This rule raises an issue when the first parameter of a class method is not an accepted name.

== Why is this an issue?

By convention, the first argument in a class method, i.e. methods decorated with ``++@classmethod++``, is named ``++cls++`` as a representation and a reminder that the argument is the class itself. If you were to name the argument something else, you would stand a good chance of confusing both users and maintainers of the code. It might also indicate that the ``++cls++`` parameter was forgotten, in which case calling the method will most probably fail. This rule also applies to methods ``++__init_subclass__++``, ``++__class_getitem__++`` and ``++__new__++`` as their first argument is always the class instead of "self".

By default this rule accepts ``++cls++`` and ``++mcs++``, which is sometime used in metaclasses, as valid names for class parameters. You can set your own list of accepted names via the parameter ``++classParameterNames++``.


== How to fix it
Follow the naming convention for the first parameter name of a class method.

=== Code examples

==== Noncompliant code example

[source,python,diff-id=1,diff-type=noncompliant]
----
class Rectangle(object):

  @classmethod
  def area(bob, height, width):  #Noncompliant
    return height * width
----


==== Compliant solution

[source,python,diff-id=1,diff-type=compliant]
----
class Rectangle(object):

  @classmethod
  def area(cls, height, width):
    return height * width
----


== Resources

=== Documentation

* PEP8 - https://www.python.org/dev/peps/pep-0008/#function-and-method-arguments[Function and Method Arguments]



ifdef::env-github,rspecator-view[]

'''
== Implementation Specification
(visible only on this page)

=== Message

Rename XXX to a valid class parameter name or add the missing class parameter.


=== Parameters

.classParameterNames
****
_string_

----
cls,mcs
----

Comma separated list of accepted values for a class parameter.
****


=== Highlighting

The first parameter of the class method.


'''
== Comments And Links
(visible only on this page)

=== on 17 Mar 2015, 08:49:57 Elena Vilchik wrote:
\[~ann.campbell.2] I would suggest label "convention" here

=== on 18 Mar 2015, 16:24:14 Ann Campbell wrote:
There is some question about class vs metaclass method vs instance method. Postponing this rule until that's settled. Updates may be needed at that time.

endif::env-github,rspecator-view[]
