Creating a method that is simultaneously an instance and class method

Question

In Python, I'd like to be able to create a function that behaves both as a class function and an instance method, but with the ability to change behaviors. The use case for this is for a set of serializable objects and types. As an example:

>>> class Thing(object):
    #...
>>> Thing.to_json()
'A'
>>> Thing().to_json()
'B'

I know that given the definition of classmethod() in funcobject.c in the Python source, this looks like it'd be simple with a C module. Is there a way to do this from within python?

Thanks!

With the hint of descriptors, I was able to do it with the following code:

class combomethod(object):
    def __init__(self, method):
        self.method = method

    def __get__(self, obj=None, objtype=None):
        @functools.wraps(self.method)
        def _wrapper(*args, **kwargs):
            if obj is not None:
                return self.method(obj, *args, **kwargs)
            else:
                return self.method(objtype, *args, **kwargs)
        return _wrapper

Thank you Alex!

there are plenty of reasons. In my particular problem, serializing the type allows us to create the specifications in an RPC system. — Mike Axiak
– Mike Axiak, Commented Apr 7, 2010 at 3:02

cs95 · Accepted Answer · 2019-02-19 02:21:46Z

9

Sure, you just need to define your own descriptor type. There's an excellent tutorial on Python descriptors here.

edited Feb 19, 2019 at 2:21

cs95

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answered Apr 7, 2010 at 2:49

Alex Martelli

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2 Comments

alkasm Over a year ago

For an update 9 years later, the Python documentation now hosts this tutorial from Raymond Hettinger, and it's no longer at the link posted. (One can check archive.org for a cached version of the original, but it's just a slightly smaller version of the one on the Python docs).

Bernd Wechner Over a year ago

This answer would be magnitudes more useful if it provided an outline solution to question not just a research pointer. It would win my upvote only then.

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