[python] add typeddict models-mode for Python HTTP client emitter

.chronus/changes/python-addTypedDict-2026-3-21-17-47-3.md

@@ -0,0 +1,8 @@
+---
+# Change versionKind to one of: internal, fix, dependencies, feature, deprecation, breaking
+changeKind: feature
+packages:
+  - "@typespec/http-client-python"
+---
+
+[python] Always generate `TypedDict` typing hints for input models in the `types.py` file, and named union aliases in the `_unions.py` file

packages/http-client-python/emitter/src/emitter.ts

@@ -198,8 +198,9 @@ async function onEmitMain(context: EmitContext<PythonEmitterOptions>) {
   const yamlMap = emitCodeModel(sdkContext);
   const parsedYamlMap = walkThroughNodes(yamlMap);
 
-  // Python emitter requires an SDK client in the TypeSpec
-  if (sdkContext.sdkPackage.clients.length === 0) {
+  // Python emitter requires an SDK client in the TypeSpec (unless models-only typeddict mode)
+  const modelsMode = (sdkContext.emitContext.options as any)["models-mode"];
+  if (sdkContext.sdkPackage.clients.length === 0 && modelsMode !== "typeddict") {
     reportDiagnostic(program, {
       code: "no-sdk-clients",
       target: NoTarget,

packages/http-client-python/emitter/src/types.ts

@@ -233,6 +233,7 @@ function emitProperty(
     // Python convert all the type of file part to FileType so clear these models' usage so that they won't be generated
     addDisableGenerationMap(context, property.type);
   }
+  const isNullable = !isMultipartFileInput && sourceType.kind === "nullable";
   return {
     clientName: getClientName(property),
     isExactName: property.isExactName,
@@ -242,6 +243,7 @@ function emitProperty(
         : property.serializationOptions?.json?.name) ?? property.name,
     type: getType(context, sourceType),
     optional: property.optional,
+    nullable: isNullable,
     description: property.summary ? property.summary : property.doc,
     addedOn: getAddedOn(context, property),
     apiVersions: property.apiVersions,