Return `None` instead `False`, and return `bool` to `None` in type stub (#39324)
Summary:
# What's this
Just a small bug fix related to typing stubs.
I haven't open an issue. I will do so if I must open it, but this PR is very small (only 6 lines diff).
## What I encountered
pytorch 1.5.0 with mypy 0.770 behaves odd. The code is following:
```python
import torch
def f() -> int: # Mypy says: `error: Missing return statement`
with torch.no_grad():
return 1
```
No mypy error is expected, but actually mypy 0.770 warns about `Missing return statement`.
## This is because
`mypy >= 0.730` with `--warn-unreachable` says it's unreachable because `torch.no_grad()` may "swallows" the error in the return statement.
http://mypy-lang.blogspot.com/2019/09/mypy-730-released.html
Here is a small "swallowing" example:
```python
from typing import Generator
from contextlib import contextmanager
contextmanager
def swallow_zerodiv() -> Generator[None, None, None]:
try:
yield None
except ZeroDivisionError:
pass
finally:
pass
def div(a: int, b: int) -> float: # This function seems `(int, int) -> float` but actually `(int, int) -> Optional[float]` because ` return a / b` may be swallowed
with swallow_zerodiv():
return a / b
if __name__ == '__main__':
result = div(1, 0)
print(result, type(result)) # None <class 'NoneType'>
```
To supress this behavior, one can tell mypy not to swallow any exceptions, with returning `Literal[False]` or `None` in `__exit__` method of the context manager.
# What I did
Return `None` instead of `bool` to tell mypy that "I never swallow your exception".
I chose `None` because I cannot interpret `Literal[False]` without typing_extensions in `python <=3.7`.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/39324
Differential Revision: D21833651
Pulled By: albanD
fbshipit-source-id: d5cad2e5e19068bd68dc773e997bf13f7e60f4de
