. ^[link]

  # baddefawesome_things@awesome_thingsend# gooddefawesome_things(index=nil)ifindex && @awesome_things@awesome_things[index]else@awesome_thingsendend

Numbers

• Use Integer 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. ^[link]

  timestamp=Time.now.to_i# badtimestamp.is_a?Fixnumtimestamp.is_a?Bignum# goodtimestamp.is_a?Integer

  • Prefer to use ranges when generating random numbers instead of integers with offsets, since it clearly states your intentions. Imagine simulating a role of a dice: ^[link]

    # badrand(6) + 1# goodrand(1..6)

Strings

• Prefer string interpolation and string formatting instead of string concatenation: ^[link]

  # bademail_with_name=user.name + ' <' + user.email + '>'# goodemail_with_name="#{user.name} <#{user.email}>"# goodemail_with_name=format('%s <%s>',user.name,user.email)

• Adopt a consistent string literal quoting style. There are two popular styles in the Ruby community, both of which are considered good—single quotes by default (Option A) and double quotes by default (Option B). ^[link]

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

    # badname="Bozhidar"# goodname='Bozhidar'

  • (Option B) Prefer double-quotes unless your string literal contains " or escape characters you want to suppress.

    # badname='Bozhidar'# goodname="Bozhidar"

  The string literals in this guide are aligned with the first style.

• Don't use the character literal syntax ?x. Since Ruby 1.9 it's basically redundant—?x would interpreted as 'x' (a string with a single character in it). ^[link]

  # badchar= ?c
  # goodchar='c'

• Don't leave out {} around instance and global variables being interpolated into a string. ^[link]

  classPersonattr_reader:first_name,:last_namedefinitialize(first_name,last_name)@first_name=first_name@last_name=last_nameend# bad - valid, but awkwarddefto_s"#@first_name #@last_name"end# gooddefto_s"#{@first_name}#{@last_name}"endend
  $global =0# badputs"$global = #$global"# goodputs"$global = #{$global}"

• Don't use Object#to_s on interpolated objects. It's invoked on them automatically. ^[link]

  # badmessage="This is the #{result.to_s}."# goodmessage="This is the #{result}."

• 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. ^[link]

  # badhtml=''html += '<h1>Page title</h1>'paragraphs.eachdo |paragraph|
  html += "<p>#{paragraph}</p>"end# good and also fasthtml=''html << '<h1>Page title</h1>'paragraphs.eachdo |paragraph|
  html << "<p>#{paragraph}</p>"end

• Don't use String#gsub in scenarios in which you can use a faster more specialized alternative. ^[link]

  url='http://example.com'str='lisp-case-rules'# badurl.gsub('http://','https://')str.gsub('-','_')# goodurl.sub('http://','https://')str.tr('-','_')

• When using heredocs for multi-line strings keep in mind the fact that they preserve leading whitespace. It's a good practice to employ some margin based on which to trim the excessive whitespace. ^[link]

  code=<<-END.gsub(/^\s+\|/,'') |def test | some_method | other_method |endEND# => "def test\n some_method\n other_method\nend\n"

• Use Ruby 2.3's squiggly heredocs for nicely indented multi-line strings. ^[link]

  # bad - using Powerpack String#strip_margincode=<<-END.strip_margin('|') |def test | some_method | other_method |endEND# also badcode=<<-ENDdef test some_method other_methodendEND# goodcode=<<~END def test some_method other_method endEND

Date & Time

• Prefer Time.now over Time.new when retrieving the current system time. ^[link]

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

  # bad - uses DateTime for current timeDateTime.now# good - uses Time for current timeTime.now# bad - uses DateTime for modern dateDateTime.iso8601('2016-06-29')# good - uses Date for modern dateDate.iso8601('2016-06-29')# good - uses DateTime with start argument for historical dateDateTime.iso8601('1751-04-23',Date::ENGLAND)

Regular Expressions

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

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

• For simple constructions you can use regexp directly through string index. ^[link]

  match=string[/regexp/]# get content of matched regexpfirst_group=string[/text(grp)/,1]# get content of captured groupstring[/text (grp)/,1]='replace'# string => 'text replace'

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

  # bad/(first|second)/# good/(?:first|second)/

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

  /(regexp)/ =~ string
  ...
  # badprocess $1
  # goodprocessRegexp.last_match(1)

• Avoid using numbered groups as it can be hard to track what they contain. Named groups can be used instead. ^[link]

  # bad/(regexp)/ =~ string# some codeprocessRegexp.last_match(1)# good/(?<meaningful_var>regexp)/ =~ string# some codeprocessmeaningful_var

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

• Be careful with ^ and $ as they match start/end of line, not string endings. If you want to match the whole string use: \A and \z (not to be confused with \Z which is the equivalent of /\n?\z/). ^[link]

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

• Use x modifier for complex regexps. This makes them more readable and you can add some useful comments. Just be careful as spaces are ignored. ^[link]

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

• For complex replacements sub/gsub can be used with a block or a hash. ^[link]

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

Percent Literals

• Use %()(it's a shorthand for %Q) for single-line strings which require both interpolation and embedded double-quotes. For multi-line strings, prefer heredocs. ^[link]

  # bad (no interpolation needed)%(<div class="text">Some text</div>)# should be '<div class="text">Some text</div>'# bad (no double-quotes)%(This is #{quality} style)# should be "This is #{quality} style"# bad (multiple lines)%(<div>\n<span class="big">#{exclamation}</span>\n</div>)# should be a heredoc.# good (requires interpolation, has quotes, single line)%(<tr><td class="name">#{name}</td>)

