Prelude

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

One thing has always bothered me as 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 such fine fellows, like us Ruby developers, should be quite capable to produce 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 3 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, and 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 that is 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 1.9" and "The Ruby Programming Language".

The guide is still a work in progress - some rules are lacking examples, some rules don't have examples that illustrate them clearly enough. In due time these issues will be addressed - just keep them in mind for now.

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

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

Table of Contents

• Source Code Layout
• Syntax
• Naming
• Comments
  • Comment Annotations
• Classes
• Exceptions
• Collections
• Strings
• 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.

• Use two spaces per indentation level. No hard tabs.

  # bad - four spacesdefsome_methoddo_somethingend# gooddefsome_methoddo_somethingend

• Use Unix-style line endings. (*BSD/Solaris/Linux/OSX users are covered by default, Windows users have to be extra careful.)

  • 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.

  # badputs'foobar';# superfluous semicolonputs'foo';puts'bar'# two expression 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.

  # 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.

  # 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 ; make 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, around { and before }. Whitespace might be (mostly) irrelevant to the Ruby interpreter, but its proper use is the key to writing easily readable code.

  sum=1 + 2a,b=1,21 > 2 ? true : false;puts'Hi'[1,2,3].each{ |e| putse}

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

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

  { and } deserve a bit of clarification, since they are used for block and hash literals, as well as embedded expressions in strings. For hash literals two styles are considered acceptable.

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

  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.

  As far as embedded expressions go, there are also two acceptable options:

  # good - no spaces"string#{expr}"# ok - arguably more readable"string#{expr}"

  The first style is extremely more popular and you're generally advised to stick with it. The second, on the other hand, is (arguably) a bit more readable. As with hashes - pick one style and apply it consistently.

• No spaces after (, [ or before ], ).

  some(arg).other[1,2,3].length

• Indent when as deep as case. I know that many would disagree with this one, but it's the style established in both "The Ruby Programming Language" and "Programming Ruby".

  casewhensong.name == 'Misty'puts'Not again!'whensong.duration > 120puts'Too long!'whenTime.now.hour > 21puts"It's too late"elsesong.playendkind=caseyearwhen1850..1889then'Blues'when1890..1909then'Ragtime'when1910..1929then'New Orleans Jazz'when1930..1939then'Swing'when1940..1950then'Bebop'else'Jazz'end

• Use empty lines between defs and to break up a method into logical paragraphs.

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

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

  # 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 at all.

  # badresult=1 - \
  2# good (but still ugly as hell)result=1 \
  - 2

• 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

• 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.

  # 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

• Add underscores to large numeric literals to improve their readability.

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

• Use RDoc and its conventions for API documentation. Don't put an empty line between the comment block and the def.

• Limit lines to 80 characters.

• Avoid trailing whitespace.

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

  # bad
  == begincommentlineanothercommentline
  == end# good# comment line# another comment line

Syntax

• Use :: only to reference constants(this includes classes and modules). Never use :: for method invocation.

  # badSomeClass::some_methodsome_object::some_method# goodSomeClass.some_methodsome_object.some_methodSomeModule::SomeClass::SOME_CONST

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

  # baddefsome_method()# body omittedend# gooddefsome_method# body omittedend# baddefsome_method_with_argumentsarg1,arg2# body omittedend# gooddefsome_method_with_arguments(arg1,arg2)# body omittedend

• Never 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.

  arr=[1,2,3]# badforeleminarrdoputselemend# goodarr.each{ |elem| putselem}

• Never use then for multi-line if/unless.

  # 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.

  # 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.

  # 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.

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

• Never use if x: ... - as of Ruby 1.9 it has been removed. Use the ternary operator instead.

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

• Never use if x; .... Use the ternary operator instead.

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

• Never use when x; .... See the previous rule.

• Use ! instead of not.

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

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

  # 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.

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

  # badifsome_conditiondo_somethingend# gooddo_somethingifsome_condition# another good optionsome_condition && do_something

• Favor unless over if for negative conditions (or control flow ||).

