Cognitive Complexity

Cognitive Complexity is a measure of how difficult a unit of code is to intuitively understand. Unlike Cyclomatic Complexity, which determines how difficult your code will be to test, Cognitive Complexity tells you how difficult your code will be to read and comprehend.

A method's cognitive complexity is based on a few simple rules:

• Code is not considered more complex when it uses shorthand that the language provides for collapsing multiple statements into one
• Code is considered more complex for each "break in the linear flow of the code"
• Code is considered more complex when "flow breaking structures are nested"

Further reading

• Cognitive Complexity docs
• Cognitive Complexity: A new way of measuring understandability

Checks that the cyclomatic complexity of methods is not higher than the configured maximum. The cyclomatic complexity is the number of linearly independent paths through a method. The algorithm counts decision points and adds one.

An if statement (or unless or ?:) increases the complexity by one. An else branch does not, since it doesn't add a decision point. The && operator (or keyword and) can be converted to a nested if statement, and ||/or is shorthand for a sequence of ifs, so they also add one. Loops can be said to have an exit condition, so they add one. Blocks that are calls to builtin iteration methods (e.g. `ary.map{...}) also add one, others are ignored.

def each_child_node(*types)               # count begins: 1
  unless block_given?                     # unless: +1
    return to_enum(__method__, *types)

  children.each do |child|                # each{}: +1
    next unless child.is_a?(Node)         # unless: +1

    yield child if types.empty? ||        # if: +1, ||: +1
                   types.include?(child.type)
  end

  self
end                                       # total: 6

Checks for assignments in the conditions of if/while/until.

AllowSafeAssignment option for safe assignment. By safe assignment we mean putting parentheses around an assignment to indicate "I know I'm using an assignment as a condition. It's not a mistake."

Safety:

This cop's autocorrection is unsafe because it assumes that the author meant to use an assignment result as a condition.

Example:

# bad
if some_var = true
  do_something
end

# good
if some_var == true
  do_something
end

Example: AllowSafeAssignment: true (default)

# good
if (some_var = true)
  do_something
end

Example: AllowSafeAssignment: false

# bad
if (some_var = true)
  do_something
end

Requires a gemspec to have rubygems_mfa_required metadata set.

This setting tells RubyGems that MFA (Multi-Factor Authentication) is required for accounts to be able perform privileged operations, such as (see RubyGems' documentation for the full list of privileged operations):

• gem push
• gem yank
• gem owner --add/remove
• adding or removing owners using gem ownership page

This helps make your gem more secure, as users can be more confident that gem updates were pushed by maintainers.

Example:

# bad
Gem::Specification.new do |spec|
  # no `rubygems_mfa_required` metadata specified
end

# good
Gem::Specification.new do |spec|
  spec.metadata = {
    'rubygems_mfa_required' => 'true'
  }
end

# good
Gem::Specification.new do |spec|
  spec.metadata['rubygems_mfa_required'] = 'true'
end

# bad
Gem::Specification.new do |spec|
  spec.metadata = {
    'rubygems_mfa_required' => 'false'
  }
end

# good
Gem::Specification.new do |spec|
  spec.metadata = {
    'rubygems_mfa_required' => 'true'
  }
end

# bad
Gem::Specification.new do |spec|
  spec.metadata['rubygems_mfa_required'] = 'false'
end

# good
Gem::Specification.new do |spec|
  spec.metadata['rubygems_mfa_required'] = 'true'
end

Checks for the presence of constructors and lifecycle callbacks without calls to super.

This cop does not consider method_missing (and respond_to_missing?) because in some cases it makes sense to overtake what is considered a missing method. In other cases, the theoretical ideal handling could be challenging or verbose for no actual gain.

Autocorrection is not supported because the position of super cannot be determined automatically.

Object and BasicObject are allowed by this cop because of their stateless nature. However, sometimes you might want to allow other parent classes from this cop, for example in the case of an abstract class that is not meant to be called with super. In those cases, you can use the AllowedParentClasses option to specify which classes should be allowed in addition to Object and BasicObject.

Example:

# bad
class Employee < Person
  def initialize(name, salary)
    @salary = salary
  end
end

# good
class Employee < Person
  def initialize(name, salary)
    super(name)
    @salary = salary
  end
end

# bad
Employee = Class.new(Person) do
  def initialize(name, salary)
    @salary = salary
  end
end

# good
Employee = Class.new(Person) do
  def initialize(name, salary)
    super(name)
    @salary = salary
  end
end

# bad
class Parent
  def self.inherited(base)
    do_something
  end
end

# good
class Parent
  def self.inherited(base)
    super
    do_something
  end
end

# good
class ClassWithNoParent
  def initialize
    do_something
  end
end

Example: AllowedParentClasses: [MyAbstractClass]

# good
class MyConcreteClass < MyAbstractClass
  def initialize
    do_something
  end
end

Checks for classes and metaclasses without a body. Such empty classes and metaclasses are typically an oversight or we should provide a comment to be clearer what we're aiming for.

