Handle types.UnionType in _serialize_python_type for Python 3.10-3.13 (#2848)

Author: koxudaxi

src/datamodel_code_generator/__main__.py

@@ -632,12 +632,29 @@ def _serialize_python_type(tp: type) -> str | None:
 
     Returns None if the type doesn't need to be preserved (e.g., standard dict, list).
     """
+    import types  # noqa: PLC0415
     from typing import get_args, get_origin  # noqa: PLC0415
 
     origin = get_origin(tp)
     args = get_args(tp)
     preserved_origins = _get_preserved_type_origins()
 
+    # Handle types.UnionType (X | Y syntax) in Python 3.10-3.13
+    # In Python 3.10-3.13, get_origin(X | Y) returns types.UnionType which is distinct from typing.Union
+    # In Python 3.14+, types.UnionType is the same as typing.Union, so this check is skipped
+    from typing import Union  # noqa: PLC0415
+
+    if (
+        hasattr(types, "UnionType")
+        and types.UnionType is not Union  # Only applies to Python 3.10-3.13
+        and origin is types.UnionType
+    ):
+        if args:
+            nested = [_serialize_python_type(a) for a in args]
+            if any(n is not None for n in nested):
+                return " | ".join(n or _simple_type_name(a) for n, a in zip(nested, args, strict=False))
+        return None  # pragma: no cover
+
     if origin in preserved_origins:
         type_name = preserved_origins[origin]
         if args:

tests/data/python/input_model/pydantic_models.py

@@ -33,6 +33,7 @@ class ModelWithPythonTypes(BaseModel):
     nested_in_list: list[Set[int]]
     optional_set: Optional[Set[str]]
     nullable_frozenset: Union[None, FrozenSet[str]]
+    optional_mapping: Mapping[str, str] | None
 
 
 class RecursiveNode(BaseModel):

tests/test_input_model.py

@@ -590,3 +590,17 @@ def test_input_model_union_none_frozenset(tmp_path: Path) -> None:
         output_path=tmp_path / "output.py",
         expected_output_contains=["frozenset[str] | None", "nullable_frozenset:"],
     )
+
+
+@SKIP_PYDANTIC_V1
+def test_input_model_optional_mapping_union_syntax(tmp_path: Path) -> None:
+    """Test that Mapping[str, str] | None using | syntax is preserved correctly.
+
+    In Python 3.10-3.13, get_origin(X | Y) returns types.UnionType.
+    This test verifies that types.UnionType is handled correctly in those versions.
+    """
+    run_input_model_and_assert(
+        input_model="tests.data.python.input_model.pydantic_models:ModelWithPythonTypes",
+        output_path=tmp_path / "output.py",
+        expected_output_contains=["Mapping[str, str] | None", "optional_mapping:"],
+    )