Duck typing and class methods (or, how to use a method from both a class and an instance?)

Question

I have some code which I would like to pass instances or classes interchangeably. All I will do in that code is to call a method that I expect both classes and instances to have (the method go() in the example below).

Unfortunately, I can't create a classmethod with the same name of a regular method... See example below. I initially expected the second call to produce an a instead of a b.

Any advice on how to achieve this?

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>>> class A(object):
...     def go(self):
...             print "a"
...     @classmethod
...     def go(cls):
...             print "b"
... 
>>> a=A()
>>> a.go()
b
>>> A.go()
b

Answer 1

Consider reusing the classinstancemethod decorator from formencode.

https://bitbucket.org/formencode/official-formencode/src/06d52c5b33c9/formencode/declarative.py

class classinstancemethod(object):
    """
    Acts like a class method when called from a class, like an
    instance method when called by an instance.  The method should
    take two arguments, 'self' and 'cls'; one of these will be None
    depending on how the method was called.
    """

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

    def __get__(self, obj, type=None):
        return _methodwrapper(self.func, obj=obj, type=type)


class _methodwrapper(object):

    def __init__(self, func, obj, type):
        self.func = func
        self.obj = obj
        self.type = type

    def __call__(self, *args, **kw):
        assert 'self' not in kw and 'cls' not in kw, (
            "You cannot use 'self' or 'cls' arguments to a "
            "classinstancemethod")
        return self.func(*((self.obj, self.type) + args), **kw)

    def __repr__(self):
        if self.obj is None:
            return ('<bound class method %s.%s>'
                    % (self.type.__name__, self.func.func_name))
        else:
            return ('<bound method %s.%s of %r>'
                    % (self.type.__name__, self.func.func_name, self.obj))

Answer 2

You could create an own method type with a specially crafted __get__() method.

In this method, you could do something like this:

class combimethod(object):
    def __init__(self, func):
        self._func = func
    def classmethod(self, func):
        self._classfunc = classmethod(func)
        return self
    def __get__(self, instance, owner):
        if instance is None:
            return self._classfunc.__get__(instance, owner)
        else:
            return self._func.__get__(instance, owner)

class A(object):
    @combimethod
    def go(self):
        print "instance", self
    @go.classmethod
    def go(cls):
        print "class", cls

a=A()
print "i:",
a.go()
print "c:",
A.go()

NOTE: The above is not very thoroughly tested, but seems to work. Nevertheless, it should be seen as a kind of "solution-near pseudo-code", not as a solution. It should give you an idea how to achieve your goal.

Answer 3

How about something like:

import inspect

class A(object):
   @staticmethod
   def go(obj):
      if inspect.isclass(obj):
         print 'class'
      else:
         print 'instance'

A.go(int) # class
A.go(1)   # instance
A.go(A)   # class
A.go(A()) # instance