A Python decorator that makes a class immutable after initialization.

frozen.py

# SPDX-License-Identifier: MIT
# SPDX-FileNotice: The Expat/MIT variant is the one without the "next paragraph" parenthetical but with the "MIT License" header.
# Copyright (C) 2021 nfitzen <https://github.com/nfitzen>

# Note that this code is very basic and likely buggy, as with everything I
# write. Specifically, the help message is a bit weird and slightly inaccurate.


import functools
from typing import Any

__all__ = (
    "FrozenInstanceError",
    "frozen",
)


class FrozenInstanceError(AttributeError):
    pass


def frozen(cls: type) -> type:
    """Freezes an object after calling its __init__ method."""

    def setat(self, name: str, value: Any) -> None:
        """Raises FrozenInstanceError after initialization."""
        if getattr(self, "__is_frozen__", False):
            raise FrozenInstanceError
        object.__setattr__(self, name, value)

    def delat(self, name: str) -> None:
        """Raises FrozenInstanceError after initialization."""
        if getattr(self, "__is_frozen__", False):
            raise FrozenInstanceError
        object.__delattr__(self, name)

    @functools.wraps(cls.__init__)
    def init(self, *args, **kwargs):

        return_val = self.__frozen_init__(*args, **kwargs)

        self.__is_frozen__ = True

        return return_val

    orig_init = cls.__init__

    setattr(cls, "__init__", init)
    setattr(cls, "__frozen_init__", orig_init)
    setattr(cls, "__delattr__", delat)
    setattr(cls, "__setattr__", setat)

    return cls