Subclassing a model with a different namespace map causes errors

[seweissman]

We recently upgraded from 2.14.3 to 2.20 and we are seeing some strange errors with subclasses. We subclass in our code so that we can parse XML files from a 3rd party API that are created with different versions of a namespace without having to duplicate our model definitions. Here is a simple example that demonstrates what I am seeing

doc.xml:

<Company trade-name="Beboop" xmlns:pd="http://www.company.com/prod">

    <hq:headquarters xmlns:hq="http://www.company.com/hq">
        <hq:country>US</hq:country>
        <hq:state>West Virginia</hq:state>
        <hq:city>Almost Heaven</hq:city>
    </hq:headquarters>

</Company>

model.py:

import pathlib

from pydantic_xml import BaseXmlModel, attr, element

NSMAPv1 = {
    "hq": "http://www.company.com/hq/v1",
    "pd": "http://www.company.com/prod",
}
NSMAP = {
    "hq": "http://www.company.com/hq",
    "pd": "http://www.company.com/prod",
}


class Headquarters(BaseXmlModel, ns="hq", nsmap=NSMAP):
    country: str = element()
    state: str = element()
    city: str = element()


class HeadquartersV1(Headquarters, ns="hq", nsmap=NSMAPv1):
    """V1 of headquarters"""

class Company(BaseXmlModel, tag="Company", nsmap=NSMAP):
    trade_name: str = attr(name="trade-name")
    headquarters: Headquarters


xml_doc = pathlib.Path("./doc.xml").read_text()
company = Company.from_xml(xml_doc)

HeadquartersV1 is defined as a subclass of Headquarters but it isn't used anywhere. When I run this code I get the error:

Traceback (most recent call last):
  File "/python3.14/site-packages/pydantic_xml/serializers/factories/model.py", line 234, in deserialize
    return self._model.model_validate(result, strict=False, context=context)
           ~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/python3.14/site-packages/pydantic/main.py", line 731, in model_validate
    return cls.__pydantic_validator__.validate_python(
           ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^
        obj,
        ^^^^
    ...<5 lines>...
        by_name=by_name,
        ^^^^^^^^^^^^^^^^
    )
    ^
pydantic_core._pydantic_core.ValidationError: 1 validation error for Company
headquarters
  Field required [type=missing, input_value={'trade_name': 'Beboop'}, input_type=dict]
    For further information visit https://errors.pydantic.dev/2.13/v/missing

It looks like pydantic is trying to validate the wrong section of the XML against the wrong model?

If I comment out the HeadquartersV1 class it runs fine and there are no errors. The expected behavior would be that this would run without errors, since the class is defined but not used.

I have experimented with a few different versions and it seems like this behavior started in 2.15.0.

[seweissman]

I suspect that this is where the problem is: bb30d8b

In BaseXMLModel.__init_subclass__ the parent nsmap is updated. I think we probably want to set the child nsmap to a merged mapping without updating the parent:

        if parent_nsmap := getattr(cls, '__xml_nsmap__', None):
            parent_nsmap.update(nsmap or {})
            cls.__xml_nsmap__ = parent_nsmap
        else:
            cls.__xml_nsmap__ = nsmap

-->

        if parent_nsmap := getattr(cls, '__xml_nsmap__', None):
            cls.__xml_nsmap__ = parent_nsmap | (nsmap or {})
        else:
            cls.__xml_nsmap__ = nsmap

[seweissman]

Note that this may be related to this other issue: #254.

[dapper91]

fixed in release 2.21.0