Using a metaclass to substitute a class definition?

Question

Python 3.6

I'm trying to modify the behavior of a third party library.

I don't want to directly change the source code.

Considering this code below:

class UselessObject(object):
    pass


class PretendClassDef(object):
    """
    A class to highlight my problem
    """

    def do_something(self):

        # Allot of code here

        result = UselessObject()

        return result

I'd like to substitute my own class for UselessObject

I'd like to know if using a metaclass in my module to intercept the creation of UselessObject is a valid idea?

EDIT

This answer posted by Ashwini Chaudhary on the same question, may be of use to others. As well as the below answer.

P.S. I also discovered that 'module' level __metaclass__ does't work in python 3. So my initial question of it 'being a valid idea' is False

Answer 1

FWIW, here's some code that illustrates Rawing's idea.

class UselessObject(object):
    def __repr__(self):
        return "I'm useless"

class PretendClassDef(object):
    def do_something(self):
        return UselessObject()

# -------

class CoolObject(object):
    def __repr__(self):
        return "I'm cool"

UselessObject = CoolObject

p = PretendClassDef()
print(p.do_something())

output

I'm cool

---

We can even use this technique if CoolObject needs to inherit UselessObject. If we change the definition of CoolObject to:

class CoolObject(UselessObject):
    def __repr__(self):
        s = super().__repr__()
        return "I'm cool, but my parent says " + s

we get this output:

I'm cool, but my parent says I'm useless

This works because the name UselessObject has its old definition when the CoolObject class definition is executed.