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

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

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

In Ruby 2.4, String#match?, Regexp#match? and Symbol#match? have been added. The methods are faster than match. Because the methods avoid creating a MatchData object or saving backref. So, when MatchData is not used, use match? instead of match.

Example:

# bad
def foo
  if x =~ /re/
    do_something
  end
end

# bad
def foo
  if x.match(/re/)
    do_something
  end
end

# bad
def foo
  if /re/ === x
    do_something
  end
end

# good
def foo
  if x.match?(/re/)
    do_something
  end
end

# good
def foo
  if x =~ /re/
    do_something(Regexp.last_match)
  end
end

# good
def foo
  if x.match(/re/)
    do_something($~)
  end
end

# good
def foo
  if /re/ === x
    do_something($~)
  end
end

In Ruby 2.4, String#match?, Regexp#match? and Symbol#match? have been added. The methods are faster than match. Because the methods avoid creating a MatchData object or saving backref. So, when MatchData is not used, use match? instead of match.

Example:

# bad
def foo
  if x =~ /re/
    do_something
  end
end

# bad
def foo
  if x.match(/re/)
    do_something
  end
end

# bad
def foo
  if /re/ === x
    do_something
  end
end

# good
def foo
  if x.match?(/re/)
    do_something
  end
end

# good
def foo
  if x =~ /re/
    do_something(Regexp.last_match)
  end
end

# good
def foo
  if x.match(/re/)
    do_something($~)
  end
end

# good
def foo
  if /re/ === x
    do_something($~)
  end
end

Checks for unused block arguments.

Example:

# bad
do_something do |used, unused|
  puts used
end

do_something do |bar|
  puts :foo
end

define_method(:foo) do |bar|
  puts :baz
end

# good
do_something do |used, _unused|
  puts used
end

do_something do
  puts :foo
end

define_method(:foo) do |_bar|
  puts :baz
end

Example: IgnoreEmptyBlocks: true (default)

# good
do_something { |unused| }

Example: IgnoreEmptyBlocks: false

# bad
do_something { |unused| }

Example: AllowUnusedKeywordArguments: false (default)

# bad
do_something do |unused: 42|
  foo
end

Example: AllowUnusedKeywordArguments: true

# good
do_something do |unused: 42|
  foo
end

Checks for redundant access modifiers, including those with no code, those which are repeated, and leading public modifiers in a class or module body. Conditionally-defined methods are considered as always being defined, and thus access modifiers guarding such methods are not redundant.

This cop has ContextCreatingMethods option. The default setting value is an empty array that means no method is specified. This setting is an array of methods which, when called, are known to create its own context in the module's current access context.

It also has MethodCreatingMethods option. The default setting value is an empty array that means no method is specified. This setting is an array of methods which, when called, are known to create other methods in the module's current access context.

Example:

# bad
class Foo
  public # this is redundant (default access is public)

  def method
  end
end

# bad
class Foo
  # The following is redundant (methods defined on the class'
  # singleton class are not affected by the private modifier)
  private

  def self.method3
  end
end

# bad
class Foo
  protected

  define_method(:method2) do
  end

  protected # this is redundant (repeated from previous modifier)

  [1,2,3].each do |i|
    define_method("foo#{i}") do
    end
  end
end

# bad
class Foo
  private # this is redundant (no following methods are defined)
end

# good
class Foo
  private # this is not redundant (a method is defined)

  def method2
  end
end

# good
class Foo
  # The following is not redundant (conditionally defined methods are
  # considered as always defining a method)
  private

  if condition?
    def method
    end
  end
end

# good
class Foo
  protected # this is not redundant (a method is defined)

  define_method(:method2) do
  end
end

Example: ContextCreatingMethods: concerning

# Lint/UselessAccessModifier:
#   ContextCreatingMethods:
#     - concerning

# good
require 'active_support/concern'
class Foo
  concerning :Bar do
    def some_public_method
    end

    private

    def some_private_method
    end
  end

  # this is not redundant because `concerning` created its own context
  private

  def some_other_private_method
  end
end

Example: MethodCreatingMethods: delegate

# Lint/UselessAccessModifier:
#   MethodCreatingMethods:
#     - delegate

# good
require 'active_support/core_ext/module/delegation'
class Foo
  # this is not redundant because `delegate` creates methods
  private

  delegate :method_a, to: :method_b
end

Checks that deprecated attributes are not set in a gemspec file. Removing deprecated attributes allows the user to receive smaller packed gems.

