Replace a parent class with another class without keeping the old parent

Question

What I have in an external library:

# external package content


class Foo:
    
    def func(): ...


class Bar(Foo):
    
    def func():
        super().func()

What I need in my code:

from external_library import Bar

class MyOwnCustomFoo:

    def func(): ...


# Do some magic here to replace a parent class with another class without keeping the old parent


# Class name can be not 'Bar', but the class must have all 'Bar' attributes
class Bar(MyOwnCustomFoo):

    def func():
        super().func()

Goals I need to achieve:

I must have all Bar methods without copy/pasting them.
Bar must inherit from my own class, not from another one.

What you want is not possible, not even with metaclasses. Maybe someone might find a hack with dirty monkey-patching to do this, that I cannot come up with right now. But this whole idea is a non-starter. I think that you might have an architectual problem that might be solved in another way. To address this, we'd need some background on the issue you're trying to solve. — Richard Neumann, Feb 09 '22 at 17:50
Just because you want a class that's like `Bar`, but inheriting from `MyOwnCustomFoo` instead, does not mean you should actually change `Bar`. Assume that classes are defined the way they are for a reason. Classes are not infinitely malleable. — chepner, Feb 09 '22 at 17:53

Programmer · Accepted Answer · 2022-02-09T17:59:06.043

1

You could assign the methods from the external library class to your own class, mimicking inheritance (AFAIK this is called monkey-patching):

from external_library import Bar as _Bar

class Bar(MyOwnCustomFoo):
    func = _Bar.func
    ...

edited Feb 09 '22 at 17:59

answered Feb 09 '22 at 17:56

Programmer

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That is... elegant! :0 – ibarrond Feb 09 '22 at 17:58
I may be worng, but instead of copy/pasting all methods names, it is probably better to just add ```def __getattr__(self, name): return getattr(_Bar, name)```. – Nairum Feb 09 '22 at 18:17
Well, since there are not better answers, I am accepting this for now. – Nairum Feb 10 '22 at 12:45
@Nairum, yep, that would probably be the shorter and better solution ! – Programmer Feb 10 '22 at 17:40

score -3 · Answer 2 · edited Feb 09 '22 at 18:06

-3

You need to use what's called a "monkey patch" to override the bar.foo()in the imported instance of the class. This link has a sufficient example:

See Monkey Patching in Python (Dynamic Behavior) - GeeksforGeeks

edited Feb 09 '22 at 18:06

martineau

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answered Feb 09 '22 at 17:51

justtryingmybest

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In order to be immediately helpful to readers (and avoid [link-rot](http://en.wikipedia.org/wiki/Link-rot)), answers that provide at least a summary of the solution directly are preferred, with links used to offer additional information. – martineau Feb 09 '22 at 17:56

Replace a parent class with another class without keeping the old parent

2 Answers2