Overloading getitem() for two key types in Python

Question

I've got a collection of users with two types of IDs (a_id, b_id) where one is a positive integer and the other is a six-letter string. For my collection class, I'd like to be able to look up users using either type of ID.

Is there something wrong with using __contains__ and __getitem__ for either type of ID?

class UserList:
    def __init__(self, users_json):
        self.a_ids = {}
        self.b_ids = {}
        for user in users_json:
            a_id = user['a_id']
            b_id = user['b_id']
            self.a_ids[a_id] = user
            self.b_ids[b_id] = user

    def __contains__(self, some_id):
        return some_id in self.a_ids or some_id in self.b_ids

    def __getitem__(self, some_id):
        try:
            return self.a_ids[some_id]
        except KeyError:
            return self.b_ids[some_id]

Update: This is for Python 3.x, and there is no implementation of __setitem__; updating users is handled in separate API functions.

Would you be willing to provide more code? Do you have a __setitem__? Are you using Python 3.3+? The answer to all of these could make a drastically different answer. — Peilonrayz
– Peilonrayz ♦, Commented Oct 24, 2017 at 9:49
@Peilonrayz: Thanks for your input. I've clarified on those two questions: No, and yes. — sshine
– sshine, Commented Oct 24, 2017 at 13:35

Peilonrayz · Accepted Answer · 2017-10-24 13:52:40Z

From Python 3.3, the collections module added collections.ChainMap. Utilizing this class can simplify your code.
You should probably make self.a_ids and self.b_ids a list. And index to get which one you want, such as self.ids[0]. This list is created when using ChainMap anyway too.

And so I'd change your code to something more like:

from collections import ChainMap


class UserList:
    def __init__(self, users):
        self._lookup = ChainMap({}, {})
        self.ids = self._lookup.maps
        for user in users:
            self.ids[0][user['a_id']] = user
            self.ids[1][user['b_id']] = user

    def __contains__(self, some_id):
        return some_id in self._lookup

    def __getitem__(self, some_id):
        return self._lookup[some_id]

Alternately, you could subclass ChainMap, and not need to write any of the methods.

from collections import ChainMap


class UserList(ChainMap):
    def __init__(self, users):
        super().__init__({}, {})
        for user in users:
            self.maps[0][user['a_id']] = user
            self.maps[1][user['b_id']] = user

However, at that point, there's not much benefit to using a class. And so I'd just use a raw ChainMap.

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