Reference before assignment errors

[jrab89]

Bug Report

mypy doesn't catch "reference before assignment" errors

To Reproduce

import os
from typing import Optional


def greet(name: Optional[str]) -> None:
    if name:
        name_str = f", {name}."

    print(f"Hello{name_str}")
    name_str = "bar"


def main() -> None:
    greet(os.environ.get("NAME"))


if __name__ == "__main__":
    main()

Expected Behavior

If the above program is ran without NAME set, it'll crash:

$ NAME='Jeff' python reference_before_assign.py
Hello, Jeff.
$ python reference_before_assign.py
Traceback (most recent call last):
  File "/Users/jeff.rabovsky/git_repos/notes/reference_before_assign.py", line 17, in <module>
    main()
  File "/Users/jeff.rabovsky/git_repos/notes/reference_before_assign.py", line 13, in main
    greet(os.environ.get("NAME"))
  File "/Users/jeff.rabovsky/git_repos/notes/reference_before_assign.py", line 9, in greet
    print(f"Hello{name_str}")
UnboundLocalError: local variable 'name_str' referenced before assignment

My expectation was that mypy would catch this error.

Actual Behavior

$ mypy --strict reference_before_assign.py
Success: no issues found in 1 source file

Your Environment

• Mypy version used: 0.982
• Mypy command-line flags: --strict
• Mypy configuration options from mypy.ini (and other config files): none
• Python version used: 3.10.5

Thanks for your help!

[hauntsaninja]

Thanks, duplicate of #2400 , #686 , etc. Is actively being worked on these days

[AlexWaygood]

(Sorry, reopened by accident...)

[PeterJCLaw]

Connecting the dots as the linked issues are closed: looks like #4019 tracks this case now.