How are methods, `classmethod`, and `staticmethod` implemented in Python?

Question

At what point do methods in Python acquire a get property? —As soon as they’re defined in the class? Why does Python let me define a method without any arguments (not even a first self argument)?

I know how to use classmethod and staticmethod, and I know that they’re built-in functions, but what happens to a function that is so-decorated?

Essentially, I’m wondering about the “magic” that happens between class definition and class construction.

Asked By: Neil G

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Source

Answer 1

Check this out.

https://docs.python.org/3/howto/descriptor.html#class-methods

You can also take a look at the source code for class and static method objects, in funcobject.c:

http://hg.python.org/cpython/file/69b416cd1727/Objects/funcobject.c

Class method object definition starts on line 694, while static method object definition starts on line 852. (I do find it kind of funny that they have items titled "method" in funcobject.c when methodobject.c also exists.)

Answered By: JAB

Answer 2

For reference, from the first link in @JAB’s answer

Using the non-data descriptor protocol, a pure Python version of staticmethod() would look like this:
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

…

Using the non-data descriptor protocol, a pure Python version of classmethod() would look like this:

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

Answered By: Christian Reall-Fluharty

How are methods, `classmethod`, and `staticmethod` implemented in Python?

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