GitHub

Source Code Layout
Comments
Naming
General Syntax
Strings
Conditionals
Collections
Iteration
Methods
Exceptions
Regular Expressions
Blocks / Procs
Classes
Misc
Rails
References

##1. Source Code Layout

Use UTF-8 as the source file encoding.
Use two spaces per indentation level, no tabs.
Leave no trailing whitespace of any kind.
Do not leave a blank line at the bottom of the file.
Use Unix-style line endings. (*BSD/Solaris/Linux/OSX users are covered by default, Windows users have to be extra careful.)
Keep lines 94 characters wide or less.

##2. Comments

Write self-documenting code. If feel you need a comment, refactor the code until a comment feels like overkill. Then, ignore the rest of this section.
Comments longer than a word are capitalized and use punctuation.

Avoid superfluous comments.

# bad
counter += 1 # increments counter by one

Keep existing comments up-to-date. An outdated is worse than no comment at all.
Avoid writing comments to explain bad code. Refactor the code to make it self-explanatory.

Avoid code banners. Your class is probably too big if you need them. Refactor!

################################################
# Code banners are a code smell
################################################

Parameter explanations, examples, return descriptions, etc., are often overkill, but you should consider adding them to public API methods. If you do add them, follow tomdoc's conventions.

Write inline comments like this:

# This is a description of the line.
@pages.each { |p| puts p.name }

Write comments for methods, classes, modules, etc., like this:

# Print a log line to STDOUT. You can customize the output by specifying
# a block.
#
# msgs  - Zero or more String messages that will be printed to the log
#         separated by spaces.
# block - An optional block that can be used to customize the date format.
#         If it is present, it will be sent a Time object representing
#         the current time. Your block should return a String version of
#         the time, formatted however you please.
#
# Examples
#
#   log("An error occurred.")
#
#   log("No such file", "/var/log/server.log") do |time|
#     time.strftime("%Y-%m-%d %H:%M:%S")
#   end
#
# Returns nothing.
#
def log(*msgs, &block)
  ...
end

##3. Naming

Use snake_case for methods and variables.
Use CamelCase for classes and modules. (Keep acronyms like HTTP, RFC, XML uppercase.)
Use SCREAMING_SNAKE_CASE for other constants.
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 and only if there exists a safe version of that dangerous method.
```
# bad - there is not matching 'safe' method
class Person
  def update!
  end
end

# good
class Person
  def update
  end
end

# good
class Person
  def update!
  end

  def update
  end
end
```

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

class Array
  def flatten_once!
    res = []

    each do |e|
      [*e].each { |f| res << f }
    end

    replace(res)
  end

  def flatten_once
    dup.flatten_once!
  end
end

When defining binary operators, name the argument other.
```
def +(other)
  # body omitted
end
```

##4. General Syntax

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 + 2
a, b = 1, 2
1 > 2 ? true : false; puts 'Hi'
[1, 2, 3].each { |e| puts e }
```
The only exception is when using the exponent operator:
```
# bad
e = M * c ** 2

# good
e = M * c**2
```
No spaces after (, [ or before ], ).
```
some(arg).other
[1, 2, 3].length
```

##5. Strings

Prefer string interpolation instead of string concatenation:

# bad
email_with_name = user.name + ' <' + user.email + '>'

# good
email_with_name = "#{user.name} <#{user.email}>"

Prefer single-quoted strings when you don't need string interpolation or special symbols such as \t, \n, ', etc.
```
# bad
name = "Bozhidar"

# good
name = 'Bozhidar'
```

Don't use {} around instance variables being interpolated into a string.

class Person
  attr_reader :first_name, :last_name

  def initialize(first_name, last_name)
    @first_name = first_name
    @last_name = last_name
  end

  # bad
  def to_s
    "#{@first_name} #{@last_name}"
  end

  # good
  def to_s
    "#@first_name #@last_name"
  end
end

Use %() 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 and %Q.
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 fast
html = ''
html << '<h1>Page title</h1>'

paragraphs.each do |paragraph|
  html << "<p>#{paragraph}</p>"
end
```

##6. Conditionals

Indent when as deep as case.

case
when song.name == 'Misty'
  puts 'Not again!'
when song.duration > 120
  puts 'Too long!'
when Time.now.hour > 21
  puts "It's too late"
else
  song.play
end

kind = case year
       when 1850..1889 then 'Blues'
       when 1890..1909 then 'Ragtime'
       when 1910..1929 then 'New Orleans Jazz'
       when 1930..1939 then 'Swing'
       when 1940..1950 then 'Bebop'
       else 'Jazz'
       end

Use when x then ... for one-line conditions in case blocks.

