Variadic tuple unpacking requires a star target -- why?
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Description
Bug Report
When attempting to unpack a tuple that uses Unpack, if you don't use a star target to capture the variadic tuple as a list, you get the error Variadic tuple unpacking requires a star target.
This is overly strict and prescriptivist and is not even applied consistently between similar type concepts within mypy.
- You definitely can do this if the lengths check out.
pyrightdoesn't raise an error like this. You are simply on your own for length checking the unpacking of indeterminate lengthtuples, just like you are forlist, another indeterminate length unpackable sequence.- In fact, this is also how
mypytreats unpackinglist. It doesn't raise an error if you don't use a star target. - I have a real use case where I want to do exactly this. I lay out my use case in this bug report to
pyright: https://github.com/microsoft/pyright/issues/7987
To Reproduce
from typing import Tuple
from typing_extensions import Unpack
def foo() -> Tuple[int, Unpack[Tuple[str, ...]]]:
return 1, "bar", "baz"
a, b, c = foo() # error: Variadic tuple unpacking requires a star target [misc]
d, e, f = [1, "bar", "baz"] # OK
Gist URL: https://gist.github.com/mypy-play/4014976ca069e54b5c1fe86a45353f58
Playground URL: https://mypy-play.net/?mypy=master&python=3.12&gist=4014976ca069e54b5c1fe86a45353f58
Expected Behavior
No errors in the above example. a's type should be revealed to be int, and b and c's types should be revealed to be str.
Actual Behavior
An error, and a, b, and c are all revealed to be Any.
Contributor guide
First steps
- Read the whole issue, then the project's contributing guide.
- Comment on the issue to say you are picking it up — it saves two people doing the same work.
- Fork the repository and make your change on a branch.
- Open a pull request that references the issue number.
Research direction
Start with the minimal reproduction in the report or its mypy-play URL, and compare the diagnostic and revealed types for tuple and list unpacking. Done means the tuple assignment produces no error, reveals a as int and b and c as str, and does not regress the list case.
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
- Stale
- Clarity
- Mostly clear
- Newbie friendliness
- 42/100