Replace method with method of instance of different class

Question

I just tried to make a system with some classes with similar parent class, which dynamically assigns their method between them, It looks like one function "transform(from, to)", which doing smth like from.foo = to.foo

So, you see, that my problem is difficult with Python, so I'm asking for help

My first try was

def transform(a, b):
    a.foo = b.foo


class A:
    def foo(self):
        pass


class B(A):
    def foo(self):
        print("bar")
        transform(self, C())


class C(A):
    def foo(self):
        print("foo")
        transform(self, B())

b = C()
b.foo()
b.foo()
b.foo()
b.foo()

I want to get from it smth like

foo
bar
foo
bar

But I get

foo
bar
bar
bar

The funniest moment of all is that first conversion working, but next.

My next try was

class A:
    def foo(self):
        pass

    def transform(self, to):
        self.foo = to.foo


class B(A):
    def foo(self):
        print("bar")
        self.transform(C())


class C(A):
    def foo(self):
        print("foo")
        self.transform(B())

b = C()
b.foo()
b.foo()
b.foo()
b.foo()

and I got the same result. Do you have any idea how to manage this conversion?

Ruzihm · Accepted Answer · 2019-11-08 16:52:52Z

1

In your code, it first runs a.foo = b.foo, where a was the object created at b = C(), and b was the object created at B(). Then when a.foo() is next run, it runs B's foo(self) where self is that B() object. There, it runs transform(self, C()) where self is still the object created with B(), not the object at b. Basically, b is only modified once then never modified again.

Instead, you should get the unbound function object of foo with b.foo.__func__ before assigning it to the other one, or the self in foo will refer to the object B() instead of b. Then, you can bind it back to the destination object with types.MethodType:

from types import MethodType

def transform(a, b):
    a.foo = MethodType(b.foo.__func__, a)

class A:
    def foo(self):
        pass


class B(A):
    def foo(self):
        print("bar")
        transform(self, C())


class C(A):
    def foo(self):
        print("foo")
        transform(self, B())

b = C()
b.foo()
b.foo()
b.foo()
b.foo()

foo
bar
foo
bar

Your 2nd attempt is also fixed by doing the same thing:

from types import MethodType

class A:
    def foo(self):
        pass

    def transform(self, to):
        self.foo = MethodType(to.foo.__func__, self)


class B(A):
    def foo(self):
        print("bar")
        self.transform(C())


class C(A):
    def foo(self):
        print("foo")
        self.transform(B())

b = C()
b.foo()
b.foo()
b.foo()
b.foo()

foo
bar
foo
bar

edited Nov 8, 2019 at 16:52

answered Nov 8, 2019 at 16:37

Ruzihm

20.3k5 gold badges40 silver badges53 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

geckos Over a year ago

So he was changing the self object between calls this is why it wasn't working?

Ruzihm Over a year ago

@geckos yes, in asker's original code, the first time it ran a.foo = b.foo, where a was the object created at b = C(), and b was the object created at B()--then when a.foo() was next run, it would run B's foo(self) where self was that B() object. There, it would run transform(self, C()) where self was the object created by B(). Basically, b was only modified once then never modified again.

Collectives™ on Stack Overflow

Replace method with method of instance of different class

1 Answer 1

2 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

2 Comments

Your Answer

Sign up or log in

Post as a guest

Related