[Python-Dev] Dataclasses, frozen and __post_init__

[Eric V. Smith]

> On Feb 20, 2018, at 5:38 PM, Guido van Rossum <guido at python.org> wrote:
> 
>> On Tue, Feb 20, 2018 at 1:37 PM, Eric V. Smith <eric at trueblade.com> wrote:
>>> On 2/17/2018 2:35 PM, Guido van Rossum wrote:
>>> PS. I have to ponder why frozen dataclasses don't use `__new__`.
>> 
>> As I'm sure everyone is now aware after the rest of this discussion: it's because the returned object isn't really immutable.
>> 
>> That said, I have threatened to create a decorator similar to typing.NamedTuple that has the @dataclass features (except maybe __post_init__) and returns a class that does inherit from tuple, in which case it really would use `__new__`. I'll save that for 3.8, if it ever happens. There's a lot there to think about, first. For now, typing.NamedTuple is the way to go if you want something based on namedtuple yet using type hints.
> 
> But then the class would also inherit a bunch of misfeatures from tuple (like being indexable and having a length).

Right. That’s the “lots to think about” part. It might just be unworkable. 

> It would be nicer if it used __slots__ instead. (Also, the problem with __slots__ is the same as the problem with inheriting from tuple, and it should just be solved right, somehow.)

Agreed. 

Eric
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