Class: RuboCop::Cop::Lint::UnexpectedBlockArity

Relationships & Source Files
Super Chains via Extension / Inclusion / Inheritance
Class Chain:
self, ::RuboCop::Cop::Base, ::RuboCop::ExcludeLimit, NodePattern::Macros, RuboCop::AST::Sexp
Instance Chain:
Inherits: RuboCop::Cop::Base
Defined in: lib/rubocop/cop/lint/unexpected_block_arity.rb


Checks for a block that is known to need more positional block arguments than are given (by default this is configured for Enumerable methods needing 2 arguments). Optional arguments are allowed, although they don’t generally make sense as the default value will be used. Blocks that have no receiver, or take splatted arguments (ie. *args) are always accepted.

Keyword arguments (including **kwargs) do not get counted towards this, as they are not used by the methods in question.

Method names and their expected arity can be configured like this:

  inject: 2
  reduce: 2


# bad
values.reduce {}
values.min { |a| a }
values.sort { |a; b| a + b }

# good
values.reduce { |memo, obj| memo << obj }
values.min { |a, b| a <=> b }
values.sort { |*x| x[0] <=> x[1] }

Cop Safety Information:

  • This cop matches for method names only and hence cannot tell apart methods with same name in different classes, which may lead to a false positive.

Constant Summary

::RuboCop::Cop::Base - Inherited


Class Attribute Summary

::RuboCop::Cop::Base - Inherited

.gem_requirements, .lint?,

Returns if class supports autocorrect.


Override if your cop should be called repeatedly for multiple investigations Between calls to on_new_investigation and on_investigation_end, the result of processed_source will remain constant.

Class Method Summary

::RuboCop::Cop::Base - Inherited


List of cops that should not try to autocorrect at the same time as this cop.



.callbacks_needed, .cop_name, .department,

Returns an url to view this cops documentation online.


Call for abstract Cop classes.


Override and return the Force class(es) you need to join.


Returns true if the cop name or the cop namespace matches any of the given names.


Register a version requirement for the given gem name.


::RuboCop::ExcludeLimit - Extended


Sets up a configuration option to have an exclude limit tracked.


Instance Attribute Summary

Instance Method Summary

::RuboCop::Cop::Base - Inherited


Adds an offense that has no particular location.


Adds an offense on the specified range (or node with an expression) Unless that offense is disabled for this range, a corrector will be yielded to provide the cop the opportunity to autocorrect the offense.


Called before any investigation.


Configuration Helpers.

#cop_name, #excluded_file?,

This method should be overridden when a cop’s behavior depends on state that lives outside of these locations:


Gets called if no message is specified when calling add_offense or add_global_offense Cops are discouraged to override this; instead pass your message directly.


Alias for Base#cop_name.


Called after all on_…​


Called before all on_…​


Called instead of all on_…​


There should be very limited reasons for a Cop to do it’s own parsing.


Called between investigations.

#relevant_file?, #target_rails_version, #target_ruby_version, #annotate, #apply_correction, #attempt_correction,

Reserved for Cop::Cop.


Called to complete an investigation.

#correct, #current_corrector,

Reserved for Commissioner:

#current_offenses, #currently_disabled_lines, #custom_severity, #default_severity, #disable_uncorrectable, #enabled_line?, #file_name_matches_any?, #find_message, #find_severity, #range_for_original, #range_from_node_or_range,

Actually private methods.


::RuboCop::Cop::AutocorrectLogic - Included

::RuboCop::Cop::IgnoredNode - Included

Constructor Details

This class inherits a constructor from RuboCop::Cop::Base

Instance Method Details

#acceptable?(node) ⇒ Boolean (private)

[ GitHub ]

# File 'lib/rubocop/cop/lint/unexpected_block_arity.rb', line 63

def acceptable?(node)
  !(included_method?(node.method_name) && node.receiver)

#arg_count(node) (private)

[ GitHub ]

# File 'lib/rubocop/cop/lint/unexpected_block_arity.rb', line 75

def arg_count(node)
  return node.children[1] if node.numblock_type? # the maximum numbered param for the block

  # Only `arg`, `optarg` and `mlhs` (destructuring) count as arguments that
  # can be used. Keyword arguments are not used for these methods so are
  # ignored.
  node.arguments.count do |arg|
    return Float::INFINITY if arg.restarg_type?

    arg.arg_type? || arg.optarg_type? || arg.mlhs_type?

#expected_arity(method) (private)

[ GitHub ]

# File 'lib/rubocop/cop/lint/unexpected_block_arity.rb', line 71

def expected_arity(method)

#included_method?(name) ⇒ Boolean (private)

[ GitHub ]

# File 'lib/rubocop/cop/lint/unexpected_block_arity.rb', line 67

def included_method?(name)

#methods (private)

[ GitHub ]

# File 'lib/rubocop/cop/lint/unexpected_block_arity.rb', line 59

def methods
  cop_config.fetch('Methods', [])

#on_block(node) Also known as: #on_numblock

[ GitHub ]

# File 'lib/rubocop/cop/lint/unexpected_block_arity.rb', line 44

def on_block(node)
  return if acceptable?(node)

  expected = expected_arity(node.method_name)
  actual = arg_count(node)
  return if actual >= expected

  message = format(MSG, method: node.method_name, expected: expected, actual: actual)
  add_offense(node, message: message)


Alias for #on_block.

[ GitHub ]

# File 'lib/rubocop/cop/lint/unexpected_block_arity.rb', line 55

alias on_numblock on_block