[Python] inspect.signature on class methods

Can someone help me understand why the Signature.bind(...).arguments are different when I call directly for a class method vs. when used in a decorator?

I have the decorator below.

def decorator(func):
@functools.wraps(func)
def inner(*args, **kwargs):
print(inspect.signature(func).bind(*args, **kwargs).arguments)
return func(*args, **kwargs)
return inner


When this decorator runs on a class method, I get the cls parameter in the signature like below.

class MyClass:

@classmethod
@decorator
def mymethod(cls, a, b):
return a + b

MyClass.mymethod(1, 2)

# returns:
# {'cls': <class '__main__.MyClass'>, 'a': 1, 'b': 2}
# 3


If I call inspect.signature on the class method directly, I get a different result or an error.

This first block runs successfully but does not return the class parameter.

print(inspect.signature(MyClass.mymethod).bind(1, 2).arguments)
# returns {'a': 1, 'b': 2}


This errors out.

print(inspect.signature(MyClass.mymethod).bind(MyClass, 1, 2).arguments)
# TypeError: too many positional arguments

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