Overriding a class's method definition

Question

Let's say I have a class, FooClass, that has a method, foo_method, defined. This class is in a third party library, and I want to explicitly override it. I have a number of classes that inherit from FooClass, so I don't want to override foo_method for every single subclass. Instead, I'd like to override the class's method definition without digging into the third party library code. When I try the obvious way,

from thirdparty import FooClass

class FooClass(object):

  def foo_method():
    newstuff

I get strange behavior -- NotImplementedErrors and such. Am I missing something?

Answer 1

You are redefining the class locally; that won't change the original class or affect anyone using it elsewhere.

You can monkeypatch the method on the class:

from thirdparty import FooClass

orig_foo_method = FooClass.foo_method

def new_foo_method(self):
    # do new stuff
    # perhaps even call orig_foo_method(self)

FooClass.foo_method = new_foo_method

Now any code that tries to call .foo_method() on instances of FooClass anywhere in your Python interpreter will find and call your new_foo_method() method instead.