How to type hints for a genetic input tuple, with list[T] map to T #2063

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1. baitian752

   added

   topic: featureDiscussions about new features for Python's type annotationsDiscussions about new features for Python's type annotations

   on Aug 6, 2025
2. RBerga06 commented on Aug 6, 2025

   

   RBerga06

   on Aug 6, 2025

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   I believe this is not yet supported because in some cases it can be ambiguous. Given:

   def func[T, *Ts1, *Ts2](values: tuple[*Ts1, list[T], *Ts2]) -> tuple[*Ts1, T, *Ts2]: ...

   what should type checkers reveal for the following code?

   y = func((5, [3], ["foo"], 3.14))
   reveal_type(y)

   There are two different options that are compatible with func's signature but are not consistent with each other:

   1. The type checker infers:

      • Ts1 = tuple[int]
      • T = int
      • Ts2 = tuple[list[str], float]
      • return type is tuple[int, int, list[str], float]

   2. The type checker infers:

      • Ts1 = tuple[int, list[int]]
      • T = str
      • Ts2 = tuple[float]
      • return type is tuple[int, list[int], str, float]

   Actually, because TypeVarTuples can also bind to empty tuples, I think this counterexample can be further simplified:

   z = func(([3], ["foo"]))
   reveal_type(z)  # `tuple[int, list[str]]` or `tuple[list[int], str]`?

   See the Python Typing specification for another example of ambiguity with TypeVarTuples.
3. RBerga06 commented on Aug 6, 2025

   

   RBerga06

   on Aug 6, 2025

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   Maybe introducing TypeVarTuple bounds could allow for multiple TypeVarTuples in the same type argument list?

   def split_tuple[*Ts1: int, *Ts2: str](values: tuple[*Ts1, *Ts2]) -> tuple[tuple[*Ts1], tuple[*Ts2]]: ...
   
   class MyInt(int): ...
   class MyStr(str): ...
   
   w = split_tuple((MyInt(3), 14, MyStr("hey"), "there"))
   reveal_type(w)  # tuple[tuple[MyInt, int], tuple[MyStr, str]]

   I think this is not ambiguous, but note that I'm relying on the fact that it's impossible (at least in CPython) to construct an object that is both an instance of int and an instance of str.
4. python

   locked and limited conversation to collaborators on Aug 8, 2025
5. srittau

   converted this issue into a discussion #2064 on Aug 8, 2025