Python - how can I override the functionality of a class before it's imported by a different module?

Question

I have a class that's being imported in module_x for instantiation, but first I want to override one of the class's methods to include a specific feature dynamically (inside some middleware that runs before module_x is loaded.

Answer 1

Neither AndiDog's nor Andrew's answer answer your question completely. But they have given the most important tools to be able to solve your problem (+1 to both). I will be using one of their suggestions in my answer:

You will need 3 files:

File 1: myClass.py

class C:
    def func(self):
        #do something

File 2: importer.py

from myClass import *
def changeFunc():
    A = C()
    A.func = lambda : "I like pi"
    return A

if __name__ == "importer":
    A = changeFunc()

File 3: module_x.py

from importer import *
print A.func()

The output of module_x would print "I like pi"

Hope this helps

Answer 2

You should know that each class type (like C in class C: ...) is an object, so you can simply overwrite the class methods. As long as instances don't overwrite their own methods (won't happen too often because that's not really useful for single inntances), each instance uses the methods as inherited from its class type. This way, you can even replace a method after an instance has been created.

For example:

class C:
    def m(self):
        print "original"

c1 = C()
c1.m() # prints "original"

def replacement(self):
    print "replaced!"

C.m = replacement

c1.m() # prints "replaced!"
C().m() # prints "replaced!"