Optional[T] in {None,...} does not drop the Optional

[Carreau]

On latest mypy; of course the code can use if not var: which would have the same effect.

$ cat foo.py
import os

var = os.environ.get('ENV')
if var in {None, ''}:
    var = 'default'
else:
    var =  var.lower()

$ mypy foo.py
foo.py:7: error: Item "None" of "Optional[str]" has no attribute "lower"
Found 1 error in 1 file (checked 1 source file)

$ mypy --version
mypy 0.770+dev.2d06ba71aa087e8d8a6eb4a37bb9a6254f7dbb70

Thanks

[EkanshMangal]

Hello Sir
Can i fix this issue?

[Carreau]

Hi @EkanshMangal,

I see there is a Contributing file on this repository I would suggest starting there for instructions.

[AlexWaygood]

Closing as a duplicate of #10977, which is a newer issue but has a lot more discussion attached to it.