unpack dataclasses/types or subsets of them to act as typed dict

[RonnyPfannschmidt]

Feature

often times helper methods take all the fields or subsets of the fields of a type/dataclass

in coordination with #4441

@dataclass
class MyModel:
   id: uuid
   name: str
   hostname: str
   memory_size: int

and i want to declare helpers like

# bad names
Magic = SubsetTypedDict(MyModel, exclude=["id"], total=False)

def wait_for_update(model_or_id: Model|uuid, **kw: Magic) -> Model:
   ...

** context **

spin out of #4441, based on my comment and the suggestion in

[AlexWaygood]

There's already a PR to make this happen: #8583

[RonnyPfannschmidt]

@AlexWaygood i strongly disagree

my understanding is that the pr makes mypy pretends that the return value of the as_dict methods is a matching typed dict
it does not in any way give access to that type for other usage in the actual code

[AlexWaygood]

I see, apologies. Your proposed new way of constructing a TypedDict (a SubsetTypedDict), however, looks like it would probably require a PEP. It should probably be discussed over at the https://github.com/python/typing issue tracker rather than here, since it would impact the typing ecosystem as a whole rather than mypy.

[AlexWaygood]

I'm closing, because I don't think this is the appropriate forum for this kind of feature request, but please feel free to open a new issue at https://github.com/python/typing or to start a discussion on the typing-sig mailing list.

[tony]

@RonnyPfannschmidt I propose as a next step, writing this out on the typing-sig list and seeing what the reception is

I think the conversation it'd generate (whether there's overlap with someone else or a PEP) would be fruitful. Would you like to write a message there?

[RonnyPfannschmidt]

Unfortunately my bandwidth for this is currently zero, and likely to stay like that for the next few months

[tony]

@RonnyPfannschmidt No problem. Is it okay if in the future if someone stumbles upon this they can cite this as an idea on the typing-sig list if they want to?

[RonnyPfannschmidt]

Yes