LGTM.com - false positive when raising Optional guarded by flag #3066
Description
Description of the false positive
LGTM complains that the raise exc_val will raise a NoneType (which would be wrong indeed). However, the type is actually exc_val: Optional[BaseException] with a flag reraise_exc indicating whether exc_val is actually filled. The critical code section cannot be entered unless exc_val actually contains a valid value.
A simplified version of the code in question (unrelated parts removed) looks like this. Note that reraise_exc and exc_val are always set together.
reraise_exc = False
exc_type = exc_val = tb = ...
for callback in callbacks:
try:
if callback(exc_type, exc_val, tb):
reraise_exc = False
exc_type = exc_val = tb = None
except BaseException as exc:
reraise_exc = True
exc_type, exc_val, tb = type(exc), exc, exc.__traceback__
if reraise_exc:
raise exc_val
URL to the alert on the project page on LGTM.com