Example:

# bad
Gem::Specification.new do |spec|
  spec.name = 'your_cool_gem_name'
  spec.test_files = Dir.glob('test/**/*')
end

# bad
Gem::Specification.new do |spec|
  spec.name = 'your_cool_gem_name'
  spec.test_files += Dir.glob('test/**/*')
end

# good
Gem::Specification.new do |spec|
  spec.name = 'your_cool_gem_name'
end

Checks for every useless assignment to local variable in every scope. The basic idea for this cop was from the warning of ruby -cw:

assigned but unused variable - foo

Currently this cop has advanced logic that detects unreferenced reassignments and properly handles varied cases such as branch, loop, rescue, ensure, etc.

NOTE: Given the assignment foo = 1, bar = 2, removing unused variables can lead to a syntax error, so this case is not autocorrected.

Safety:

This cop's autocorrection is unsafe because removing assignment from operator assignment can cause NameError if this assignment has been used to declare local variable. For example, replacing a ||= 1 to a || 1 may cause "undefined local variable or method `a' for main:Object (NameError)".

Example:

# bad

def some_method
  some_var = 1
  do_something
end

Example:

# good

def some_method
  some_var = 1
  do_something(some_var)
end

Checks for private or protected access modifiers which are applied to a singleton method. These access modifiers do not make singleton methods private/protected. private_class_method can be used for that.

Example:

# bad

class C
  private

  def self.method
    puts 'hi'
  end
end

Example:

# good

class C
  def self.method
    puts 'hi'
  end

  private_class_method :method
end

Example:

# good

class C
  class << self
    private

    def method
      puts 'hi'
    end
  end
end

This cop identifies places where gsub can be replaced by tr or delete.

Example:

# bad
'abc'.gsub('b', 'd')
'abc'.gsub('a', '')
'abc'.gsub(/a/, 'd')
'abc'.gsub!('a', 'd')

# good
'abc'.gsub(/.*/, 'a')
'abc'.gsub(/a+/, 'd')
'abc'.tr('b', 'd')
'a b c'.delete(' ')

Checks for the use of Kernel#open and URI.open with dynamic data.

Kernel#open and URI.open enable not only file access but also process invocation by prefixing a pipe symbol (e.g., open("| ls")). So, it may lead to a serious security risk by using variable input to the argument of Kernel#open and URI.open. It would be better to use File.open, IO.popen or URI.parse#open explicitly.

NOTE: open and URI.open with literal strings are not flagged by this cop.

Safety:

This cop could register false positives if open is redefined in a class and then used without a receiver in that class.

Example:

# bad
open(something)
open("| #{something}")
URI.open(something)

# good
File.open(something)
IO.popen(something)
URI.parse(something).open

# good (literal strings)
open("foo.text")
open("| foo")
URI.open("http://example.com")

Checks for excessive nesting of conditional and looping constructs.

You can configure if blocks are considered using the CountBlocks option. When set to false (the default) blocks are not counted towards the nesting level. Set to true to count blocks as well.

The maximum level of nesting allowed is configurable.

Checks for excessive nesting of conditional and looping constructs.

You can configure if blocks are considered using the CountBlocks option. When set to false (the default) blocks are not counted towards the nesting level. Set to true to count blocks as well.

The maximum level of nesting allowed is configurable.

Checks for private or protected access modifiers which are applied to a singleton method. These access modifiers do not make singleton methods private/protected. private_class_method can be used for that.

Example:

# bad

class C
  private

  def self.method
    puts 'hi'
  end
end

Example:

# good

class C
  def self.method
    puts 'hi'
  end

  private_class_method :method
end

Example:

# good

class C
  class << self
    private

    def method
      puts 'hi'
    end
  end
end