Monkey Patching an attribute within a class?

Question

Is it possible to Monkey Patch an attribute within a Python class? A quick google for "monkey patching python" returns a ton of results about patching methods, but no mention of changing fields within a class.

Patching Methods:

However, like I said, no mention of changing Class attributes/fields.

To give a somewhat contrived example, let's say I have a class which has a multiprocessing queue, and I want to patch it with a threading Queue.Queue.

import threading 
import multiprocessing

class MyClassWithAQueue(object):
        def__init__(self):
                self.q = multiprocessing.Queue()

Is there a way to patch this? Attempting to simply assign it via the class name before construction appears to do nothing.

if __name__ == '__main__':
        MyClassWithAQueue.q = Queue.Queue()
        myclass = MyClassWithAQueue()

        print myclass.q 
            # still shows as a multiprocessing queue
        >>><multiprocessing.queues.Queue object at 0x10064493>

Is there a way to do this?

You must either create instance of the original class and then rewrite the actual value of the field, or monkey patch the class by replacing the constructor. But I don’t recommend using monkey patching at all. There are often much better ways to solve the problem you have. — Palec, Feb 08 '14 at 01:32

score 1 · Accepted Answer · answered Feb 08 '14 at 01:37

The problem is the order in which the code runs.

__init__ runs on instantiation, and is setting self.q, regardless of what if anything it was before. You can do one of the following:

Change it to a class attribute:

class MyClassWithAQueue(object):
    q = multiprocessing.Queue()
    def __init__(self):
        pass

MyClassWithAQueue.q = Queue.Queue()
myclass = MyClassWithAQueue()
print myclass.q

Or change the instance attribute:

class MyClassWithAQueue(object):
    def __init__(self):
        self.q = multiprocessing.Queue()

myclass = MyClassWithAQueue()
myclass.q = Queue.Queue()
print myclass.q

Monkey Patching an attribute within a class?

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