python / python/typing

TypeVarTuple Transformations Before Unpack

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topic: feature
Dominant language
Python
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Description

It would be useful to be able to apply type transformations to TypeVarTuple's types. Some motivating examples:

 Ts = TypeVarTuple('Ts')

 class Example(Generic[*Ts]):
     def __init__(self, *types: *type[Ts]) -> None:
         self.types = types

     def create_defaults(self) -> tuple[*Ts]:
         return tuple(t() for t in self.types)

     def create_lists(self) -> tuple[*list[Ts]]:
         return tuple([t()] for t in self.types)

     def create_callables(self) -> tuple[*Callable[[], Ts]]:
         return tuple(lambda: t() for t in self.types)

 e = Example(int, str)

 assert_type(e.create_defaults(), tuple[int, str])
 assert_type(e.create_lists(), tuple[list[int], list[str]])
 assert_type(e.create_callables(), tuple[Callable[[], int], Callable[[], str]])

At a high level, everything between the * and the TypeVarTuple it expands would be applied per type in the tuple before it's unpacked.

There are some decisions to be made about how the expansion is done if there are multiple TypeVarTuples in the expand expression, but I think it can be supported and I think the intuitive thing would be to zip up all the TypeVarTuples. For example,
tuple[*tuple[Ts, Us]] with Ts = (int, str) and Us = (float, bytes) would be tuple[tuple[int, float], tuple[str, bytes]], with the implementation enforcing equal-length TypeVarTuples.

If there's a workaround for this using the existing logic, let me know. My current use case involves keeping a reference to the runtime types, so *type[Ts] is what I'm looking for at the moment.

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First steps

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  3. Fork the repository and make your change on a branch.
  4. Open a pull request that references the issue number.

Research direction

No files, tests, or entry points are named. Start by reviewing the existing TypeVarTuple unpacking behavior and the motivating examples, then define how transformations apply before unpacking, including the proposed equal-length zip semantics for multiple TypeVarTuples. Done means the feature's behavior and unresolved expansion decisions are specified clearly enough to implement.

Written by the indexing model from the issue text.

Assessment

Tech stack
python
Domain
devtools
Issue type
Feature
Difficulty
5/5
Estimated time
Over a week
Activity status
Stale
Clarity
Mostly clear
Newbie friendliness
30/100

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