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

  # badname=%q(Bruce Wayne)time=%q(8 o'clock)question=%q("What did you say?")# goodname='Bruce Wayne'time="8 o'clock"question='"What did you say?"'quote=%q(<p class='quote'>"What did you say?"</p>)

• Use %r only for regular expressions matching at least one '/' character. ^[link]

  # bad%r{\s+}# good%r{^/(.*)$}%r{^/blog/2011/(.*)$}

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

  # baddate=%x(date)# gooddate=`date`echo=%x(echo `date`)

• Avoid the use of %s. It seems that the community has decided :"some string" is the preferred way to create a symbol with spaces in it. ^[link]

• Prefer () as delimiters for all % literals, except %r. Since parentheses often appear inside regular expressions in many scenarios a less common character like { might be a better choice for a delimiter, depending on the regexp's content. ^[link]

  # bad%w[onetwothree]%q{"Test's king!", John said.}# good%w(onetwothree)%q("Test's king!", John said.)

Metaprogramming

• Avoid needless metaprogramming. ^[link]

• Do not mess around in core classes when writing libraries. (Do not monkey-patch them.) ^[link]

• The block form of class_eval is preferable to the string-interpolated form. ^[link]

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

    class_eval'def use_relative_model_naming?; true; end',__FILE__,__LINE__

  • define_method is preferable to class_eval{ def ... }

• 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): ^[link]

  # from activesupport/lib/active_support/core_ext/string/output_safety.rbUNSAFE_STRING_METHODS.eachdo |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 EOTendend

• Avoid using method_missing for metaprogramming because backtraces become messy, the behavior is not listed in #methods, and misspelled method calls might silently work, e.g. nukes.launch_state = false. Consider using delegation, proxy, or define_method instead. If you must use method_missing: ^[link]

  • Be sure to also define respond_to_missing?

  • Only catch methods with a well-defined prefix, such as find_by_* -- make your code as assertive as possible.

  • Call super at the end of your statement

  • Delegate to assertive, non-magical methods:

    # baddefmethod_missing?(meth, *params, &block)if/^find_by_(?<prop>.*)/ =~ meth# ... lots of code to do a find_byelsesuperendend# gooddefmethod_missing?(meth, *params, &block)if/^find_by_(?<prop>.*)/ =~ methfind_by(prop, *params, &block)elsesuperendend# best of all, though, would to define_method as each findable attribute is declared

• Prefer public_send over send so as not to circumvent private/protected visibility. ^[link]

  # We have an ActiveModel Organization that includes concern ActivatablemoduleActivatableextendActiveSupport::Concernincludeddobefore_create:create_tokenendprivatedefreset_token# some codeenddefcreate_token# some codeenddefactivate!# some codeendendclassOrganization < ActiveRecord::BaseincludeActivatableendlinux_organization=Organization.find(...)# BAD - violates privacylinux_organization.send(:reset_token)# GOOD - should throw an exceptionlinux_organization.public_send(:reset_token)

• Prefer __send__ over send, as send may overlap with existing methods. ^[link]

  require'socket'u1=UDPSocket.newu1.bind('127.0.0.1',4913)u2=UDPSocket.newu2.connect('127.0.0.1',4913)# Won't send a message to the receiver obj.# Instead it will send a message via UDP socket.u2.send:sleep,0# Will actually send a message to the receiver obj.u2.__send__ ...

Misc

• Write ruby -w safe code. ^[link]

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

• Avoid methods longer than 10 LOC (lines of code). Ideally, most methods will be shorter than 5 LOC. Empty lines do not contribute to the relevant LOC. ^[link]

• Avoid parameter lists longer than three or four parameters. ^[link]

• If you really need "global" methods, add them to Kernel and make them private. ^[link]

• Use module instance variables instead of global variables. ^[link]

  # bad
  $foo_bar =1# goodmoduleFooclass << selfattr_accessor:barendendFoo.bar=1

• Use OptionParser for parsing complex command line options and ruby -s for trivial command line options. ^[link]

• Code in a functional way, avoiding mutation when that makes sense. ^[link]

• Do not mutate parameters unless that is the purpose of the method. ^[link]

• Avoid more than three levels of block nesting. ^[link]

• Be consistent. In an ideal world, be consistent with these guidelines. ^[link]

• Use common sense. ^[link]

Tools

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

RuboCop

RuboCop is a Ruby code style checker based on this style guide. RuboCop already covers a significant portion of the Guide, supports both MRI 1.9 and MRI 2.0 and has good Emacs integration.

RubyMine

RubyMine's code inspections are partially based on this guide.

Contributing

The guide is still a work in progress—some rules are lacking examples, some rules don't have examples that illustrate them clearly enough. Improving such rules is a great (and simple way) to help the Ruby community!

In due time these issues will (hopefully) be addressed—just keep them in mind for now.

Nothing written in this guide is set in stone. It's my desire to work together with everyone interested in Ruby coding style, so that we could ultimately create a resource that will be beneficial to the entire Ruby community.

Feel free to open tickets or send pull requests with improvements. Thanks in advance for your help!

You can also support the project (and RuboCop) with financial contributions via Gratipay.

How to Contribute?

It's easy, just follow the contribution guidelines.

License

This work is licensed under a Creative Commons Attribution 3.0 Unported License

Spread the Word

A community-driven style guide is of little use to a community that doesn't know about its existence. Tweet about the guide, share it with your friends and colleagues. Every comment, suggestion or opinion we get makes the guide just a little bit better. And we want to have the best possible guide, don't we?

Cheers,
Bozhidar