Patch a python object to add default to method kwargs

Question

I want to patch some code that uses an object from an external module.

One method of this object is called all over the place, and I need to set a new default kwarg in all those calls.

Rather than add so much duplicate code, I thought it would be better to change the object method. What is the cleanest way to do this?

score 3 · Answer 1 · edited May 23 '17 at 11:50

3

This is called monkey-patching and there is no "clean" version of it.

If you need to replace a method bar in a class Foo, use this code:

oldMethod = Foo.bar
def newMethod(self, **kwargs):
    ... fix kwargs as necessary ...
    oldMethod(self, **kwargs)
Foo.bar = newMethod

First, we save the old method handle in a variable
Then we define the new method as a function. The first argument has to be self, just as if this function was inside of a class.
To call the original method, we use oldMethod(self, ...). This will take the method pointer, and call it with the instance as first argument. self.oldMethod() doesn't work since we're not in a class context (I think).
Lastly, we install the patched method in the class.

http://mflerackers.wordpress.com/2012/02/01/modifying-python-classes-at-run-time/

edited May 23 '17 at 11:50

Community

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answered Jan 05 '15 at 17:05

Aaron Digulla

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Where does `self.oldMethod` get defined/set? – martineau Jan 05 '15 at 18:13
Wouldn't this be much neater as a decorator? – sapi Jan 05 '15 at 22:38
@sapi if you were able to modify the source. But, I doubt that's what the OP wants here because if that were the case, then the OP would have done that in the first place. – OozeMeister Jan 05 '15 at 23:32

score 0 · Answer 2 · answered Jan 05 '15 at 22:36

0

I decided to write a wrapper class which inherits from the object I needed to modify.

class Wrapper(Target):
    def method(self, *args, **kwargs):
        kwargs['option'] = True
        return super(Wrapper, self).method(*args, **kwargs)

instance = Target()
instance.__class__ = Wrapper

answered Jan 05 '15 at 22:36

so12311

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why not just: `instance = Wrapper()`? – OozeMeister Jan 05 '15 at 23:34
in my real situation, the instance is returned by a function from an external module that I don't want to edit. quite possibly there is a better way to do this though I know very little about classes. – so12311 Jan 06 '15 at 01:18

Patch a python object to add default to method kwargs

2 Answers2