[python] correctly deserialize xml errors (#9488)

Co-authored-by: iscai-msft <isabellavcai@gmail.com>

Author: iscai-msft

.chronus/changes/python-xmlErrorDeserialization-2026-0-28-11-40-34.md

@@ -0,0 +1,7 @@
+---
+changeKind: fix
+packages:
+  - "@typespec/http-client-python"
+---
+
+`_failsafe_deserialize_xml` xml errors

packages/http-client-python/generator/pygen/codegen/models/operation.py

@@ -448,7 +448,19 @@ def imports(  # pylint: disable=too-many-branches, disable=too-many-statements
             elif self.need_deserialize:
                 file_import.add_submodule_import(relative_path, "_deserialize", ImportType.LOCAL)
             if self.default_error_deserialization(serialize_namespace) or self.non_default_errors:
-                file_import.add_submodule_import(relative_path, "_failsafe_deserialize", ImportType.LOCAL)
+                xml_non_default_errors = any(
+                    xml_serializable(str(e.default_content_type)) for e in self.non_default_errors
+                )
+                try:
+                    default_error = next(e for e in self.exceptions if "default" in e.status_codes and e.type)
+                except StopIteration:
+                    default_error = None
+                if xml_non_default_errors or (
+                    default_error and xml_serializable(str(default_error.default_content_type))
+                ):
+                    file_import.add_submodule_import(relative_path, "_failsafe_deserialize_xml", ImportType.LOCAL)
+                else:
+                    file_import.add_submodule_import(relative_path, "_failsafe_deserialize", ImportType.LOCAL)
         return file_import
 
     def get_response_from_status(self, status_code: Optional[Union[str, int]]) -> ResponseType:

packages/http-client-python/generator/pygen/codegen/serializers/builder_serializer.py

@@ -1045,7 +1045,9 @@ def handle_error_response(  # pylint: disable=too-many-statements, too-many-bran
             retval.extend([f"    {l}" for l in response_read])
         retval.append("    map_error(status_code=response.status_code, response=response, error_map=error_map)")
         error_model = ""
-        if builder.non_default_errors and self.code_model.options["models-mode"]:
+        if (  # pylint: disable=too-many-nested-blocks
+            builder.non_default_errors and self.code_model.options["models-mode"]
+        ):
             error_model = ", model=error"
             condition = "if"
             retval.append("    error = None")
@@ -1062,9 +1064,11 @@ def handle_error_response(  # pylint: disable=too-many-statements, too-many-bran
                             is_operation_file=True, skip_quote=True, serialize_namespace=self.serialize_namespace
                         )
                         if self.code_model.options["models-mode"] == "dpg":
-                            retval.append(
-                                f"        error = _failsafe_deserialize({type_annotation},{pylint_disable}\n  response)"
-                            )
+                            if xml_serializable(str(e.default_content_type)):
+                                fn = "_failsafe_deserialize_xml"
+                            else:
+                                fn = "_failsafe_deserialize"
+                            retval.append(f"        error = {fn}({type_annotation},{pylint_disable}\n  response)")
                         else:
                             retval.extend(
                                 [
@@ -1130,9 +1134,14 @@ def handle_error_response(  # pylint: disable=too-many-statements, too-many-bran
             if builder.non_default_errors:
                 retval.append("    else:")
             if self.code_model.options["models-mode"] == "dpg":
+                default_exception = next(e for e in builder.exceptions if "default" in e.status_codes and e.type)
+                if xml_serializable(str(default_exception.default_content_type)):
+                    fn = "_failsafe_deserialize_xml"
+                else:
+                    fn = "_failsafe_deserialize"
                 retval.extend(
                     [
-                        f"{indent}error = _failsafe_deserialize(",
+                        f"{indent}error = {fn}(",
                         f"{indent}    {default_error_deserialization}",
                         f"{indent}    response,",
                         f"{indent})",