Fix custom encoders for BaseModel python-mode dump

fastapi/encoders.py

@@ -225,7 +225,7 @@ def jsonable_encoder(
         exclude = set(exclude)  # type: ignore[assignment]  # ty: ignore[unused-ignore-comment]
     if isinstance(obj, BaseModel):
         obj_dict = obj.model_dump(
-            mode="json",
+            mode="python",
             include=include,
             exclude=exclude,
             by_alias=by_alias,
@@ -237,6 +237,7 @@ def jsonable_encoder(
             obj_dict,
             exclude_none=exclude_none,
             exclude_defaults=exclude_defaults,
+            custom_encoder=custom_encoder,
             sqlalchemy_safe=sqlalchemy_safe,
         )
     if dataclasses.is_dataclass(obj):

tests/test_jsonable_encoder.py

@@ -225,6 +225,19 @@ def test_custom_encoders():
     assert encoded_instance2["dt_field"] == instance["dt_field"].isoformat()
 
 
+
+def test_custom_encoders_applied_to_model_dump_python_values():
+    class ModelWithDecimal(BaseModel):
+        value: Decimal
+
+    instance = ModelWithDecimal(value=Decimal("1.23"))
+
+    encoded_instance = jsonable_encoder(
+        instance, custom_encoder={Decimal: lambda o: f"{o} custom"}
+    )
+    assert encoded_instance == {"value": "1.23 custom"}
+
+
 def test_custom_enum_encoders():
     def custom_enum_encoder(v: Enum):
         return v.value.lower()