python / python/mypy

pre-3.12 python_versions do not complain about f-strings syntax, but they should

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feature topic-runtime-semantics
Dominant language
Python
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Description

Bug Report

pre-3.12 python_versions do not complain about f-strings syntax, but they should, as those pythons could not parse those f-strings.

To Reproduce

Gist URL: https://gist.github.com/mypy-play/4606f2bda8d271532cac71318aa3eb84
Playground URL: https://mypy-play.net/?mypy=latest&python=3.9&flags=strict&gist=4606f2bda8d271532cac71318aa3eb84

ignored_codes = ["1","2"]

f'Error {', '.join(ignored_codes)}'

Expected Behavior

[Syntax] error

Actual Behavior

Success!

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 Gist and Playground reproducer using Python 3.9, then trace how the checker handles f-string syntax for pre-3.12 python_versions. The fix is complete when the example produces a Syntax error for those versions and the behavior is covered by a regression test.

Written by the indexing model from the issue text.

Assessment

Tech stack
python
Domain
compilers
Issue type
Bug
Difficulty
3/5
Estimated time
1-2 days
Activity status
Stale
Clarity
Mostly clear
Newbie friendliness
38/100

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