Python – Can StaticMethod and ClassMethod Decorators Be Implemented in Pure Python?

class-methoddecoratorpythonstatic-methods

This is an academic question, more than a practical one. I'm trying to delve into the foundations of Python, and I wonder: could I implement the staticmethod and classmethod decorators in pure Python if I wanted to?

For staticmethod maybe I could keep a reference to the original function and call it omitting the first argument?

For classmethod maybe I could keep a reference to the original function and call it passing the type of the first argument, rather than the argument itself? (How would I handle it being called directly on the class?)

Again, it's not that I want to actually implement it in my programs; I want to learn how the language works from it. If it's possible to do I would really appreciate code samples. If not, I would appreciate explanations why it can't be done theoretically. Thanks!

Edit: Thanks for the answer and for the reference to the descriptor protocol! As a means to further extend my understanding of Python I'd like to ask two more things:

  • Could this be done without the descriptor protocol, for example by implementing the __ call __ special method in our classes?

  • Does the descriptor protocol work the same way in Python 2.7 and in Python 3.x?

Thanks again!

Best Answer

There are examples in the docs:

class StaticMethod(object):
    "Emulate PyStaticMethod_Type() in Objects/funcobject.c"

    def __init__(self, f):
        self.f = f

    def __get__(self, obj, objtype=None):
        return self.f

And:

class ClassMethod(object):
    "Emulate PyClassMethod_Type() in Objects/funcobject.c"

    def __init__(self, f):
        self.f = f

    def __get__(self, obj, klass=None):
        if klass is None:
            klass = type(obj)
        def newfunc(*args):
            return self.f(klass, *args)
        return newfunc
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