python3: bind method to class instance with .__get__(), it works but why?

Question

I know if you want to add a method to a class instance you can\'t do a simple assignment like this:

>>> def print_var(self): # method to be added
               


        

Answer 1

The information you're looking for is in the Descriptor HowTo Guide:

To support method calls, functions include the __get__() method for binding methods during attribute access. This means that all functions are non-data descriptors which return bound or unbound methods depending whether they are invoked from an object or a class. In pure Python, it works like this:

class Function(object):
    . . .
    def __get__(self, obj, objtype=None):
        "Simulate func_descr_get() in Objects/funcobject.c"
        return types.MethodType(self, obj)

So there really isn't anything strange going on -- the __get__ method of a function object calls types.MethodType and returns the result.