Python: how to monkey patch class method to other class method

Question

I have got the following code:

class A:
    def __init__(self):
        self.a = "This is mine, "

    def testfunc(self, arg1):
        print self.a + arg1

class B:
    def __init__(self):
        self.b = "I didn't think so"
        self.oldtestfunc = A.testfunc
        A.testfunc = self.testfuncPatch

    def testfuncPatch(self, arg):
        newarg = arg + self.b # B instance 'self'
        self.oldtestfunc(self, newarg) # A instance 'self'

instA = A()
instB = B()
instA.testfunc("keep away! ")

I want to do the following:

Some class A consists of a function with arguments. I want to monkey patch this function to a function in class B do some manipulate the arguments and accessing class B's variables, my problem being the patched function actually needs two different 'self' objects, namely the instance of class A as well as the instance of class B.

Is this possible?

Answer 1

the issue is that when you override a class function with an already bound method, trying to bind to other instances just ignore the second instance:

print(instA.testfunc)
#<bound method B.testfuncPatch of <__main__.B object at 0x1056ab6d8>>

so the method basically is treated as a staticmethod meaning you would have to call it with the instance as the first argument:

instA.testfunc(instA,"keep away! ")

I first ran into this issue when trying to import random.shuffle directly into a class to make it a method:

class List(list):
    from random import shuffle #I was quite surprised when this didn't work at all

a = List([1,2,3])
print(a.shuffle)
#<bound method Random.shuffle of <random.Random object at 0x1020c8c18>>
a.shuffle()

Traceback (most recent call last):
  File "/Users/Tadhg/Documents/codes/test.py", line 5, in <module>
    a.shuffle()
TypeError: shuffle() missing 1 required positional argument: 'x'

To fix this issue I created a function that can be rebound to a second instance on top of the first:

from types import MethodType

def rebinder(f):
    if not isinstance(f,MethodType):
        raise TypeError("rebinder was intended for rebinding methods")
    def wrapper(*args,**kw):
        return f(*args,**kw)
    return wrapper

class List(list):
    from random import shuffle
    shuffle = rebinder(shuffle) #now it does work :D

a = List(range(10))
print(a.shuffle)
a.shuffle()
print(a)

#output:
<bound method rebinder.<locals>.wrapper of [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]>
[5, 6, 8, 2, 4, 1, 9, 3, 7, 0]

So you can apply this to your situation just as easily:

from types import MethodType

def rebinder(f):
    if not isinstance(f,MethodType):
        raise TypeError("rebinder was intended for rebinding methods")
    def wrapper(*args,**kw):
        return f(*args,**kw)
    return wrapper
...

class B:
    def __init__(self):
        self.b = "I didn't think so"
        self.oldtestfunc = A.testfunc
        A.testfunc = rebinder(self.testfuncPatch) #!! Edit here

    def testfuncPatch(selfB, selfA, arg): #take the instance of B first then the instance of A
        newarg = arg + selfB.b
        self.oldtestfunc(selfA, newarg)