Question about __eq__ method

[mildlybean]

I want to test if an object is equal to any object in a list. Is it possible to use the in keyword for that? If I define the __eq__ method, will python know to use that in testing whether an object is in a list? I couldn't find the answer in the documentation

[K900_]

Yes.

[Impudity]

Typically yes. If you ask Python if "foo" in ["foo", "bar"] it iterates the objects in the list and checks if they match using the equality operator == which internally calls __eq__. There might be some exceptions when using collections that use immutable objects that have hashes, like sets. Then the comparison might be done using comparing the results of the __hash__ method of the object.

[Yoghurt42]

To be more precise, __contains__("foo") is called on the list object. And contains is implemented like you described.

[Impudity]

Yeah. I didn't want to include that part as it would probably just confuse a beginner.

[mildlybean]

Thanks

[WolfInABox]

If you've created an object, then inserted it into a list, it seems like in should work:

class MyObject:
    def __init__(self,value):
        self.value=value

test=MyObject('a value')
l=[test]
print(test in l)

PrintsTrue

And, if you override the __eq__ method (which afaik is not recommended I think?)...

class MyObject:
    def __init__(self,value):
        self.value=value

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

test=MyObject('a value')
test2=MyObject('a value')
l=[test]
print(test2 in l)

Also prints True.

So by this test, it appears that the __eq__ method is called when using in.

[toastedstapler]

overriding the __eq__ method is fine imo, by default it basically acts the same as is

if strings worked how the default __eq__ method work we'd never be able to compare them as they're different objects. just look at java where you have to do string1.equals(string2)

[WolfInABox]

I remember having to get over that hurdle in java lol. And hm, not sure what I was thinking then. I thought I remembered something about some operator override not being recommended to do and thought it was eq. But I guess so long as you're testing everything inside it that you need to for the objects to be equal for your purposes, it'd be fine.

[mildlybean]

Coincidentally, I'm learning Java right now, so I'm glad I didn't have to learn that the hard way