Monkey patching a class in another module in Python

Question

I\'m working with a module written by someone else. I\'d like to monkey patch the __init__ method of a class defined in the module. The examples I have found sh

Answer 1

One only slightly-less-hacky version uses global variables as parameters:

sentinel = False

class SomeClass(object):
    def __init__(self):
        global sentinel
        if sentinel:
            
        else:
            # Original code
            self.a = 42
    def show(self):
        print self.a

when sentinel is false, it acts exactly as before. When it's true, then you get your new behaviour. In your code, you would do:

import thirdpartymodule_b

thirdpartymodule_b.sentinel = True    
thirdpartymodule.dosomething()
thirdpartymodule_b.sentinel = False

Of course, it is fairly trivial to make this a proper fix without impacting existing code. But you have to change the other module slightly:

import thirdpartymodule_a
def dosomething(sentinel = False):
    sc = thirdpartymodule_a.SomeClass(sentinel)
    sc.show()

and pass to init:

class SomeClass(object):
    def __init__(self, sentinel=False):
        if sentinel:
            
        else:
            # Original code
            self.a = 42
    def show(self):
        print self.a

Existing code will continue to work - they will call it with no arguments, which will keep the default false value, which will keep the old behaviour. But your code now has a way to tell the whole stack on down that new behaviour is available.