Why does Inheritance polymorphism works depending on how we import in python

Question

Consider these three files:

base.py

class Base():
    def method(self):
        print('Base Class')

object = Base()

main.py

from base import object

def main():
    object.method()

if __name__ == '__main__':
    main()

test.py

import main
import base

class Derived(base.Base):
    def method(self):
        print('Derived Class')

base.object = Derived()

main.main()

I would expect that launching test.py will invoke the method() function of derived class but it is not.

$ python3.8 test.py
Base Class

But it works if I change the way I import object in main.py

new main.py

import base

def main():
    base.object.method()

if __name__ == '__main__':
    main()

new output:

$ python3.8 test.py
Derived Class

TanjiroLL · Accepted Answer · 2023-04-14 05:09:29Z

2

Short answer: because they are different object.

In the first case: When you import main, you are creating object, which I call it main_object. Then you are importing base, which will create base_object. Those object are not the same(you can use id to check).

In the second case: You are referencing to the same object, thus it will produce the predicted result. try the following:

base.py

from base import obj
print(f"object from main has id = {id(obj)}")
def main():
    obj.method()

if __name__ == '__main__':
    main()

main.py

from base import obj
print(f"object from main has id = {id(obj)}")
def main():
    obj.method()

if __name__ == '__main__':
    main()

test.py

import base

import main

class Derived(base.Base):
    def method(self):
        print('Derived Class')
base.obj = Derived()
print(f"base.obj in test.py has id = {id(base.obj)}")

main.main()

Also, object is a key word in python. It's better to use different name for variables for example obj.

answered Apr 14, 2023 at 5:09

TanjiroLL

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1 Comment

jsbueno Over a year ago

just noticing that "object" is not a keyword, it is the name of a builtin - there is nothing in the rules of the language preventing its reuse, but common sense. Which means, indeed, its usage as variable name should be avoided, nonetheless.

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