Example:

# bad
class Foo
end

class Bar
  class << self
  end
end

class << obj
end

# good
class Foo
  def do_something
    # ... code
  end
end

class Bar
  class << self
    attr_reader :bar
  end
end

class << obj
  attr_reader :bar
end

Example: AllowComments: false (default)

# bad
class Foo
  # TODO: implement later
end

class Bar
  class << self
    # TODO: implement later
  end
end

class << obj
  # TODO: implement later
end

Example: AllowComments: true

# good
class Foo
  # TODO: implement later
end

class Bar
  class << self
    # TODO: implement later
  end
end

class << obj
  # TODO: implement later
end

Checks for assignments in the conditions of if/while/until.

AllowSafeAssignment option for safe assignment. By safe assignment we mean putting parentheses around an assignment to indicate "I know I'm using an assignment as a condition. It's not a mistake."

Safety:

This cop's autocorrection is unsafe because it assumes that the author meant to use an assignment result as a condition.

Example:

# bad
if some_var = true
  do_something
end

# good
if some_var == true
  do_something
end

Example: AllowSafeAssignment: true (default)

# good
if (some_var = true)
  do_something
end

Example: AllowSafeAssignment: false

# bad
if (some_var = true)
  do_something
end

Checks for literals with extremely many entries. This is indicative of configuration or data that may be better extracted somewhere else, like a database, fetched from an API, or read from a non-code file (CSV, JSON, YAML, etc.).

Example:

# bad
# Huge Array literal
[1, 2, '...', 999_999_999]

# bad
# Huge Hash literal
{ 1 => 1, 2 => 2, '...' => '...', 999_999_999 => 999_999_999}

# bad
# Huge Set "literal"
Set[1, 2, '...', 999_999_999]

# good
# Reasonably sized Array literal
[1, 2, '...', 10]

# good
# Reading huge Array from external data source
# File.readlines('numbers.txt', chomp: true).map!(&:to_i)

# good
# Reasonably sized Hash literal
{ 1 => 1, 2 => 2, '...' => '...', 10 => 10}

# good
# Reading huge Hash from external data source
CSV.foreach('numbers.csv', headers: true).each_with_object({}) do |row, hash|
  hash[row["key"].to_i] = row["value"].to_i
end

# good
# Reasonably sized Set "literal"
Set[1, 2, '...', 10]

# good
# Reading huge Set from external data source
SomeFramework.config_for(:something)[:numbers].to_set

Checks for rescue blocks targeting the Exception class.

Example:

# bad

begin
  do_something
rescue Exception
  handle_exception
end

Example:

# good

begin
  do_something
rescue ArgumentError
  handle_exception
end

Checks for rescue blocks with no body.

Example:

# bad
def some_method
  do_something
rescue
end

# bad
begin
  do_something
rescue
end

# good
def some_method
  do_something
rescue
  handle_exception
end

# good
begin
  do_something
rescue
  handle_exception
end

Example: AllowComments: true (default)

# good
def some_method
  do_something
rescue
  # do nothing
end

# good
begin
  do_something
rescue
  # do nothing
end

Example: AllowComments: false

# bad
def some_method
  do_something
rescue
  # do nothing
end

# bad
begin
  do_something
rescue
  # do nothing
end

Example: AllowNil: true (default)

# good
def some_method
  do_something
rescue
  nil
end

# good
begin
  do_something
rescue
  # do nothing
end

# good
do_something rescue nil

Example: AllowNil: false

# bad
def some_method
  do_something
rescue
  nil
end

# bad
begin
  do_something
rescue
  nil
end

# bad
do_something rescue nil

Checks that there are no repeated bodies within if/unless, case-when, case-in and rescue constructs.

With IgnoreLiteralBranches: true, branches are not registered as offenses if they return a basic literal value (string, symbol, integer, float, rational, complex, true, false, or nil), or return an array, hash, regexp or range that only contains one of the above basic literal values.

With IgnoreConstantBranches: true, branches are not registered as offenses if they return a constant value.

Example:

# bad
if foo
  do_foo
  do_something_else
elsif bar
  do_foo
  do_something_else
end

# good
if foo || bar
  do_foo
  do_something_else
end

# bad
case x
when foo
  do_foo
when bar
  do_foo
else
  do_something_else
end

# good
case x
when foo, bar
  do_foo
else
  do_something_else
end

# bad
begin
  do_something
rescue FooError
  handle_error
rescue BarError
  handle_error
end

# good
begin
  do_something
rescue FooError, BarError
  handle_error
end

Example: IgnoreLiteralBranches: true

# good
case size
when "small" then 100
when "medium" then 250
when "large" then 1000
else 250
end

Example: IgnoreConstantBranches: true

# good
case size
when "small" then SMALL_SIZE
when "medium" then MEDIUM_SIZE
when "large" then LARGE_SIZE
else MEDIUM_SIZE
end