TYP: Remove Sequence[str] in type annotations

In various places, we use Sequence[str] as a type annotation. The problem is that a regular string also matches Sequence[str] . So we need to look at the various places we use Sequence[str] and change the type accordingly. Maybe just List or ArrayLike, dependent on the context.

[Dr-Irv]

Mostly it is this:

the implementation doesn’t actually work with a “sequence of characters”?

So we document a method like DataFrame.to_string() with columns and header being a list of columns/names, and type it as Sequence[str], but you can’t just pass a single string (it would match for typing). What we want to accept there is something array-like, but not a plain string.

[Dr-Irv]

• Should this be the case for only the public API or are also for the internal code? If it is only the public code: should it be pandas and pandas-stubs?

For pandas-stubs, I’m taking out all references to Sequence[str] and replacing it with List[str]. Maybe that will be too narrow, but if someone has an argument x for some method that is a sequence and not a list, they could just do list(x) and the type checker will be happy.

I think the debate here is more about what we do internally in the code. For the PR’s I submitted, narrowing the type to List[str] still passes the mypy type checking.

The other issue is more of a documentation one, where we are not necessarily precise in the meanings of list-like, array-like. I think that many times where we write in the docs array-like, we mean 1-D array-like, as we wouldn’t accept a higher dimensional array for those arguments.