feature: Support new DeserializeAs setting

Author: NiklasRosenstein

databind.json/.changelog/_unreleased.toml

@@ -0,0 +1,5 @@
+[[entries]]
+id = "5d31c345-35ef-40f4-bf02-bfca907f0517"
+type = "feature"
+description = "Support new `DeserializeAs` setting"
+author = "@NiklasRosenstein"

databind.json/src/databind/json/converters.py

@@ -11,6 +11,7 @@
 from databind.core.settings import (
     Alias,
     DateFormat,
+    DeserializeAs,
     ExtraKeys,
     Precision,
     Remainder,
@@ -413,7 +414,12 @@ def _get_alias_setting(ctx: Context, field_name: str) -> Alias:
         return ctx.get_setting(Alias) or Alias(field_name)
 
     def _get_schema(self, ctx: Context) -> Schema:
-        datatype = _unwrap_annotated(ctx.datatype)
+        deserialize_as = ctx.get_setting(DeserializeAs)
+        if deserialize_as is not None:
+            datatype = TypeHint(deserialize_as.type)
+        else:
+            datatype = _unwrap_annotated(ctx.datatype)
+
         try:
             return self.convert_to_schema(datatype)
         except ValueError as exc:

databind.json/tests/test_converters.py

@@ -12,6 +12,7 @@
 from databind.core.mapper import ObjectMapper
 from databind.core.settings import (  # noqa: F401
     Alias,
+    DeserializeAs,
     ExtraKeys,
     Flattened,
     Remainder,
@@ -379,6 +380,38 @@ class A:
         assert mapper.deserialize({"a": 1, "spam": 2}, A) == A(1, {"spam": 2})
 
 
+def test_deserialize_as() -> None:
+    mapper = make_mapper([SchemaConverter(), PlainDatatypeConverter()])
+
+    @dataclasses.dataclass
+    class A:
+        a: int
+
+    @dataclasses.dataclass
+    class B(A):
+        a: int = 42
+
+    @dataclasses.dataclass
+    class MyClass:
+        a: te.Annotated[A, DeserializeAs(B)]
+
+    with pytest.raises(ConversionError) as excinfo:
+        assert mapper.serialize(MyClass(A(1)), MyClass) == {"a": {"a": 1}}
+    assert (
+        str(excinfo.value)
+        == """expected test_converters.test_deserialize_as.<locals>.B, got test_converters.test_deserialize_as.<locals>.A instead
+
+Trace:
+    $: TypeHint(test_converters.test_deserialize_as.<locals>.MyClass)
+    .a: TypeHint(typing_extensions.Annotated[test_converters.test_deserialize_as.<locals>.A, DeserializeAs(type=<class 'test_converters.test_deserialize_as.<locals>.B'>, priority=<Priority.NORMAL: 1>)])"""
+    )
+
+    assert mapper.serialize(MyClass(B(2)), MyClass) == {"a": {"a": 2}}
+    assert mapper.deserialize({"a": {"a": 1}}, MyClass) == MyClass(B(1))
+    assert mapper.deserialize({"a": {"a": 2}}, MyClass) == MyClass(B(2))
+    assert mapper.deserialize({"a": {}}, MyClass) == MyClass(B(42))
+
+
 def test_deserialize_union_dataclass_subclass():
     """Tests that a subclass of a dataclass marked as a union is deserialized as a dataclass."""