Overriding method of a class instance in Python-CodePudding

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!

CodePudding user response：

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.

CodePudding user response：

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()