How does Python check if a dictionary is in a list

Question

I was confused weather or not dictionaries are hashable since you can check if a dictionary is in a list using the "in" operator

>>> test_list = [{'key1': 'a'}, {'key2': 'b'}]
>>> test_dict = {'key1': 'a'}
>>> test_dict in test_list
True
>>> {'key1': 'b'} in test_list
False
>>> {'key2': 'a'} in test_list
False

Running set on a dictionary doesn't exactly give a good result for this so this wasn't happening either behind the scenes

>>> set(test_dict)
{'key1'}

And it doesn't seem to be only checking the keys/values/items

>>> test_dict.keys() in test_list
False
>>> test_dict.values() in test_list
False
>>> test_dict.items() in test_list
False

`set(test_dict)` does not "hash a dictionary". Dictionaries aren't hashable — juanpa.arrivillaga, Aug 28 '23 at 21:03
So I've seen but it does make something that can be hashed out of it — Filip, Aug 28 '23 at 21:06
`set` is the `set` constructor, which takes *any iterable* and add the values to the set. `dict` objects are iterable, when you iterate over a dict, you iterate over it's keys, e.g. `for x in test_dict: print(s)`, so this was equvialent to `set(["key1"])` — juanpa.arrivillaga, Aug 28 '23 at 22:09
_grumbles about the PEP proposing `frozendict` having been rejected_ — Charles Duffy, Aug 28 '23 at 23:53
That would have honestly solved me the problem that made me investigate this thing — Filip, Aug 30 '23 at 00:33

Filip · Answer 1 · 2023-08-28T21:00:56.047

After a little digging around it looks like what I was looking for is an explanation for the list class's __contains__ magic function

This function can be found here: https://github.com/python/cpython/blob/main/Objects/listobject.c#L443C5-L443C5

But to quote a roughly equivalent version from another answer https://stackoverflow.com/a/67409987/2337471

>>> any(test_dict is item or test_dict == item for item in test_list)
True

This can be further deduced to only this part in this case

>>> any(test_dict == item for item in test_list)
True

So, when we check for the items in the list

>>> for item in test_list:
...   print(item)
{'key1': 'a'}
{'key2': 'b'}

It becomes apparent that

>>> {'key1': 'a'} == {'key1': 'a'}
True

How does Python check if a dictionary is in a list

1 Answers1