python / python/mypy

Unpacked TypeVarTuple as a function argument is not compatible with empty tuple as its default

Open
#17,113 0 comments 0 reactions 0 assignees View on GitHub

Nobody has claimed this yet.

bug topic-pep-646
Dominant language
Python
Stars
20.6k
Forks
3.3k
PR merge metrics
PR metrics pending

Description

Bug Report

Mypy complains when default an unpacked TypeVarTuple (tuple[*Ts]) to an empty tuple, but accepts one as input, which seems a conflicting behaviour at least.

To Reproduce

https://mypy-play.net/?mypy=latest&python=3.12&gist=9c2fa71781c45ed24b1901b8e9252e64

from collections.abc import Callable
from typing import TypeVarTuple

Ts = TypeVarTuple("Ts")


def foo1(func: Callable[[*Ts], None], args: tuple[*Ts] = ()) -> None:  # error: Incompatible default for argument "args" (default has type "tuple[()]", argument has type "tuple[*Ts]")  [assignment]
    func(*args)


def foo2(func: Callable[[*Ts], None], args: tuple[*Ts]) -> None:
    func(*args)

def bar() -> None: ...

foo2(bar, ())  # ok

Expected Behavior

args: tuple[*Ts] can have an empty tuple as default.

Actual Behavior

args: tuple[*Ts] cannot default to an empty tuple.

Your Environment

  • Mypy version used: 1.9.0
  • Mypy command-line flags: N/A
  • Mypy configuration options from mypy.ini (and other config files): N/A
  • Python version used: 3.11

Contributor guide

Open the contributing guide

First steps

  1. Read the whole issue, then the project's contributing guide.
  2. Comment on the issue to say you are picking it up — it saves two people doing the same work.
  3. Fork the repository and make your change on a branch.
  4. Open a pull request that references the issue number.

Research direction

Start with the linked mypy-play reproducer and the foo1/foo2 example using an unpacked TypeVarTuple. Trace the type-checking behavior for the empty default versus the accepted empty-tuple argument; done means foo1 accepts the default without an incompatible-default diagnostic while foo2 remains valid.

Written by the indexing model from the issue text.

Assessment

Tech stack
python
Domain
devtools
Issue type
Bug
Difficulty
4/5
Estimated time
3-5 days
Activity status
Stale
Clarity
Mostly clear
Newbie friendliness
42/100

Get new issues in your inbox

A short digest of beginner-friendly GitHub issues.