Use Element[Any] instead of Element in ElementTree

[MatthewMckee4]

Resolves #14036

[srittau]

I haven't looked too deeply into this, but I don't think changing the Element instances from Element (which is Element[str]) to just Element[Any] is the right course of action. This will probably make previous type safe code unsafe. For example:

from xml.etree import ElementTree

tree = ElementTree.parse("foo.xml")
el = tree.find(".//*")
assert el is not None
print(type(el.tag))

el.tag is str in this case and it's typed as such. Now, it is Any.

This needs a more nuanced approach. I'm not sure what problems @JelleZijlstra alluded to in #14036, though.

[JelleZijlstra]

The problem is primarily in argument types, not return types. For example, the tostring function is (still with this PR) marked as accepting only Element, which means only Element[str]. And because Element is invariant, that means any other Element is not allowed.

For return types, returning Element[str] might be fine though it should be verified that that's what these methods actually return.

I'm becoming skeptical that the way we're using TypeVar defaults has been a good idea; it can lead to usability issues quite easily.

[MatthewMckee4]

because append accepts Element[Any] it seems appropriate to accept Element[Any] for insert, remove and __setitem__

[tungol]

I'm becoming skeptical that the way we're using TypeVar defaults has been a good idea; it can lead to usability issues quite easily.

As the person who added that part to the xml stubs, I believe my thought process was that tag is str in the normal use cases, so I didn't want downstream consumers to have to worry about it unless they were using one of the weirder use cases.

This particular issue came up because it seems like I didn't look at the uses of Element without parameters that existed in the stubs before my MR, which was definitely an oversight.

[srittau]

Thanks, and sorry for the delay.

[srittau]

stubtest errors are unrelated.

[github-actions]

According to mypy_primer, this change has no effect on the checked open source code. 🤖🎉