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Detect impossible condition caused by user defined __bool__ #7008
Description
Eg with the following:
from typing import *
class Foo:
def __bool__(self) -> Literal[True]:
return True
def some_foo() -> Foo:
...
def throw_error():
raise Exception
if __name__ == '__main__':
print(some_foo() or throw_error())
# ^^^^^^^^^^^^^ impossible condition, unreachablemypy should be able to detect that the call to throw_error is unreachable
Activity
ilevkivskyi commented on Jun 17, 2019
Well mypy doesn't detect even simpler unreachable function calls like True or print(). It may be a useful feature to warn about unreachable code.
Related to #2395
ilevkivskyi commented on Jun 17, 2019
#6990 is another example where this may be useful.
Michael0x2a commented on Jun 17, 2019
FWIW I've started sort of looking into adding in some kind of flag for this during the past week (I'm already poking around this corner of the codebase for literal and enum reachability stuff anyways).
I think in principle it should be straightforward: to avoid false-positives with things like sys.platform checks, we don't alert on blocks that were marked as unreachable as a part of semantic analysis and only look at results from the type-checking phase.
We'll also probably want to adjust mypy so it distinguishes between intentionally and unintentionally unreachable blocks. E.g. if somebody does if False: ... that was probably intentional so we'll probably need to do something different there. Probably we'll need to refactor find_isinstance_check a bit to make this all work cleanly, but I was maybe thinking of doing that anyways.
TBH I think the hardest/most annoying part will be constructing an understandable error message for subexpressions that were skipped due to being unreachable, just due to the "line-by-line" nature of error reporting in mypy.
Possibly adding in a "fyi this expression is always True/always False" error message might be sufficient instead, as opposed to indicating specific ignored subexpressions. Not sure, still need to do more poking around.
ilevkivskyi commented on Jun 19, 2019
On Python 3.8 there is a simple solution for that: ast.get_source_segment() gives exact source code for every AST node. But maybe a more generic error message would be sufficient.
On the other hand adding a # type: ignore to that code of switching it to if typing.TYPE_CHECKING: ... is probably a fine solution too.
graingert commented on Jun 19, 2019
Add flag to warn about unreachable branches and exprs
Add flag to warn about unreachable branches and exprs (#7050)
ilevkivskyi commented on Sep 5, 2019
It looks like #7454 will help with this. @Michael0x2a is there something else apart from better error location needed for this issue? If not, I would propose to close this one.
ilevkivskyi commented on Sep 5, 2019
Actually I think -> Literal[True] (currently not supported by --warn-unreachable) part is enough to keep this open.
intgr commented on Mar 21, 2023
FWIW mypy is now capable of warning on ilevkivskyi's case with --warn-unreachable (Playground link)
It doesn't handle def __bool__(self) -> Literal[True] for objects.
Should we close this or keep it open for extra __bool__ handling?
ilevkivskyi commented on Mar 27, 2023
Yeah, let's keep this open for __bool__ support.
Support __bool__ with Literal in --warn-unreachable (python#15645)