[FIXED] How can I check if an instance of a class exists in a List in Python 3 according to value semantics instead of object identity?

Issue

In Python 3, I have a list (my_array) that contains an instance of the Demo class with a certain attribute set on that instance. In my case, the attribute is value: int = 4.

Given an instance of the Demo class, how can I determine if that instance already exists in my_array with the same properties. Here's a MCRE of my issue:

from typing import List
class Demo:
    def __init__(self, value: int):
        self.value = value
        
my_array: List[Demo] = [Demo(4)]

print(Demo(4) in my_array)  # prints False

However, Demo(4) clearly already exists in the list, despite that it prints False.

I looked through the Similar Questions, but they were unrelated.

How can I check if that already exists in the list?

Solution

You need to add an equality function to Demo:

    def __eq__(self, other):
        return isinstance(other, Demo) and self.value == other.value

P.S, since Python 3.9 you don't need to import List and just use:

my_array: list[Demo] = [Demo(4)]

Answered By - Adam.Er8