intentionally empty generator marked as unreachable

[asottile]

Bug Report

this seems to be a common pattern:

def f() -> Generator[None, None, None]:
 return
 yield

it can instead be rewritten as:

def f() -> Generator[None, None, None]:
 yield from ()

To Reproduce

see above

Expected Behavior

(no errors)

Actual Behavior

$ mypy --warn-unreachable t.py
t.py:5: error: Statement is unreachable [unreachable]
Found 1 error in 1 file (checked 1 source file)

Your Environment

• Mypy version used: 1.3.0
• Mypy command-line flags: --warn-unreachable
• Mypy configuration options from mypy.ini (and other config files): N/A
• Python version used: 3.10.6

[Hayashi-Yudai]

Uum... I couldn't reproduce this bug.

• mypy: 1.3.0
• with --warn-unreachable flag
• w/o mypy.ini
• python 3.10.5

❯ mypy --warn-unreachable t.py
Success: no issues found in 1 source file

❯ cat t.py
from typing import Generator


def f() -> Generator[None, None, None]:
 yield from ()

[asottile]

yes you've used the workaround.

[ikonst]

What could be a good heuristic here? That an empty yield statement (without expression) is explicitly allowed to trail a return statement?

[asottile]

I'm not sure -- I think the only case is specifically the one in the issue -- I can't think of others since if it were conditional then there'd probably be a yield in the other arm. maybe this is specifically a cutout for return; yield as the total body?

[ikonst]

How about every function having a budget for one bare yield that doesn't get flagged?

[ikonst]

Planning to address it on top of #15386.