Should it be possible to use Unpack in Union?

[sobolevn]

I have the following function:

def random_tuple_element(t: tuple):
     return random.choice(t)

My tuples can have any structure. [int, int, int] or [str, int, bool, CustomObject] or whatever.

I thought that something like this should be possible:

import random
from typing import Union
from typing_extensions import TypeVarTuple

Ts = TypeVarTuple('Ts')

def random_tuple_element(t: tuple[*Ts]) -> Union[*Ts]:
     return random.choice(t)

But, right now it is not possible: https://mypy-play.net/?mypy=latest&python=3.12&gist=5ee956588f2eb33faba76e87902cf59a

# E: Unpack is only valid in a variadic position

Does it make sense to allow Unpack in Union?

[erictraut]

Unpacking a TypeVarTuple within a Union was supported in earlier drafts of PEP 646, but it was removed before the final draft was submitted to the SC for approval.

I implemented support for this in pyright (based on the earlier draft PEP). It was quite difficult to add, so I empathize with the PEP authors who justified the removal based on a lack of use case plus feedback from the pyre team. I'll note that pyre and pytype still don't have support for TypeVarTuple in general, and it has been four years since PEP 646 was finalized.

There have been some requests to add back this capability. See here and here. None of these have progressed beyond an initial discussion.

Pyright still contains my original implementation (I didn't have the heart to rip it out completely after spending so much time getting it working), but it's enabled only if you set enableExperimentalFeatures to true. Here's an example in the pyright playground.

If you'd like to see this added to the typing spec, you could propose an update to the spec. This change is probably sufficiently small that it wouldn't require a full PEP, but there should be a discussion with an opportunity for type checker maintainers to weigh in.