python / python/mypy

Literal `(*tuple_of_strings, "string")` not assignable to `tuple[str, *tuple[str, ...]]`

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bug topic-pep-646
Dominant language
Python
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Description

Bug Report

Mypy doesn't allow assigning a literal constructed by unpacking a tuple of unknown length mixed with more literals to a variable with a matching tuple type, when the value's unpacking order doesn't exactly match the type expression, even if the resulting tuple's type should match.

To Reproduce

a: tuple[str, ...]
b: tuple[str, *tuple[str, ...]]
b = ("first", *a) # ok
b = (*a, "last") # Incompatible types in assignment (expression has type "tuple[str, ...]", variable has type "tuple[str, *tuple[str, ...]]")

https://mypy-play.net/?mypy=latest&python=3.14&gist=7f1d80e082b57c2c0aa7a3aa38f094be

Actual Behavior

main.py:4: error: Incompatible types in assignment (expression has type "tuple[str, ...]", variable has type "tuple[str, *tuple[str, ...]]") [assignment]

Your Environment

  • Mypy version used: 2.0.0
  • Mypy command-line flags: None
  • Mypy configuration options from mypy.ini (and other config files): None
  • Python version used: 3.14

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 by reproducing the assignment in main.py with the Python 3.14 example and the linked mypy-play case. Trace the tuple unpacking and type-checking path, then add a regression test showing that both unpacking orders are accepted when their resulting types match.

Written by the indexing model from the issue text.

Assessment

Tech stack
python
Domain
compilers
Issue type
Bug
Difficulty
4/5
Estimated time
3-5 days
Activity status
Quiet
Clarity
Clearly specified
Newbie friendliness
55/100

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