override.py

#!/usr/bin/env python3

class Prop:
    def __get__(self, instance, owner):
        return 'foo'


class Foo:
    foo = Prop()


x = Foo()
x.foo = 'bar'
print(x.foo)

def set_foo(self, instance, value):
    pass

Prop.__set__ = set_foo
print(x.foo)