Switching to Dataclasses - #5334

So I had this drafted for a bit of time now, I intended to send it earlier on to get your feedback on it, especially the logic around the after-processing of generated __init__ and whether that path seems acceptable to you.

Then I was tempted to walk till (near) the finish line and see how (simple v.s complicated) it will turn to be.

Future Stability Policy handling
arg: InitVar and _arg = field(init=False)

This did not age well.

from dataclasses import InitVar, dataclass, field


@dataclass(frozen=True)
class Example:
    value: InitVar[int | None] = None
    _value: int | None = field(init=False, default=None)

    def __post_init__(self, value: int | None, /):
        object.__setattr__(self, "_value", value)

    @property
    def value(self) -> int | None:
        print("value is deprecated and will be removed")
        return self._value


a = Example()
print(a._value)  # output: <property object at 0x7ff25e629e40>

b = Example(10)
print(b._value)  # output: 10

When not providing an explicit argument for value, the default None is overwritten by the property object.
A relevant CI issue also is that, both linting (ruff) and type checking (mypy) report:
error: Name "value" already defined/Redefinition of unused \value``

So i tried giving this alias Field specifier parameters a shot:

alias is an optional str parameter that provides an alternative name for the field. This alternative name is used in the synthesized init method.

The outcome of it is pretty neat and less verbose, however, and just like converter we have to provide runtime impl

# No runtime support fo alias, this is just to demonstrate type checkers support
from dataclasses import MISSING, dataclass, field
from typing import Any, TypeVar, dataclass_transform

_T = TypeVar("_T")


def tg_field(
    *,
    default: Any = MISSING,
    alias: str | None = None,
) -> Any:
    return field(default=default)


@dataclass_transform(field_specifiers=(tg_field,))
def tg_dataclass(cls: type[_T]) -> type[_T]:
    return dataclass(frozen=True)(cls)


@tg_dataclass
class Example:
    _value: int | None = tg_field(alias="value", default=None)

    @property
    def value(self) -> int | None:
        print("value is deprecated and will be removed")
        return self._value


# Wroks in runtime
c = Example(_value=10)

# TypeError: Example.__init__() got an unexpected keyword argument 'value'
d = Example(value=10)

Its well supported in type checkers that common IDEs use

IDE Support

Pyright

VSCode Pylance

Pycharm, should be supported from 2024.3 onwards. https://youtrack.jetbrains.com/issue/PY-73832/Pycharm-doesnt-properly-recognizes-aliases-for-Pydantic-dataclass

mypy also support aliases running it through the above MWE gives:
Unexpected keyword argument "_value" for "Example"; did you mean "value"? [call-arg]

I honestly don't know what to do about temp unfreezing ...

I just did nothing. Excluding test files, there was very minimal unfreezing of objects outside of old __init__s, so I just object.__seattr__ these. For tests, mutations use a combination of a new with _unfrozen(obj) helper and object.__seattr__

CI

Sadly pylint and mypy have a bunch of false positives that we'll have to fight some way.
pylint: pylint-dev/pylint#4899, pylint-dev/pylint#7437
mypy: python/mypy#17547

I still got some polishing to do, specifically I want to:

• Remove the enum converters and simplify with functool.partial
• Add a TO._set(). object.__setattr__ was still a one-liner but it takes too much horizontal space
• Add deprecation notes to the changelog, mostly parameter order changes
• Add tests for _utils/dataclass.py
• Update Checklist for PRs
• Adapt test_official as needed.
• Experiment with no longer tampering with the generated __init__ signatures and annotations and instead make TO._de_json() dataclasses-first, meaning to look up the correct annotation from dataclasses.fields(obj) directly rather than inpecting __init__. (For later)
• Apply similar treatment to test_official (For later)

The last two points can further simplify _utils/dataclass.py, however the current implementation of the helper is the most truthful about what __init__ actually accepts but also fights python and standard dataclasses behaviour.

I'm going to be working on these items in the following days, would be great to get some initial review since tests/ are passing now

Doesn't have to be the whole thing, just _utils/dataclass.py and the changelog.