Python: avoid defining both a classmethod and an instancemethod

Question

NOTE on the question below. I think the 'proper' pythonic idiom is to a) create module functions, such as foo_math below, and then call their specific action against an instance within the class itself. The bottom piece of code reflects that approach.

I want to define a classmethod which takes two arguments and returns a value. I want the same method to be able to be called on a class instance with the instance value pass as one of the arguments. Can I do this without defining two distinct methods as I have done here?

class Foo(object):
    __init__(x):
        self.x = x

    @classmethod
    def foo_math(cls, x, y):
         return x + y

    def math(self, y):
        return Foo.foo_math(self.x, y)

What I would like is:

>>> Foo.math(3, 4)
7
>>> f = Foo()
>>> f.x = 3
>>> f.math(4)
7

Short of subtyping int, here is my conclusion to this question:

def foo_math(cls, x, y):
     return x + y

class Foo(object):
    __init__(x):
        self.x = x

    def foo_math(self, y):
        return foo_math(self, y)

What you want unfortunately conflicts with the heavy expectation that if f.math(4) works, type(f).math(f, 4) will do the same thing. — user2357112
– user2357112, Commented Feb 24, 2014 at 20:37
I'm unfamiliar with the equivalence of f.math(4) and type(f).math(f, 4) .. another item for my learn python to-do list. Since I want to conform with 'heavy expectation', is there a conventional way to handle naming a very similar classmethod and instancemethod? — Cole
– Cole, Commented Feb 24, 2014 at 20:42
why would you want to do this? it seems like the problem you're trying to solve might not be the best one to actually solve. — acushner
– acushner, Commented Feb 24, 2014 at 22:49
@AdamCushner How so? I'm still in the thinking about how I want to do this phase so any additional feedback would be great. — Cole
– Cole, Commented Feb 25, 2014 at 0:06
oh i just meant like could you give a good use case? like it seems like you'd either want to do one or the other. could you show why you needed to do both? also, you might want to consider operator overloading. — acushner
– acushner, Commented Feb 25, 2014 at 14:33

acushner · Accepted Answer · 2014-02-25 14:56:03Z

1

i don't recommend doing this, but if you really want, it's this (thank you other guy on stackoverflow for first part):

class staticorinstancemethod(object):
    def __init__(self, func):
        self.func = func

    def __get__(self, instance, owner):
        return functools.partial(self.func, instance)

then, do something like

class F(object):
    @staticorinstancemethod
    def math(instOrNone, v1, v2=None):
        return instOrNone.x + v1 if instOrNone else v1 + v2

but maybe you just want to define the __add__ and __radd__ methods...

answered Feb 25, 2014 at 14:56

acushner

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1 Comment

Cole Over a year ago

Nice idea. I agree with your non-recommendation. But thanks for the thoughtfulness.

anon582847382 · Accepted Answer · 2014-02-24 20:32:08Z

1

I don't think that you can call a method from a class without defining an object of that class (class methods don't belong inside the methods of any one class), so things like Foo.math(3, 4) will return a NameError as Foo has not been defined.

With this in mind, you should modify your code to be like this (even though with the problem solved there are still some issues with the code):

# A class method would probably go here somewhere.

class Foo(object):
    def __init__(self, x):
        self.x = x

    def foo_math(self, x, y):
         return x + y

    def math(self, y):
        return self.foo_math(self.x, y)

Then you can do:

>>> f = Foo(3)
>>> f.math(4)
7

answered Feb 24, 2014 at 20:32

anon582847382

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