Argument "nsmap" to "Element" has incompatible type

[keith-gray-powereng]

This simplified code reproduces an error I am seeing when running mypy over my project.

from lxml import etree


def fcn() -> None:
    namespaces = {"a": "http://www.w3.org/2001/XMLSchema"}
    etree.Element(
        "{{{scl_namespaces['xs']}}}pattern", attrib={"value": "0"}, nsmap=namespaces
    )


if __name__ == "__main__":
    fcn()

The following error is produced.

c:\>mypy --strict -m lxml_mypy
lxml_mypy.py:7: error: Argument "nsmap" to "Element" has incompatible type "Dict[str, str]"; expected "Union[Dict[Optional[bytes], bytes], Dict[Optional[str], str], None]"
Found 1 error in 1 file (checked 1 source file)

I am using:

• python 3.9.5
• lxml 4.6.3
• lxml-stubs 0.2.0

I did some searching and found TypeVar and TypedDict but neither of those seemed to be able to address the issue. Does anyone have suggestions on how to resolve this typing error?

Please let me know if you need more info and thanks in advance.

[scoder]

Hmm, the error looks wrong to me. I would expect
Union[Dict[Optional[bytes], bytes], Dict[Optional[str], str], None]
to include
Dict[str, str].
Seems more something to ask on the mypy side, maybe?

[JelleZijlstra]

This is a variance issue; Dict is invariant in its key type. See https://mypy.readthedocs.io/en/stable/common_issues.html#invariance-vs-covariance.

[scoder]

Mostly a duplicate of the more general #29

[abelcheung]

Just noticed this part:

https://github.com/lxml/lxml/blob/182e0c92f7fd32701f85cad532f29c2e559757b5/src/lxml/_elementpath.py#L260-L267

In this code path which seems to be used by ET.iterfind(), _Element.iterfind() and their callers, lxml can support None as default namespace key, like {None: 'http://my.ns/'}. XPath and friends pose no problem as they don't support default namespace.

If it is decided lxml shouldn't support it in annotation, this issue can be treated as the same as #29, otherwise namespace mappings would need extra consideration of its own.

[scoder]

The type hints are incomplete anyway. Anything that helps users will probably be fine.

Let's mark this as duplicate of #29 and wait for someone to actually complain.