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

• Never use unless with else. Rewrite these with the positive case first.

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

• Don't use parentheses around the condition of an if/unless/while/until.

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

• Never use while/until condition do for multi-line while/until.

  # 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.

  # badwhilesome_conditiondo_somethingend# gooddo_somethingwhilesome_condition

• Favor until over while for negative conditions.

  # baddo_somethingwhile !some_condition# gooddo_somethinguntilsome_condition

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

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

• Omit parentheses around parameters for methods that are part of an internal DSL (e.g. Rake, Rails, RSpec), methods that have "keyword" status in Ruby (e.g. attr_reader, puts) and attribute access methods. Use parentheses around the arguments of all other method invocations.

  classPersonattr_reader:name,:age# omittedendtemperance=Person.new('Temperance',30)temperance.nameputstemperance.agex=Math.sin(y)array.delete(e)bowling.score.should == 0

• Omit parentheses for method calls with no arguments.

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

• Prefer {...} over do...end for single-line blocks. Avoid using {...} for multi-line blocks (multiline 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.

  names=['Bozhidar','Steve','Sarah']# 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{ |name| name.upcase}

  Some will argue that multiline 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?

• Avoid return where not required for flow of control.

  # 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.)

  # 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.

  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.

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

• Use ||= freely to initialize variables.

  # 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.)

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

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

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

• Avoid using Perl-style special variables (like $0-9, $, etc. ). They are quite cryptic and their use in anything but one-liner scripts is discouraged.

• Never put a space between a method name and the opening parenthesis.

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

• If the first argument to a method begins with an open parenthesis, always use parentheses in the method invocation. For example, write f((3 + 2) + 1).

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

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

  # 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

• Prefer proc over Proc.new.

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

• Use _ for unused block parameters.

  # badresult=hash.map{ |k,v| v + 1}# goodresult=hash.map{ |_,v| v + 1}

• 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.

• 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).

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

  # bad'%d %d' % [20,10]# => '20 10'# goodsprintf('%d %d',20,10)# => '20 10'format('%d %d',20,10)# => '20 10'

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

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

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

  # badpaths=[paths]unlesspaths.is_a?Arraypaths.each{ |path| do_something(path)}# good[*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.

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

• Avoid the use of BEGIN blocks.

• Never use END blocks. Use Kernel#at_exit instead.

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

• Avoid the use of flip-flops.

Naming

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

• Name identifiers in English.

  # bad - variable name written in Bulgarian with latin characterszaplata=1_000# goodsalary=1_000

• Use snake_case for symbols, methods and variables.

  # bad:'some symbol':SomeSymbol:someSymbolsomeVar=5defsomeMethod
  ...
  enddefSomeMethod
  ...
  end# good:some_symboldefsome_method
  ...
  end

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

  # badclassSomeclass
  ...
  endclassSome_Class
  ...
  endclassSomeXml
  ...
  end# goodclassSomeClass
  ...
  endclassSomeXML
  ...
  end

• Use SCREAMING_SNAKE_CASE for other constants.

  # 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?).

• 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.

  # bad - there is not matching 'safe' methodclassPersondefupdate!endend# goodclassPersondefupdateendend# goodclassPersondefupdate!enddefupdateendend

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

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

• When using reduce with short blocks, name the arguments |a, e| (accumulator, element).

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

  def +(other)# body omittedend

• 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.

• 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.

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

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!

• Write comments in English.

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

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

• Avoid superfluous comments.

  # badcounter += 1# increments counter by one

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

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

Comment Annotations

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

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

• If multiple lines are required to describe the problem, subsequent lines should be indented two spaces after the #.

  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.

  defbarsleep100# OPTIMIZEend

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

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

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

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

• 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?

