A defaultdict that can be frozen after which it will not add new missing keys but still return a default object

freezable_defaultdict.py

from collections import defaultdict


class freezable_defaultdict(dict):
    def __init__(self, default_factory, *args, **kwargs):
        self.frozen = False
        self.default_factory = default_factory
        super(freezable_defaultdict, self).__init__(*args, **kwargs)

    def __missing__(self, key):
        if self.frozen:
            return self.default_factory()
        else:
            self[key] = value = self.default_factory()
            return value

    def freeze(self):
        self.frozen = True

An alternative is to use defaultdict.get(key, default_value), which doesn't add a new entry for a missing key in defaultdict

Another alternative is to set defaultdict default_factory to None. This will raise an error instead of adding a new default. See also https://stackoverflow.com/a/13465745 and https://docs.python.org/3/library/collections.html#collections.defaultdict