how to make the mypy accept List[Optional[str]] as input arg for filter()?

Question

I have these lines:

tags: List[Optional[str]] = [None, "adf"]
x: List[str] = list(filter(lambda x: x is not None, tags))

the mypy errors out Argument 2 to "filter" has incompatible type "List[Optional[str]]"; expected "Iterable[str]" Is there anyway to type hint the lambda?

Answer 1

I've run into the same issue and the workaround I used was to basically do a list comprehension to remove None, which in your case would be

x: List[str] = [t for t in tags if t is not None]

I know it's a little bit less pythonic and less readable than the filter approach you were trying to use, but it seems like this is what mypy would prefer.