Question

How can I tell if a class instance method and function are the same?

I have a simple class that uses a decorator:

@decorate_class
class Hello:

    @decorate_func
    def world(self):
        return

And these are my decorators:

# A global variable
global_func = None

def decorate_func(func):
    global_func = func
    return func

def decorate_class(clazz):
    print clazz.__dict__["world"] == global_func
    return clazz

The above returns False, possible because type(func) in decorate_func is function, but in decorate_class it is instancemethod. But printing both of them gives me:

<function world at 0x7f490e59ce60>

As in, the same memory address. How do I compare them to know that they are the same function? Is comparing by the memory address safe (and correct)?

Answer 1

Your problem here is actually your use of global_func.

If you change decorate_class to:

def decorate_class(clazz):
    print global_func, clazz.__dict__["world"] == global_func
    return clazz

You’ll see that global_func is None.

To fix this, explicitly declare global_func as global in decorate_func:

def decorate_func(func):
    global global_func
    global_func = func
    return func

And everything will work.

This is (basically) because Python assumes that, if you assign to a variable in a function, that variable is assumed to be local to that function, unless it is explicitly declared to be global.