Apply Mutations on Dataclass Fields

mutation_descriptors.py

from __future__ import annotations

import json
from dataclasses import dataclass, MISSING, field as _field
from typing import Any, Callable


class MutationDescriptor:
    def __init__(self, mutator: Callable[[Any], Any]):
        self._mutator = mutator
        self._attr = None

    def __set_name__(self, owner, name):
        self._attr = name

    def __set__(self, instance, value):
        instance.__dict__[self._attr] = self._mutator(value)


def field(*, default=MISSING, default_factory=MISSING, init=True, repr=True,
          hash=None, compare=True, metadata=None, kw_only=MISSING, mutation=MISSING):
    kwargs = locals()  # Gets the field args while maintaining IDE autocomplete/type hinting
    del kwargs["mutation"]
    if mutation is not MISSING:
        if not callable(mutation):
            raise TypeError(f"Mutation callable '{mutation!r}' is not a callable.")

        kwargs["default"] = MutationDescriptor(mutation)

    return _field(**kwargs)


@dataclass
class Hello:
    a: int = field(mutation=lambda x: x ** 2)
    b: dict = field(mutation=json.dumps)


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

Inspired by @rednafi's tweet

This is one honking great idea.