NSMapArg type definition is incorrect.

[dapper91]

nsmap argument of Element function is of type Optional[_NSMapArg] where _NSMapArg is Mapping[str, str].

But according to the documentation here namespace key for the default namespace supposed to be None:

...
>>> NSMAP = {None : XHTML_NAMESPACE} # the default namespace (no prefix)

>>> xhtml = etree.Element(XHTML + "html", nsmap=NSMAP) # lxml only!
...

So it seems like _NSMapArg should be Mapping[Optional[str], str]

[ajnelson-nist]

I also just encountered this issue, and agree that the documentation disagrees with the type stubs.