[asukaminato0721]

Summary

Fixes #3352

unpack-length checking rejects string-like unpack sources (str, string literals, LiteralString) with bad-unpacking, which catches for k, v in {"x": 1} because dict iteration yields keys.

Test Plan

update && add test

[Copilot]

Pull request overview

Fixes a false-negative where tuple-unpacking in for loop targets did not report an unpacking error when the iterated element type was string-like (e.g., dict key iteration yielding str), by adding a string-like guard to unpack-length checking and updating/adding regression tests.

Changes:

• Add is_string_like helper and emit bad-unpacking when unpack-length checking is applied to string-like types.
• Update existing string-literal unpacking tests to assert the expected error.
• Add a new regression test covering for k, v in {"x": 1} and for a, b in list[str].

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
pyrefly/lib/alt/solve.rs	Add string-like detection and produce BadUnpacking in BindingExpect::UnpackedLength for string-like types.
pyrefly/lib/test/assign.rs	Update existing unpack-string tests and add for-loop unpacking regression coverage.

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[yangdanny97]

the added errors for apprise look fishy, both of them are typed as list[tuple[str, str]] which should be legal

https://github.com/caronc/apprise/blob/dc93c2a3747ce595032949db75e9dc48a05296d8/apprise/plugins/xmpp/base.py#L202

https://github.com/caronc/apprise/blob/dc93c2a3747ce595032949db75e9dc48a05296d8/apprise/plugins/xmpp/adapter.py#L232

[asukaminato0721]

from __future__ import annotations
x: list[ (str, str) ]= []

Expected 1 type argument for `list`, got 2 [[bad-specialization](https://pyrefly.org/en/docs/error-kinds/#bad-specialization)]

[asukaminato0721]

Looks like the problem with tuple handling.

[codspeed]

Merging this PR will not alter performance

✅ 35 untouched benchmarks
⏩ 9 skipped benchmarks¹

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_{Comparing asukaminato0721:3352 (b01e367) with main (1af3067)}

Footnotes

1. 9 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

[stroxler]

Thanks! I think this has a number of improvements it needs before we can merge:

Parenthesized generic arguments change meaning

expr.rs:5367–5375

Python treats dict[(str, int)] as dict[str, int]. The new helper instead constructs one tuple-type argument.

Confirmed on the PR head:

• dict[(str, int)] errors: “Expected 2 type arguments for dict, got 1.”
• For class Box[T], Box[(int,)] becomes Box[tuple[int]] instead of Box[int].

Fix: Remove the tuple reinterpretation and preserve Ast::unpack_slice semantics. The new fixture should use list[tuple[str, str]]; list[(str, str)] supplies two arguments to list, not one tuple type.

Star-only string unpacking is rejected despite always being valid

solve.rs:2789–2805

def f(s: str) -> None:
    *chars, = s

The PR reports “Cannot unpack str into 0+ values.” This assignment succeeds for every string length, including zero. Both new rejection branches ignore the SizeExpectation::Ge(0) case; LiteralString has the same problem.

Fix: At minimum, exempt Ge(0) and retain ordinary iteration.

Coverage gap

Union members bypass the new string rejection: unpacking elements of list[str | tuple[int, int]] produces no diagnostic. If the broader string-unpacking policy is retained, apply it consistently across branches.

[github-actions]

Diff from mypy_primer, showing the effect of this PR on open source code:

============================================================
SUMMARY
============================================================
Total: +15 new errors, -2 fixed errors
By preset: +15/-0 (default), +15/-0 (strict)

Projects with changes (10):
  apprise: +4 -0
  setuptools: +1 -0
  streamlit: +1 -0
  cloud-init: +4 -0
  aiohttp: +1 -0
  pywin32: +1 -0
  werkzeug: +1 -1
  spack: +1 -0
  pylint: +1 -0
  porcupine: +0 -1
============================================================

FULL DIFF DETAILS
------------------------------------------------------------

apprise (https://github.com/caronc/apprise)
+ ERROR apprise/plugins/blink1.py:328:9-16: Cannot unpack str into 3 values [bad-unpacking]
+ ERROR apprise/plugins/xmpp/adapter.py:254:25-38: Cannot unpack str into 2 values [bad-unpacking]
+ ERROR apprise/plugins/xmpp/adapter.py:964:17-30: Cannot unpack str into 2 values [bad-unpacking]
+ ERROR apprise/plugins/xmpp/base.py:374:17-28: Cannot unpack str into 2 values [bad-unpacking]

setuptools (https://github.com/pypa/setuptools)
+ ERROR setuptools/tests/test_build_ext.py:387:11-27: Cannot unpack str into 2 values [bad-unpacking]

streamlit (https://github.com/streamlit/streamlit)
+ ERROR lib/streamlit/runtime/state/query_params.py:917:13-23: Cannot unpack str into 2 values [bad-unpacking]

cloud-init (https://github.com/canonical/cloud-init)
+ ERROR tests/unittests/cmd/devel/test_make_mime.py:40:5-17: Cannot unpack str into 1 value [bad-unpacking]
+ ERROR tests/unittests/test_net.py:2128:17-37: Cannot unpack str into 2 values [bad-unpacking]
+ ERROR tests/unittests/test_net.py:2804:17-37: Cannot unpack str into 2 values [bad-unpacking]
+ ERROR tests/unittests/test_net.py:3053:17-37: Cannot unpack str into 2 values [bad-unpacking]

aiohttp (https://github.com/aio-libs/aiohttp)
+ ERROR aiohttp/cookiejar.py:492:13-25: Cannot unpack str into 2 values [bad-unpacking]

pywin32 (https://github.com/mhammond/pywin32)
+ ERROR win32/Demos/getfilever.py:28:5-19: Cannot unpack str into 2 values [bad-unpacking]

werkzeug (https://github.com/pallets/werkzeug)
- ERROR src/werkzeug/datastructures/headers.py:504:78-104: Unused `# type: ignore` comment [unused-type-ignore]
+ ERROR src/werkzeug/middleware/shared_data.py:115:13-23: Cannot unpack str into 2 values [bad-unpacking]

spack (https://github.com/spack/spack)
+ ERROR lib/spack/spack/spec.py:591:13-47: Cannot unpack str into 3 values [bad-unpacking]

pylint (https://github.com/pycqa/pylint)
+ ERROR pylint/checkers/variables.py:341:9-34: Cannot unpack str into 2 values [bad-unpacking]

porcupine (https://github.com/Akuli/porcupine)
- ERROR porcupine/plugins/directory_tree.py:410:64-78: Unused `# type: ignore` comment [unused-type-ignore]