Used when a metaclass method has a first argument named differently than the value specified in valid-classmethod-first-arg option (default to cls ), recommended to easily differentiate them from regular instance methods. Weird enough but usually naming of the first argument is mcs and pylint documentation says so but when someone specifies it then the lib throws C0203 error.

Noncompliant Code:

class Meta(type):
    def __new__(some):
        pass

Compliant Code:

class Meta(type):
    def __new__(mcs):
        pass