dapper91 · dapper91 · Aug 18, 2023 · Aug 18, 2023
diff --git a/docs/source/pages/misc.rst b/docs/source/pages/misc.rst
@@ -49,22 +49,10 @@ Since xml format doesn't support ``null`` type natively it is not obvious how to
 (ignore it, encode it as an empty string or mark it as ``xsi:nil``) the library doesn't implement
 ``None`` type encoding by default.
 
-You can define your own encoding format for the model:
+You can define your own encoding format:
 
-.. code-block:: python
-
-    from typing import Annotated
-    from pydantic import PlainSerializer
-
-    InnerType = TypeVar('InnerType')
-    XmlOptional = Annotated[Optional[InnerType], PlainSerializer(lambda val: val if val is not None else '')]
-
-    class Company(BaseXmlModel):
-        title: XmlOptional[str] = element(default=None)
-
-
-    company = Company()
-    assert company.to_xml() == b'<Company><title></title></Company>'
+.. literalinclude:: ../../../examples/snippets/py3.9/serialization.py
+  :language: python
 
 
 or drop ``None`` fields at all:

diff --git a/examples/snippets/py3.9/serialization.py b/examples/snippets/py3.9/serialization.py
@@ -0,0 +1,29 @@
+from typing import Annotated, Optional, TypeVar
+from xml.etree.ElementTree import canonicalize
+
+from pydantic import BeforeValidator, PlainSerializer
+
+from pydantic_xml import BaseXmlModel, element
+
+InnerType = TypeVar('InnerType')
+XmlOptional = Annotated[
+    Optional[InnerType],
+    PlainSerializer(lambda val: val if val is not None else 'null'),
+    BeforeValidator(lambda val: val if val != 'null' else None),
+]
+
+
+class Company(BaseXmlModel):
+    title: XmlOptional[str] = element(default=None)
+
+
+xml_doc = '''
+<Company>
+    <title>null</title>
+</Company>
+'''
+
+company = Company.from_xml(xml_doc)
+
+assert company.title is None
+assert canonicalize(company.to_xml(), strip_text=True) == canonicalize(xml_doc, strip_text=True)
diff --git a/tests/test_encoder.py b/tests/test_encoder.py
@@ -3,12 +3,14 @@
 import sys
 from decimal import Decimal
 from enum import Enum
+from typing import Any
 from uuid import UUID
 
 import pytest
 from helpers import assert_xml_equal
 from pydantic import field_serializer
-from pydantic.functional_serializers import PlainSerializer
+from pydantic.functional_serializers import PlainSerializer, WrapSerializer
+from pydantic.functional_validators import AfterValidator, BeforeValidator, WrapValidator
 
 from pydantic_xml import BaseXmlModel, element
 
@@ -123,12 +125,16 @@ def serialize_dt(self, value: dt.datetime) -> float:
 
 
 @pytest.mark.skipif(sys.version_info < (3, 9), reason="requires python 3.9 and above")
-def test_plain_serializer():
+@pytest.mark.parametrize(
+    'Serializer', [
+        PlainSerializer(lambda val: val.timestamp(), return_type=float),
+        WrapSerializer(lambda val, nxt: val.timestamp(), return_type=float),
+    ],
+)
+def test_serializer(Serializer: Any):
     from typing import Annotated
 
-    Timestamp = Annotated[
-        dt.datetime, PlainSerializer(lambda val: val.timestamp(), return_type=float),
-    ]
+    Timestamp = Annotated[dt.datetime, Serializer]
 
     class TestSubModel(BaseXmlModel, tag='submodel'):
         field1: Timestamp = element(tag='field1')
@@ -155,3 +161,44 @@ class TestModel(BaseXmlModel, tag='model'):
 
     actual_xml = obj.to_xml()
     assert_xml_equal(actual_xml, xml.encode())
+
+
+@pytest.mark.skipif(sys.version_info < (3, 9), reason="requires python 3.9 and above")
+@pytest.mark.parametrize(
+    'Validator', [
+        AfterValidator(lambda val: val),
+        BeforeValidator(lambda val: dt.datetime.fromtimestamp(float(val), tz=dt.timezone.utc)),
+        WrapValidator(lambda val, nxt: dt.datetime.fromtimestamp(float(val), tz=dt.timezone.utc)),
+    ],
+)
+def test_validator(Validator: Any):
+    from typing import Annotated
+
+    Timestamp = Annotated[dt.datetime, Validator]
+
+    class TestSubModel(BaseXmlModel, tag='submodel'):
+        field1: Timestamp = element(tag='field1')
+
+    class TestModel(BaseXmlModel, tag='model'):
+        field1: Timestamp = element(tag='field1')
+        field2: TestSubModel
+
+    xml = '''
+    <model>
+        <field1>1675468800.0</field1>
+        <submodel>
+            <field1>1675468800.0</field1>
+        </submodel>
+    </model>
+    '''
+
+    actual_obj = TestModel.from_xml(xml)
+
+    expected_obj = TestModel.model_construct(
+        field1=dt.datetime(2023, 2, 4, tzinfo=dt.timezone.utc),
+        field2=TestSubModel.model_construct(
+            field1=dt.datetime(2023, 2, 4, tzinfo=dt.timezone.utc),
+        ),
+    )
+
+    assert actual_obj == expected_obj
diff --git a/tests/test_examples.py b/tests/test_examples.py
@@ -1,4 +1,5 @@
 import importlib.machinery
+import sys
 from pathlib import Path
 
 import pytest
@@ -14,6 +15,13 @@ def test_snippets(snippet: Path):
     loader.load_module('snippet')
 
 
+@pytest.mark.skipif(sys.version_info < (3, 9), reason="requires python 3.9 and above")
+@pytest.mark.parametrize('snippet', list((EXAMPLES_PATH / 'snippets' / 'py3.9').glob('*.py')), ids=lambda p: p.name)
+def test_snippets_py39(snippet: Path):
+    loader = importlib.machinery.SourceFileLoader('snippet', str(snippet))
+    loader.load_module('snippet')
+
+
 @pytest.fixture(
     params=[
         'custom-encoder',