[Python] Why does all(isinstance(x, str) for x in value) not help Pyright infer Iterable[str]...

I'm working with Pyright in strict mode and want to check if a function parameter value of type object is an Iterable[str]. I tried using:

if isinstance(value, Iterable) and all(isinstance(v, str) for v in value):
# Pyright complains: 'Type of "v" is unknown'


However, looking at the elements, Pyright complains that the type of v is still Unknown, even after the isinstance check on each element. Why doesn't all(isinstance(...)) refine the type of value to Iterable[str]?

For context: I'm implementing __contains__ in a class that inherits from collections.abc.MutableSequence.
Therefore, the method signature must remain def __contains__(self, value: object) -> bool, I can't change the type annotation of value to Iterable[str].

Here's my current implementation:

def __contains__(self, value: object) -> bool:
if isinstance(value, Iterable) and all(isinstance(v, str) for v in value):
value = "".join(value).upper()
return value in "".join(self.sequence) # self.sequence is an Iterable[str]
return False


Is there a way to get Pyright to properly infer the type of value here without using an explicit cast() or defining a separate TypeGuard function?
It seems to me that a separate TypeGuard should make no difference.

Alternatively: am I looking at this the wrong way? Should I try to avoid implementing __contains__ myself, because of the object type?

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