How to extend Class instance

Question

MyClass is defined in module.py. There is no way we can modify it. But we do know the Class definition looks like this:

class MyClass:
    def method(self, msg):
        print 'from method:', msg

I start my script with importing the module and then declaring an object's instance:

import module    
foo = module.MyClass()

Then I write my own function:

def function(msg):
    print 'from function:', msg

Now, every time foo.method('') is used I want to call function() so it prints the same message too.

Would this situation be referred as the monkey patching? How to achieve what is needed?

Answer 1

Yes, it's called monkey-patching.

This is basically decoration, but done manually after the class is already defined.

from functools import wraps

def wrapper(f):
    @wraps(f)
    def wrapped(*args, **kwargs):
        myFunction()
        return f(*args, **kwargs)
    return wrapped

MyClass.printThis = wrapper(MyClass.printThis)

It will affect all instances of MyClass, even those that were created before the patch was applied.

If you don't need to dynamically modify runtime behaviour, avoid monkey-patching and prefer to use inheritance to customise behaviour, as suggested in the comments. It's less hacky.