Mutate the fields of a dataclass by applying ad-hoc mutation callables.

mutator.py

from __future__ import annotations

import json
from collections.abc import Mapping
from dataclasses import dataclass
from typing import Callable


class Mutator:
    def __init_subclass__(
        cls, mutation: dict[str, Callable] | None = None
    ) -> None:
        if not mutation:
            return None

        if not isinstance(mutation, Mapping):
            raise TypeError(
                "Mutation must be a Mapping between attribute name "
                "and mutation callables.",
            )
        cls.__mutation__ = mutation

    def __post_init__(self) -> None:
        for attr, cb in self.__class__.__mutation__.items():
            if not isinstance(attr, str):
                raise TypeError(
                    f"Mutation attribute '{attr}' is not a string."
                )
            if not callable(cb):
                raise TypeError(f"Mutation callable '{cb}' is not a callable.")

            setattr(self, attr, cb(getattr(self, attr)))


@dataclass
class Hello(Mutator, mutation={"a": lambda x: x ** 2, "b": json.dumps}):
    a: int
    b: dict


print(Hello(4, {"a": "b"}))