fix(event_handler): parse single list items in form data

aws_lambda_powertools/event_handler/middlewares/openapi_validation.py

@@ -15,6 +15,7 @@
     _normalize_errors,
     _regenerate_error_with_loc,
     get_missing_field_error,
+    is_sequence_field,
 )
 from aws_lambda_powertools.event_handler.openapi.dependant import is_scalar_field
 from aws_lambda_powertools.event_handler.openapi.encoders import jsonable_encoder
@@ -150,11 +151,10 @@ def _parse_form_data(self, app: EventHandlerInstance) -> dict[str, Any]:
         """Parse URL-encoded form data from the request body."""
         try:
             body = app.current_event.decoded_body or ""
-            # parse_qs returns dict[str, list[str]], but we want dict[str, str] for single values
+            # NOTE: Keep values as lists; we'll normalize per-field later based on the expected type.
+            # This avoids breaking List[...] fields when only a single value is provided.
             parsed = parse_qs(body, keep_blank_values=True)
-
-            result: dict[str, Any] = {key: values[0] if len(values) == 1 else values for key, values in parsed.items()}
-            return result
+            return parsed
 
         except Exception as e:  # pragma: no cover
             raise RequestValidationError(  # pragma: no cover
@@ -388,6 +388,10 @@ def _request_body_to_args(
                 values[field.name] = deepcopy(field.default)
             continue
 
+        # Normalize lists for non-sequence fields
+        if isinstance(value, list) and not is_sequence_field(field):
+            value = value[0]
+
         # MAINTENANCE: Handle byte and file fields
 
         # Finally, validate the value

tests/functional/event_handler/_pydantic/test_openapi_validation_middleware.py

@@ -1481,20 +1481,33 @@ def handler_custom_route_response_validation_error() -> Model:
 
 def test_parse_form_data_url_encoded(gw_event):
     """Test _parse_form_data method with URL-encoded form data"""
-
+    # GIVEN an APIGatewayRestResolver with validation enabled
     app = APIGatewayRestResolver(enable_validation=True)
 
     @app.post("/form")
     def post_form(name: Annotated[str, Form()], tags: Annotated[List[str], Form()]):
         return {"name": name, "tags": tags}
 
+    # WHEN sending a POST request with URL-encoded form data
     gw_event["httpMethod"] = "POST"
     gw_event["path"] = "/form"
     gw_event["headers"]["content-type"] = "application/x-www-form-urlencoded"
     gw_event["body"] = "name=test&tags=tag1&tags=tag2"
 
     result = app(gw_event, {})
+
+    # THEN it should parse the form data correctly
+    assert result["statusCode"] == 200
+    assert result["body"] == '{"name":"test","tags":["tag1","tag2"]}'
+
+    # WHEN sending a POST request with a single value for a list field
+    gw_event["body"] = "name=test&tags=tag1"
+
+    result = app(gw_event, {})
+
+    # THEN it should parse the form data correctly
     assert result["statusCode"] == 200
+    assert result["body"] == '{"name":"test","tags":["tag1"]}'
 
 
 def test_parse_form_data_wrong_value(gw_event):