[python] enable test case of discriminated union

.chronus/changes/python-discriminated-union-2025-10-18-9-43-10.md

@@ -0,0 +1,7 @@
+---
+changeKind: fix
+packages:
+  - "@typespec/http-client-python"
+---
+
+Fix import when body parameter is union of models

packages/http-client-python/eng/scripts/ci/regenerate.ts

@@ -22,7 +22,7 @@ const argv = parseArgs({
 });
 
 // Add this near the top with other constants
-const SKIP_SPECS = ["type/union/discriminated"];
+const SKIP_SPECS = [];
 
 // Get the directory of the current file
 const PLUGIN_DIR = argv.values.pluginDir
@@ -272,6 +272,10 @@ const EMITTER_OPTIONS: Record<string, Record<string, string> | Record<string, st
     "package-name": "typetest-union",
     namespace: "typetest.union",
   },
+  "type/union/discriminated": {
+    "package-name": "typetest-union-discriminated",
+    namespace: "typetest.union.discriminated",
+  },
 };
 
 function toPosix(dir: string): string {

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

@@ -408,6 +408,9 @@ def imports(  # pylint: disable=too-many-branches, disable=too-many-statements
         file_import.merge(self.get_request_builder_import(self.request_builder, async_mode, serialize_namespace))
         if self.overloads:
             file_import.add_submodule_import("typing", "overload", ImportType.STDLIB)
+            for overload in self.overloads:
+                if overload.parameters.has_body:
+                    file_import.merge(overload.parameters.body_parameter.type.imports(**kwargs))
         if self.code_model.options["models-mode"] == "dpg":
             relative_path = self.code_model.get_relative_import_path(
                 serialize_namespace, module_name="_utils.model_base"

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

@@ -776,8 +776,14 @@ def _initialize_overloads(self, builder: OperationType, is_paging: bool = False)
         client_names = [
             overload.request_builder.parameters.body_parameter.client_name for overload in builder.overloads
         ]
-        for v in sorted(set(client_names), key=client_names.index):
-            retval.append(f"_{v} = None")
+        all_dpg_model_overloads = False
+        if self.code_model.options["models-mode"] == "dpg" and builder.overloads:
+            all_dpg_model_overloads = all(
+                isinstance(o.parameters.body_parameter.type, DPGModelType) for o in builder.overloads
+            )
+        if not all_dpg_model_overloads:
+            for v in sorted(set(client_names), key=client_names.index):
+                retval.append(f"_{v} = None")
         try:
             # if there is a binary overload, we do a binary check first.
             binary_overload = cast(
@@ -803,17 +809,20 @@ def _initialize_overloads(self, builder: OperationType, is_paging: bool = False)
                     f'"{other_overload.parameters.body_parameter.default_content_type}"{check_body_suffix}'
                 )
         except StopIteration:
-            for idx, overload in enumerate(builder.overloads):
-                if_statement = "if" if idx == 0 else "elif"
-                body_param = overload.parameters.body_parameter
-                retval.append(
-                    f"{if_statement} {body_param.type.instance_check_template.format(body_param.client_name)}:"
-                )
-                if body_param.default_content_type and not same_content_type:
+            if all_dpg_model_overloads:
+                retval.extend(f"{l}" for l in self._create_body_parameter(cast(OperationType, builder.overloads[0])))
+            else:
+                for idx, overload in enumerate(builder.overloads):
+                    if_statement = "if" if idx == 0 else "elif"
+                    body_param = overload.parameters.body_parameter
                     retval.append(
-                        f'    content_type = content_type or "{body_param.default_content_type}"{check_body_suffix}'
+                        f"{if_statement} {body_param.type.instance_check_template.format(body_param.client_name)}:"
                     )
-                retval.extend(f"    {l}" for l in self._create_body_parameter(cast(OperationType, overload)))
+                    if body_param.default_content_type and not same_content_type:
+                        retval.append(
+                            f'    content_type = content_type or "{body_param.default_content_type}"{check_body_suffix}'
+                        )
+                    retval.extend(f"    {l}" for l in self._create_body_parameter(cast(OperationType, overload)))
         return retval
 
     def _create_request_builder_call(