245 – frozendict

Python 3.15 comes with the new built-in type frozendict, an immutable dictionary type.

Trying to modify an existing key/value pair or adding a new key raises a TypeError:

>>> fd = frozendict({"one": 2})
>>> fd["one"] = 1
TypeError
>>> fd["two"] = 2
TypeError

It’s also worth noting that the immutability of a frozen dictionary is shallow. You can still mutate the underlying values:

>>> fd = frozendict({"list": []})
>>> fd
frozendict({'list': []})
>>> fd["list"].append(73)
>>> fd
frozendict({'list': [73]})

Remember that this behaviour matches the behaviour of a tuple.