Never use then for multi-line if/unless.

# bad
if some_condition then
  # body omitted
end

# good
if some_condition
  # body omitted
end

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

# bad
result = if some_condition then something else something_else end

# good
result = some_condition ? something : something_else

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

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.

# bad
some_condition ? (nested_condition ? nested_something : nested_something_else) : something_else

# good
if some_condition
  nested_condition ? nested_something : nested_something_else
else
  something_else
end

Never use if x; .... Use the ternary operator instead.
Never use when x; .... See the previous rule.
Use &&/|| for boolean expressions, and/or for control flow. (Rule of thumb: If you have to use outer parentheses, you are using the wrong operators.)
```
# boolean expression
if some_condition && some_other_condition
  do_something
end

# control flow
document.saved? or document.save!
```

Favour modifier if/unless usage when you have a single-line body.

# bad
if some_condition
  do_something
end

# good
do_something if some_condition

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

# bad
do_something if !some_condition

# good
do_something unless some_condition

# another good option
some_condition or do_something

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

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

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

Don't use parentheses around the condition of an if/unless/while, unless the condition contains an assignment (see "Using the return value of =" below).

# bad
if (x > 10)
  # body omitted
end

# good
if x > 10
  # body omitted
end

# ok
if (x = self.next_value)
  # body omitted
end

Favour ! over not. The latter binds more loosely, and can lead to confusing results.

##7. Collections

Prefer literal array and hash creation notation (unless you need to pass parameters to their constructors, that is).
```
# bad
arr = Array.new
hash = Hash.new

# good
arr = []
hash = {}
```

Prefer %w to the literal array syntax when you need an array of single-word strings.

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

# good
STATES = %w(draft open closed)

Use parentheses are the delimeters for %w.

# bad
STATES = %w[draft open closed]

# good
STATES = %w(draft open closed)

Avoid %W.

Avoid the creation of huge gaps in arrays.

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

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.

Use symbols instead of strings as hash keys.

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

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

Avoid the use of mutable objects as hash keys.

Use the new Ruby 1.9 has syntax whenever possible. For cases in which the hash keys cannot be symbolized, use the old syntax.

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

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

Rely on the fact that hashes in 1.9 are ordered.

Leave a single space padding inside the braces of a hash.

# bad
hash = {one: 1, two: 2}

# good
hash = { one: 1, two: 2 }

Leave no padding inside the brackets of an array.

# bad
array = [ 1, 2, 3 ]

# good
array = [1, 2, 3]

Use a single line for arrays and hashes if they will fit.

Format multiline hashes and arrays by indenting two-spaces.

hash = {
  name:             'peter',
  professional:     'teacher',
  favourite_color:  'green'
}

array = [
  'Happy Sauce',
  'Awesome people eating computers'
]

You may optionally align hash values if it improves readability. Especially when there are many keys.

hash = {
  name:             'peter',
  professional:     'teacher',
  favourite_color:  'green'
}

Prefer size over length for getting the number of elements.

##8. Iteration

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]

# bad
for elem in arr do
  puts elem
end

# good
arr.each { |elem| puts elem }
```

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

# bad
while some_condition
  do_something
end

# good
do_something while some_condition

Favour until over while for negative conditions.

# bad
do_something while !some_condition

# good
do_something until some_condition

Never modify a collection while traversing it.
Know the iterators provided to you by Hash, Array, and enumerable. Don't re-invent the wheel.
Prefer map over collect, find over detect, select over find_all, reduce over inject.
When using reduce with short blocks, name the arguments |a, e| (accumulator, element).

##9. Methods

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

def some_method
  data = initialize(options)

  data.manipulate!

  data.result
end

def some_method
  result
end

Indent the parameters of a method call if they span over multiple lines.

# starting point (line is too long)
def send_mail(source)
  Mailer.deliver(to: 'bob@example.com', from: 'us@example.com', subject: 'Important message', body: source.text)
end

# bad
Mailer.deliver(to: 'bob@example.com',
               from: 'us@example.com',
               subject: 'Important message',
               body: source.text)

# good
Mailer.deliver(
  to:         'bob@example.com',
  from:       'us@example.com',
  subject:    'Important message',
  body:       source.text
)

# also good
create(
  :invitation,
  email:      'bob@example.com',
  role:       :author
)

Use def with parentheses when there are arguments. Omit the parentheses when the method doesn't accept any arguments.
```
def some_method
  # body omitted
end

