Inconsistency in classmethod args in Python 3.9

[cristianmatache]

from wrapt import decorator

@decorator
def deco(wrapped, instance, args, kwargs):
    print(args)
    return wrapped(*args, **kwargs)

class X:
    @classmethod
    @deco
    def method(cls, a):
        pass

# Python 3.8 and lower
X.method(1)
# (<class '__main__.X'>, 1)

# Python 3.9
X.method(1)
# (1,)

In the example above, in Python 3.9 the injected args don't include the class while in python 3.8 and lower, args includes the class object.

[GrahamDumpleton]

The difference is because there was a long standing bug in Python which I first reported back in 2013. They finally fixed it in Python 3.9.

• https://bugs.python.org/issue19072

So the behaviour of wrapt is correct based on how the respective Python versions worked.

The docs for wrapt mention this issue:

• https://wrapt.readthedocs.io/en/latest/issues.html#classmethod-get

The docs need to be updated to mention that the behaviour in Python was finally corrected in Python 3.9 though.

[cristianmatache]

Thank you @GrahamDumpleton. This is very interesting. It is nice that regular methods defined on a metaclass and methods decorated with @classmethod on a regular class behave the same finally.