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 another possible approach, very similar to Andrew Clark's one, is to use wrapt library. Among other useful things, this library provides wrap_function_wrapper and patch_function_wrapper helpers. They can be used like this:

import wrapt
import thirdpartymodule_a
import thirdpartymodule_b

@wrapt.patch_function_wrapper(thirdpartymodule_a.SomeClass, '__init__')
def new_init(wrapped, instance, args, kwargs):
    # here, wrapped is the original __init__,
    # instance is `self` instance (it is not true for classmethods though),
    # args and kwargs are tuple and dict respectively.

    # first call original init
    wrapped(*args, **kwargs)  # note it is already bound to the instance
    # and now do our changes
    instance.a = 43

thirdpartymodule_b.do_something()

Or sometimes you may want to use wrap_function_wrapper which is not a decorator but othrewise works the same way:

def new_init(wrapped, instance, args, kwargs):
    pass  # ...

wrapt.wrap_function_wrapper(thirdpartymodule_a.SomeClass, '__init__', new_init)