def some_method_with_arguments(arg1, arg2)
  # body omitted
end
```
Omit parentheses around parameters for methods that are part of an internal DSL (e.g. Rake, Rails, RSpec), methods that are with "keyword" status in Ruby (e.g. attr_reader, puts) and attribute access methods. Use parentheses around the arguments of all other method invocations.
```
class Person
  attr_reader :name, :age

  # omitted
end

temperance = Person.new('Temperance', 30)
temperance.name

puts temperance.age

x = Math.sin(y)
array.delete(e)
```

Avoid return where not required.

# bad
def some_method(some_arr)
  return some_arr.size
end

# good
def some_method(some_arr)
  some_arr.size
end

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

# bad
def some_method(arg1=:default, arg2=nil, arg3=[])
  # do something...
end

# good
def some_method(arg1 = :default, arg2 = nil, arg3 = [])
  # do something...
end

Never put a space between a method name and the opening parenthesis.
```
# bad
f (3 + 2) + 1

# good
f(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).

If the last parameter is a hash, don't use curly brackets around the arguments.

# bad
do_something({
  param1: value1,
  param2: value2
})

# good
do_something(
  param1: value1,
  param2: value2
)

##10. Exceptions

Use e as the rescued variable:
```
rescue StandardError => e
```
Signal exceptions using the fail keyword. Use raise only when catching an exception and re-raising it (because here you're not failing, but explicitly and purposefully raising an exception).
```
begin
  fail 'Oops';
rescue => e
  raise if e.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.
```
def foo
  begin
    fail
  ensure
    return 'very bad idea'
  end
end
```

Use implicit begin blocks when possible.

# bad
def foo
  begin
    # main logic goes here
  rescue
    # failure handling goes here
  end
end

# good
def foo
  # main logic goes here
rescue
  # failure handling goes here
end

Don't suppress exceptions.

# bad
begin
  # an exception occurs here
rescue SomeError
  # the rescue clause does absolutely nothing
end

# bad
do_something rescue nil

Don't use exceptions for flow of control.

# bad
begin
  n / d
rescue ZeroDivisionError
  puts 'Cannot divide by 0!'
end

# good
if d.zero?
  puts 'Cannot divide by 0!'
else
  n / d
end

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

# bad
begin
  # calls to exit and kill signals will be caught (except kill -9)
  exit
rescue Exception
  puts "you didn't really want to exit, right?"
  # exception handling
end

# good
begin
  # a blind rescue rescues from StandardError, not Exception as many
  # programmers assume.
rescue => e
  # exception handling
end

# also good
begin
  # an exception occurs here

rescue StandardError => e
  # exception handling
end

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

# bad
begin
  # some code
rescue Exception => e
  # some handling
rescue StandardError => e
  # some handling
end

# good
begin
  # some code
rescue StandardError => e
  # some handling
rescue Exception => e
  # some handling
end

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

f = File.open('testfile')
begin
  # .. process
rescue
  # .. handle error
ensure
  f.close unless f.nil?
end

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

##11. Regular Expressions

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 regexp
first_group = string[/text(grp)/, 1] # get content of captured group
string[/text (grp)/, 1] = 'replace'  # string => 'text replace'

Use non capturing groups when you don't use captured result of parenthesis.
```
/(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
...
process meaningful_var

