Repository navigation
False positive statement is unreachable in try-finally related to KeyboardInterrupt #13104
Description
Bug Report
A KeyboardInterrupt may interleave between the start of a try block and its contained statements. mypy seems to assume that the flow of execution will not be asynchronously interrupted.
def fn() -> None:
i: Optional[int] = None
try:
i = 1
finally:
if i is None:
return # error: Statement is unreachableExpected Behavior
It's trivial to show that this statement is reachable. Insert a sleep before the assignment:
def fn() -> None:
i: Optional[int] = None
try:
time.sleep(10)
i = 1
finally:
if i is None:
print("reachable")
returnAnd then Ctrl+C (KeyboardInterrupt) the function during its execution. The print will run.
Your Environment
- Mypy version used: 0.960
- Mypy command-line flags:
--warn-unreachableseems to be the only flag necessary to reproduce this
Activity
bbatliner commentedon Jul 11, 2022
Technically any exception could be raised before the statements inside of the
tryblock execute, not justKeyboardInterrupt, via CPython's asynchronous exception state that each thread tracks: https://docs.python.org/3/c-api/init.html#c.PyThreadState_SetAsyncExcbbatliner commentedon Jul 11, 2022
What makes this extra interesting is that the unreachable block appears necessary to narrow the Optional and avoid an attribute error:
whereas such narrowing is implicit without the try-finally:
tyralla commentedon Jul 20, 2025
Fixed by #19270; included in Mypy 1.17.