Overriding method of a class instance in Python

Question

I have an instance bear of some unknown class with a method size that returns a number. So for example:

bear.size()

will do some calculation and return a number, let's say 3.

I want to override this method so it returns double whichever number it is, so bear.size() will return 6.

When I try to implement it I get a recursion error, since the new method calls itself. So when I run:

from types import MethodType

class Animal():
    def size(self):
        return 3
bear = Animal()

def new_size(self):
    return self.size() * 2

bear.size() # returns 3. 
bear.size = MethodType(new_size, bear)
bear.size() # should return 6.

I get RecursionError: maximum recursion depth exceeded. How should I overwrite the method?

Thanks!

Assigning to bear.size creates a new, callable instance attribute (the value being a new bound method) that shadows the class attribute Animal.size. Overriding methods on a per-instance basis doesn't really make sense. — chepner
– chepner, Commented Jan 4, 2023 at 18:24
Of course there is recursion, it is calling itself over and over again, you assigned it to self.size. If you want to keep that, I guess you could use Animal.size(self) but I question the wisdom of mucking with your instance like this in the first place — juanpa.arrivillaga
– juanpa.arrivillaga, Commented Jan 4, 2023 at 18:25
@juanpa.arrivillaga it's for an edge case, so it's either this or finding and changing the original block of code, which is a central piece and would have to be compatible with the rest of the project. But it's very hacky, I agree. — Oren Neumann
– Oren Neumann, Commented Jan 4, 2023 at 20:57

AKX · Accepted Answer · 2023-01-04 18:23:56Z

4

You can stash the old implementation on the instance so you can call it in the overridden version.

from types import MethodType


class Animal:
    def size(self):
        return 3


bear = Animal()


def new_size(self):
    return self._old_size() * 2


bear.size()  # returns 3.
bear._old_size = bear.size
bear.size = MethodType(new_size, bear)
print(bear.size())  # should return 6.

Alternately, call the unbound original method with self:

from types import MethodType


class Animal:
    def size(self):
        return 3


bear = Animal()


def new_size(self):
    return Animal.size(self) * 2


bear.size()  # returns 3.
bear.size = MethodType(new_size, bear)
print(bear.size())  # should return 6.

answered Jan 4, 2023 at 18:23

AKX

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Comments

Amin · Accepted Answer · 2023-01-04 18:31:59Z

0

In addition to @AKX solution, you can also use decoration and decorate your function:

def double_size(fn):
    def wrapper():
        return fn() * 2
    return wrapper

bear.size()
bear.size = double_size(bear.size)
bear.size()

answered Jan 4, 2023 at 18:31

Amin

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1 Comment

Oren Neumann Over a year ago

Thanks! Pretty elegant solution.

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