Character classes have only 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$/]   # matches
string[/\Ausername\z/] # don't match
```

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

# bad
%r(\s+)

# still bad
%r(^/(.*)$)
# should be /^\/(.*)$/

# good
%r(^/blog/2011/(.*)$)

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.

##12. Blocks / Procs

Use {...} over do...end for single-line blocks.

# good
names.each { |name| puts name }

# bad
names.each do |name|
  puts name
end

Use {...} over do...end when chaining.

# good
names.select { |name| name.start_with?('S') }.map { |name| name.upcase }

# good
expect {
  page.publish
}.to raise_error(StandardError)

# bad
names.select do |name|
  name.start_with?('S')
end.map { |name| name.upcase }

Use the old lambda syntax over the new literal syntax. NOTE: We prefer the new syntax, but are staying with the old for consistency with the existing codebase.
```
# bad
lambda = ->(a, b) { a + b }
lambda.(1, 2)

# good
lambda = lambda { |a, b| a + b }
lambda.call(1, 2)
```

Use _ for unused block parameters.

# bad
result = hash.map { |k, v| v + 1 }

# good
result = hash.map { |_, v| v + 1 }

##13. Classes

Avoid self where not required.

# bad
def ready?
  if self.last_reviewed_at > self.last_updated_at
    self.worker.update(self.content, self.options)
    self.status = :in_progress
  end
  self.status == :verified
end

# good
def ready?
  if last_reviewed_at > last_updated_at
    worker.update(content, options)
    self.status = :in_progress
  end
  status == :verified
end

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

class Foo
  attr_accessor :options

  # ok
  def initialize(options)
    self.options = options
    # both options and self.options are equivalent here
  end

  # bad
  def do_something(options = {})
    unless options[:when] == :later
      output(self.options[:message])
    end
  end

  # good
  def do_something(params = {})
    unless params[:when] == :later
      output(options[:message])
    end
  end
end

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

# bad
class Person
  def initialize(first_name, last_name)
    @first_name = first_name
    @last_name = last_name
  end

  def first_name
    @first_name
  end

  def last_name
    @last_name
  end
end

# good
class Person
  attr_reader :first_name, :last_name

  def initialize(first_name, last_name)
    @first_name = first_name
    @last_name = last_name
  end
end

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

class Parent
  @@class_var = 'parent'

  def self.print_class_var
    puts @@class_var
  end
end

class Child < Parent
  @@class_var = 'child'
end

Parent.print_class_var # => will print "child"

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

Assign proper visibility levels to methods (private, protected) in accordance with their intended usage. Don't go off leaving everything public (which is the default).
Prefer private visibility over protected unless you truly know you need the latter.

Indent private and protected methods.

class SomeClass

  def public_method
    # ...
  end

  private

    def private_method
      # ...
    end

end

Order methods in your classes by visibility: public, then protected, and private at the bottom.
Order class methods before instance methods.

Use def self.method to define singleton methods. This makes the methods more resistant to refactoring changes.

class TestClass
  # bad
  def TestClass.some_method
    # body omitted
  end

  # good
  def self.some_other_method
    # body omitted
  end

  # Also possible and convenient when you
  # have to define many singleton methods.
  class << self
    def first_method
      # body omitted
    end

    def second_method_etc
      # body omitted
    end
  end
end

##14. Misc

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 have to, add "global" methods to Kernel and make them private.

Use class instance variables instead of global variables.

#bad
$foo_bar = 1

#good
class Foo
  class << self
    attr_accessor :bar
  end
end

Foo.bar = 1

Avoid alias when alias_method will do.
Avoid needless metaprogramming.
Do not mutate arguments unless that is the purpose of the method.
Avoid more than three levels of block nesting.

Avoid line continuation (\) where not required. In practice, avoid using line continuations at all.

# bad
result = 1 - \
         2

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

Using the return value of = (an assignment) is ok, but surround the assignment with parenthesis.

# good - shows intended use of assignment
if (v = array.grep(/foo/)) ...

# bad
if v = array.grep(/foo/) ...

# also good - shows intended use of assignment and has correct precedence.
if (v = self.next_value) == 'hello' ...

Use ||= freely to initialize variables.

# set name to Bozhidar, only if it's nil or false
name ||= 'Bozhidar'

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

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

# good
enabled = true if enabled.nil?

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.
Prefer () as delimiters for all % literals.

##15. Rails

See the examples in model.rb, model_spec.rb, factory.rb, and view.html.erb for canonical examples.

##16. References

Read these books and guides:

Eloquent Ruby
The Well-grounded Rubyist
Rails guides
Clean Code (Robert C. Martin)

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