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Using same function as instance and classmethod in python

One can do something like this:

class master:
@combomethod
def foo(param):
param.bar() # Param could be type as well as object

class slaveClass( master ):
@classmethod
def bar(cls):
print("This is class method")

slaveType = slaveClass
slaveType.foo()

class slaveInstance( master ):
def __init__(self, data):
self.data = data
def bar(self):
print("This is "+self.data+" method")


slaveType = slaveInstance("instance")
slaveType.foo()


combomethod is defined in "Creating a method that is simultaneously an instance and class method".

combomethod

My question is, why is it like this, that default first parameter can't be used as parameter of comboclass? Or at least, why can't I pass object to classmethod as the first parameter? I know the difference between classmethod and instancemethods, and I know decorators, but I might not understand how built-in @classmethod and self parameter passing is made. Is there a technical limitation? Or, why isn't combomethod allready built in?

@classmethod

self

combomethod

2 Answers
2

combomethod doesn't create a method object when accessed but a specially wrapped function. Like methods each access creates a new object, in this case a new function object.

combomethod

class A:
def __init__(self):
self.data = 'instance'

@combomethod
def foo(param):
if isinstance(param, A):
print("This is an " + param.data + " method.")
elif param is A:
print("This is a class method.")

>>> a = A()
>>> A.foo
<function foo at 0x00CFE810>
>>> a.foo
<function foo at 0x00CFE858>

>>> A.foo()
This is a class method.
>>> a.foo()
This is an instance method.


It's new for each access:

>>> A.foo is A.foo
False
>>> a.foo is a.foo
False


foo is really _wrapper in disguise:

foo

_wrapper

>>> A.foo.__code__.co_name
'_wrapper'


When called from a class the closure has obj == None (note that 'self' here refers to the combomethod, which has a reference to the original function object in self.method):

self.method

>>> print(*zip(A.foo.__code__.co_freevars, A.foo.__closure__), sep='n')
('obj', <cell at 0x011983F0: NoneType object at 0x1E1DF8F4>)
('self', <cell at 0x01198530: combomethod object at 0x00D29630>)
('objtype', <cell at 0x00D29D10: type object at 0x01196858>)


When called as the attribute of an instance, obj is the instance:

>>> print(*zip(a.foo.__code__.co_freevars, a.foo.__closure__), sep='n')
('obj', <cell at 0x01198570: A object at 0x00D29FD0>)
('self', <cell at 0x01198530: combomethod object at 0x00D29630>)
('objtype', <cell at 0x00D29D10: type object at 0x01196858>)


Here is the original function stored in the combomethod:

>>> A.foo.__closure__[1].cell_contents.method
<function foo at 0x00D1CB70>
>>> A.foo.__closure__[1].cell_contents.method.__code__.co_name
'foo'


_wrapper executes self.method with either the class or instance as the first argument given the value of obj:

_wrapper

self.method

if obj is not None:
return self.method(obj, *args, **kwargs)
else:
return self.method(objtype, *args, **kwargs)


use this:

class A(object):

@classmethod
def print(cls):
print 'A'

def __print(self):
print 'B'

def __init__(self):
self.print = self.__print


a = A()
a.print()
A.print()


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