Fix type annotations for xml_field_validator/serializer

There are 2 issues with the annotations:

1. Function types are contravariant rather than covariant in their
   argument types. This means that `Callable[[BaseXmlModel], Any]` is a
   subtype rather than a supertype of `Callable[[MyModel], Any]` and so
   using `Callable[[BaseXmlModel], Any]` as a lower bound of TypeVars
   doesn't work. Since Python doesn't support upper bounds on TypeVars,
   the solution is to push the TypeVar inside. To see the issue run mypy
   on the example from the documentation[1]. Currently, you get these
   errors:

```
t.py:13: error: Value of type variable "ValidatorFuncT" of function cannot be "Callable[[type[Plot], XmlElementReader, str], list[float]]"  [type-var]
t.py:21: error: Value of type variable "SerializerFuncT" of function cannot be "Callable[[Plot, XmlElementWriter, list[float], str], None]"  [type-var]
```

   This PR resolves these (however, validate_space_separated_list needs
   to be decorated with @classmethod).

2. A recent commit[2] has a typo where ValidatorFuncT was bound by
   SerializerFunc instead of ValidatorFunc.

[1] https://pydantic-xml.readthedocs.io/en/latest/pages/misc.html#custom-xml-serialization
[2] ec4b547

Author: eltoder

examples/xml-serialization-decorator/model.py

@@ -11,9 +11,12 @@ class Plot(BaseXmlModel):
     y: List[float] = element()
 
     @xml_field_validator('x', 'y')
+    @classmethod
     def validate_space_separated_list(cls, element: XmlElementReader, field_name: str) -> List[float]:
-        if element := element.pop_element(field_name, search_mode=cls.__xml_search_mode__):
-            return list(map(float, element.pop_text().split()))
+        if (sub_element := element.pop_element(field_name, search_mode=cls.__xml_search_mode__)) and (
+            text := sub_element.pop_text()
+        ):
+            return list(map(float, text.split()))
 
         return []
 

pydantic_xml/fields.py

@@ -1,6 +1,6 @@
 import dataclasses as dc
 import typing
-from typing import Any, Callable, Optional, TypeVar, Union
+from typing import Any, Callable, Optional, Union
 
 import pydantic as pd
 import pydantic_core as pdc
@@ -292,36 +292,38 @@ def computed_element(
     return computed_entity(EntityLocation.ELEMENT, prop, path=tag, ns=ns, nsmap=nsmap, nillable=nillable, **kwargs)
 
 
-ValidatorFuncT = TypeVar('ValidatorFuncT', bound='model.SerializerFunc')
-
-
-def xml_field_validator(field: str, /, *fields: str) -> Callable[[ValidatorFuncT], ValidatorFuncT]:
+def xml_field_validator(
+    field: str, /, *fields: str
+) -> 'Callable[[model.ValidatorFuncT[model.ModelT]], model.ValidatorFuncT[model.ModelT]]':
     """
     Marks the method as a field xml validator.
 
     :param field: field to be validated
     :param fields: fields to be validated
     """
 
-    def wrapper(func: ValidatorFuncT) -> ValidatorFuncT:
+    def wrapper(func: model.ValidatorFuncT[model.ModelT]) -> model.ValidatorFuncT[model.ModelT]:
+        if isinstance(func, (classmethod, staticmethod)):
+            func = func.__func__
         setattr(func, '__xml_field_validator__', (field, *fields))
         return func
 
     return wrapper
 
 
-SerializerFuncT = TypeVar('SerializerFuncT', bound='model.SerializerFunc')
-
-
-def xml_field_serializer(field: str, /, *fields: str) -> Callable[[SerializerFuncT], SerializerFuncT]:
+def xml_field_serializer(
+    field: str, /, *fields: str
+) -> 'Callable[[model.SerializerFuncT[model.ModelT]], model.SerializerFuncT[model.ModelT]]':
     """
     Marks the method as a field xml serializer.
 
     :param field: field to be serialized
     :param fields: fields to be serialized
     """
 
-    def wrapper(func: SerializerFuncT) -> SerializerFuncT:
+    def wrapper(func: model.SerializerFuncT[model.ModelT]) -> model.SerializerFuncT[model.ModelT]:
+        if isinstance(func, (classmethod, staticmethod)):
+            func = func.__func__
         setattr(func, '__xml_field_serializer__', (field, *fields))
         return func
 

pydantic_xml/model.py

@@ -20,9 +20,12 @@
 __all__ = (
     'BaseXmlModel',
     'create_model',
+    'ModelT',
     'RootXmlModel',
     'SerializerFunc',
+    'SerializerFuncT',
     'ValidatorFunc',
+    'ValidatorFuncT',
     'XmlModelMeta',
 )
 
@@ -137,9 +140,11 @@ def _collect_xml_field_serializers_validators(mcls, cls: Type['BaseXmlModel']) -
                         cls.__xml_field_validators__[field] = func
 
 
-ValidatorFunc = Callable[[Type['BaseXmlModel'], XmlElementReader, str], Any]
-SerializerFunc = Callable[['BaseXmlModel', XmlElementWriter, Any, str], Any]
 ModelT = TypeVar('ModelT', bound='BaseXmlModel')
+ValidatorFuncT = Callable[[Type[ModelT], XmlElementReader, str], Any]
+ValidatorFunc = ValidatorFuncT['BaseXmlModel']
+SerializerFuncT = Callable[[ModelT, XmlElementWriter, Any, str], Any]
+SerializerFunc = SerializerFuncT['BaseXmlModel']
 
 
 class BaseXmlModel(BaseModel, __xml_abstract__=True, metaclass=XmlModelMeta):

tests/test_preprocessors.py

@@ -13,9 +13,12 @@ class TestModel(BaseXmlModel, tag='model1'):
         element1: List[int] = element()
 
         @xml_field_validator('element1')
+        @classmethod
         def validate_element(cls, element: XmlElementReader, field_name: str) -> List[int]:
-            if element := element.pop_element(field_name, search_mode=cls.__xml_search_mode__):
-                return list(map(int, element.pop_text().split()))
+            if (sub_element := element.pop_element(field_name, search_mode=cls.__xml_search_mode__)) and (
+                text := sub_element.pop_text()
+            ):
+                return list(map(float, text.split()))
 
             return []