Monkeypatching: replacing method on class to function

Question

I have some silly code with monkey matching part in it. The example below is only for self-studying not for production.

class MyClass:

    def some_method(self):
        print("some_method call")
        self.yet_another_method()

    def yet_another_method(self):
        print('yet_another_method call')


def some_function(self):
    print("some function call")
    self.yet_another_method()

obj = MyClass()
obj.some_method()
obj.some_method = some_function
obj.some_method()

When I execute this code I get the following error:

TypeError: non_class_some_method() missing 1 required positional argument: 'self'

It's obvious that Python interpreter can't implicitly pass obj in some_function. But when I doing introspection and getting the byte code I have the similar representation of method (before and after replace).

import dis
import inspect


class MyClass:

    def some_method(self):
        print("some_method call")
        self.yet_another_method()

    def yet_another_method(self):
        print('yet_another_method call')


def some_function(self):
    print("some function call")
    self.yet_another_method()

obj = MyClass()
dis.dis(obj.some_method)
print(inspect.getargspec(obj.some_method))
obj.some_method = some_function
print("======================================================================")
dis.dis(obj.some_method)
print(inspect.getargspec(obj.some_method))

The result:

  8           0 LOAD_GLOBAL              0 (print) 
              3 LOAD_CONST               1 ('some_method call') 
              6 CALL_FUNCTION            1 (1 positional, 0 keyword pair) 
              9 POP_TOP              

  9          10 LOAD_FAST                0 (self) 
             13 LOAD_ATTR                1 (yet_another_method) 
             16 CALL_FUNCTION            0 (0 positional, 0 keyword pair) 
             19 POP_TOP              
             20 LOAD_CONST               0 (None) 
             23 RETURN_VALUE         
ArgSpec(args=['self'], varargs=None, keywords=None, defaults=None)
======================================================================
 16           0 LOAD_GLOBAL              0 (print) 
              3 LOAD_CONST               1 ('some function call') 
              6 CALL_FUNCTION            1 (1 positional, 0 keyword pair) 
              9 POP_TOP              

 17          10 LOAD_FAST                0 (self) 
             13 LOAD_ATTR                1 (yet_another_method) 
             16 CALL_FUNCTION            0 (0 positional, 0 keyword pair) 
             19 POP_TOP              
             20 LOAD_CONST               0 (None) 
             23 RETURN_VALUE         
ArgSpec(args=['self'], varargs=None, keywords=None, defaults=None)

Please can anyone explain why it happens this way?

Well this happens because functions defined on instances are never converted to bound methods, they should be defined as class attribute.(Or you can use [`types.MethodType`](https://docs.python.org/2/library/types.html#types.MethodType) to register a function as method using instance.) — Ashwini Chaudhary, Jul 23 '14 at 14:40
If you only want to change the method for only one instance of a class , not for all instances of the class then your question is answered here: http://stackoverflow.com/questions/972/adding-a-method-to-an-existing-object?rq=1 — Ross Ridge, Jul 23 '14 at 16:45

score 4 · Accepted Answer · answered Jul 23 '14 at 14:56

Most of the answer is here : https://wiki.python.org/moin/FromFunctionToMethod. To make a long story short:

def always yield a function object
a method is just a thin callable wrapper around the function, class and instance
this wrapper is only created when the function is an attribute of the class - not when it's an attribute of the instance.

To make your code work (monkeypatching on a per-instance basis), you have to manually invoke the mechanism that would create a method from the function, class and instance. When using new-style classes the simplest solution is to directly invoke the descriptor protocol on the function, ie:

obj.some_method = some_function.__get__(obj, type(obj))

When using old-style classes like you do in your example (and which is not a good idea), you can use types.MethodType instead, but really unless you are stuck with some legacy code you'd be better making your class a new-style one (=> inherit from object).

Monkeypatching: replacing method on class to function

1 Answers1