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

Classes & Modules

• Use a consistent structure in your class definitions.

  classPerson# extend and include go firstextendSomeModuleincludeAnotherModule# 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# followed by public instance methodsdefsome_methodend# protected and private methods are grouped near the endprotecteddefsome_protected_methodendprivatedefsome_private_methodendend

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

  # badclassSomeClassdefself.some_method# body omittedenddefself.some_other_methodendend# goodmoduleSomeClassmodule_functiondefsome_method# body omittedenddefsome_other_methodendend

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

  # 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.

• Try to make your classes as [SOLID](http://en.wikipedia.org/wiki/SOLID_(object-oriented_design\)) as possible.

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

  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.

  # 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

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

  # bad - creates a single attribute accessor (deprecated in 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.

  # goodclassPersonattr_reader: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 a Struct.new - it already is a new class. Extending it introduces a superfluous class level and may also introduce weird errors if the file is required multiple times.

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

  classPersondefself.create(options_hash)# body omittedendend

• Prefer duck-typing over inheritance.

  # 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.

  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.

• Indent the public, protected, and private methods as much 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.

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

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

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

Exceptions

• Signal exceptions using the fail method. Use raise only when catching an exception and re-raising it (because here you're not failing, but explicitly and purposefully raising an exception).

  beginfail'Oops'rescue=>errorraiseiferror.message != 'Oops'end

• Never 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.

  deffoobeginfailensurereturn'very bad idea'endend

• Use implicit begin blocks where possible.

  # 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).

  # 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.

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

• Avoid using rescue in its modifier form.

  # 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.

  # 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.

  # 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.

  # badbegin# some coderescueException=>e# some handlingrescueStandardError=>e# some handlingend# goodbegin# some coderescueStandardError=>e# some handlingrescueException=>e# some handlingend

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

  f=File.open('testfile')begin# .. processrescue# .. handle errorensuref.closeunlessf.nil?end

• Favor the use of exceptions for the standard library over introducing new exception classes.

Collections

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

  # 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.

  # 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.

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

• Avoid the creation of huge gaps in arrays.

  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].

• 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.

• Prefer symbols instead of strings as hash keys.

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

• Avoid the use of mutable objects as hash keys.

• Use the hash literal syntax when your hash keys are symbols.

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

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

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

• Use fetch with second argument to use a default value

  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

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

• Never modify a collection while traversing it.

Strings

• Prefer string interpolation instead of string concatenation:

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

• Consider padding string interpolation code with space. It more clearly sets the code apart from the string.

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

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

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

• 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).

  # badchar= ?c
  # goodchar='c'

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

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

• 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.

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

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

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

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']

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

  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 captured result of parentheses.

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

• Avoid using $1-9 as it can be hard to track what they contain. Named groups can be used instead.

  # bad/(regexp)/ =~ string
  ...
  process $1
  # good/(?<meaningful_var>regexp)/ =~ string
  ...
  processmeaningful_var

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

• 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/).

  string="some injection\nusername"string[/^username$/]# matchesstring[/\Ausername\z/]# don'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.

  regexp=%r{ 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 block or hash.

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.

  # 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.

  # 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?"'

• Use %r only for regular expressions matching more than one '/' character.

  # bad%r(\s+)# still bad%r(^/(.*)$)# should be /^\/(.*)$/# good%r(^/blog/2011/(.*)$)

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

  # 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 created a symbol with spaces in it.

• Prefer () as delimiters for all % literals, except %r. Given the nature of regexp in many scenarios a less command character than ( might be a better choice for a delimiter.

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

Metaprogramming

• Avoid needless metaprogramming.

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

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

  • 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 I learned from the Rails code):

  # 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}(*args, &block) # def capitalize(*args, &block) to_str.#{unsafe_method}(*args, &block) # to_str.capitalize(*args, &block) end # end def #{unsafe_method}!(*args) # def capitalize!(*args) @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:

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

Misc

• Write ruby -w safe code.

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

• 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.

• Avoid parameter lists longer than three or four parameters.

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

• Use module instance variables instead of global variables.

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

• Avoid alias when alias_method will do.

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

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

• Do not mutate arguments unless that is the purpose of the method.

• Avoid more than three levels of block nesting.

• Be consistent. In an ideal world, be consistent with these guidelines.

• Use common sense.

Tools

Here's 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

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!

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