A dict, which remembers missing keys

RememberMissingKeysDict.py

from collections import UserDict


class RememberMissingKeysDict(UserDict):
    def __init__(self, *args, **kwargs):
        super().__init__(*args, **kwargs)
        self.missing_keys = set()

    def get(self, k, default_v=None):
        if super().get(k) is None:
            self.missing_keys.add(k)

        return super().get(k, default_v)


if __name__ == "__main__":
    d = RememberMissingKeysDict({"a": 1, "b": 2})
    d.get("c")
    d.get("d")

    assert d.missing_keys == {"c", "d"}