Monkey patching a class in another module in Python

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 showing how to do this have all assumed I'd be calling the class myself (e.g. Monkey-patch Python class). However, this is not the case. In my case the class is initalised within a function in another module. See the (greatly simplified) example below:

thirdpartymodule_a.py

    class SomeClass(object):
        def __init__(self):
            self.a = 42
        def show(self):
            print self.a

thirdpartymodule_b.py

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

mymodule.py

    import thirdpartymodule_b
    thirdpartymodule.dosomething()

Is there any way to modify the __init__ method of SomeClass so that when dosomething is called from mymodule.py it, for example, prints 43 instead of 42? Ideally I'd be able to wrap the existing method.

I can't change the thirdpartymodule*.py files, as other scripts depend on the existing functionality. I'd rather not have to create my own copy of the module, as the change I need to make is very simple.

Edit 2013-10-24

I overlooked a small but important detail in the example above. SomeClass is imported by thirdpartymodule_b like this: from thirdpartymodule_a import SomeClass.

To do the patch suggested by F.J I need to replace the copy in thirdpartymodule_b, rather than thirdpartymodule_a. e.g. thirdpartymodule_b.SomeClass.__init__ = new_init.

The following should work:

    import thirdpartymodule_a
    import thirdpartymodule_b

    def new_init(self):
        self.a = 43

    thirdpartymodule_a.SomeClass.__init__ = new_init

    thirdpartymodule_b.dosomething()

If you want the new init to call the old init replace the new_init() definition with the following:

    old_init = thirdpartymodule_a.SomeClass.__init__
    def new_init(self, *k, **kw):
        old_init(self, *k, **kw)
        self.a = 43

From: stackoverflow.com/q/19545982