[Python] Fix multipart when files part is optional (#9144)

fix #9136

Related PR:
- [ ] #9139

Author: msyyc

.chronus/changes/python-multipart-fix-optional-files-part-2025-11-5-8-29-45.md

@@ -0,0 +1,7 @@
+---
+changeKind: fix
+packages:
+  - "@typespec/http-client-python"
+---
+
+Fix multipart when files part is optional

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

@@ -679,7 +679,7 @@ def _serialize_body_parameter(self, builder: OperationType) -> list[str]:
                     ")",
                     f"_file_fields: list[str] = {file_fields}",
                     f"_data_fields: list[str] = {data_fields}",
-                    "_files, _data = prepare_multipart_form_data(_body, _file_fields, _data_fields)",
+                    "_files = prepare_multipart_form_data(_body, _file_fields, _data_fields)",
                 ]
             )
             return retval
@@ -861,7 +861,6 @@ def _create_request_builder_call(
             retval.append(f"    {client_name}=_{client_name},")
         elif request_builder.has_form_data_body:
             retval.append("    files=_files,")
-            retval.append("    data=_data,")
         elif request_builder.overloads:
             seen_body_params = set()
             for overload in request_builder.overloads:

packages/http-client-python/generator/pygen/codegen/templates/utils.py.jinja2

@@ -78,20 +78,21 @@ def serialize_multipart_data_entry(data_entry: Any) -> Any:
 
 def prepare_multipart_form_data(
     body: Mapping[str, Any], multipart_fields: list[str], data_fields: list[str]
-) -> tuple[list[FileType], dict[str, Any]]:
+) -> list[FileType]:
     files: list[FileType] = []
-    data: dict[str, Any] = {}
     for multipart_field in multipart_fields:
         multipart_entry = body.get(multipart_field)
         if isinstance(multipart_entry, list):
             files.extend([(multipart_field, e) for e in multipart_entry ])
         elif multipart_entry:
             files.append((multipart_field, multipart_entry))
 
+    # if files is empty, sdk core library can't handle multipart/form-data correctly, so
+    # we put data fields into files with filename as None to avoid that scenario.
     for data_field in data_fields:
         data_entry = body.get(data_field)
         if data_entry:
-            data[data_field] = serialize_multipart_data_entry(data_entry)
+            files.append((data_field, str(serialize_multipart_data_entry(data_entry))))
 
-    return files, data
+    return files
 {% endif %}