Add hook for custom validation error formatting

We have some older APIs that don't follow the standard error formatting,
so this allows us to still use the api decorator for these endpoints
without breaking older clients.

django_api_decorator/decorators.py

@@ -21,6 +21,7 @@
 T = typing.TypeVar("T")
 
 Annotation = Any
+ExceptionHandler = Callable[[ValidationError | pydantic.ValidationError], HttpResponse]
 
 
 def api(
@@ -32,6 +33,7 @@ def api(
     atomic: bool | None = None,
     auth_check: Callable[[HttpRequest], bool] | None = None,
     serialize_by_alias: bool = False,
+    validation_error_handler: ExceptionHandler | None = None,
 ) -> Callable[[Callable[P, T]], Callable[P, HttpResponse]]:
     """
     Defines an API view. This handles validation of query parameters, parsing of
@@ -75,6 +77,7 @@ def api(
         if atomic is not None
         else getattr(settings, "API_DECORATOR_DEFAULT_ATOMIC", True)
     )
+    validation_error_handler = validation_error_handler or handle_validation_error
 
     def default_auth_check(request: HttpRequest) -> bool:
         return hasattr(request, "user") and request.user.is_authenticated
@@ -138,7 +141,7 @@ def inner(request: HttpRequest, *args: Any, **kwargs: Any) -> HttpResponse:
                 pydantic.ValidationError,
             ) as e:
                 # Normalize and return a unified error message payload
-                return handle_validation_error(exception=e)
+                return validation_error_handler(e)
 
             try:
                 if atomic:
@@ -155,7 +158,7 @@ def inner(request: HttpRequest, *args: Any, **kwargs: Any) -> HttpResponse:
             except ValidationError as e:
                 # Normalize and return a unified error message payload, but only
                 # for Django's ValidationError error, not DRF or Pydantic
-                return handle_validation_error(exception=e)
+                return validation_error_handler(e)
 
             except PublicAPIError as exc:
                 # Raised custom errors to frontend
@@ -290,7 +293,6 @@ class PydanticErrorDict(TypedDict):
 
 
 def handle_validation_error(
-    *,
     exception: (ValidationError | pydantic.ValidationError),
 ) -> HttpResponse:
     errors: list[str]

tests/test_decorator.py

@@ -8,6 +8,7 @@
 from django.test.utils import override_settings
 from django.urls import path
 from pydantic import BaseModel
+from pytest_mock import MockerFixture
 
 from django_api_decorator.decorators import api
 
@@ -344,3 +345,35 @@ def view(request: HttpRequest, body: list[Body]) -> JsonResponse:
         )
         assert response.status_code == 400
         assert response.json()["field_errors"].keys() == {"0.num", "1.num", "1.d"}
+
+
+@override_settings(ROOT_URLCONF=__name__)
+def test_custom_exception_handler(client: Client, mocker: MockerFixture) -> None:
+    """
+    Test that a custom validation error handler is called
+    """
+
+    def handle_exception(e: Exception) -> JsonResponse:
+        return JsonResponse({"error": "Something is wrong here"}, status=400)
+
+    class Body(BaseModel):
+        num: int
+        d: datetime.date
+
+    @api(
+        method="POST",
+        login_required=False,
+        validation_error_handler=handle_exception,
+    )
+    def view(request: HttpRequest, body: list[Body]) -> JsonResponse:
+        return JsonResponse({})
+
+    urls = [
+        path("", view),
+    ]
+
+    mocker.patch(f"{__name__}.urlpatterns", urls)
+
+    response = client.post("/", data={}, content_type="application/json")
+    assert response.status_code == 400
+    assert response.json() == {"error": "Something is wrong here"}