doc (Azure#21830)

doc/dev/customize_code/how-to-patch-sdk-code.md

@@ -0,0 +1,64 @@
+This doc shows some cases for developers about how to write customized code based on SDK generated by autorest.
+
+# Where to write your code?
+
+SDK provides mechanism to develop your code. You just need to write your code in `_patch.py` then your code will override the generated code. For example: [webpubsub patch](https://github.com/Azure/azure-sdk-for-python/blob/main/sdk/webpubsub/azure-messaging-webpubsubservice/azure/messaging/webpubsubservice/_patch.py)
+
+
+
+# Customization Scenario
+
+Here are some samples about how to customize code in different scenario. Some samples use `directive`, which is useful method to do some customized change. For more info about `directive`, please see [directive doc](https://github.com/Azure/autorest/blob/main/docs/generate/built-in-directives.md).
+
+## How to customize client?
+
+With the help of `importlib`, it is easy to use your own client to override generated client. For example: [webpubsub customized client](https://github.com/Azure/azure-sdk-for-python/blob/0d071b55cde1eb1ba89032b14adfb94e8d52f873/sdk/webpubsub/azure-messaging-webpubsubservice/azure/messaging/webpubsubservice/_patch.py#L378-L379)
+
+![img.png](client.png)
+
+## How to rename operation?
+
+Use `directive` in `readme.md`, you could redefine operation name. Then run autorest again, the operation name will be changed. For example: [webpubsub rename operation](https://github.com/Azure/azure-sdk-for-python/tree/0d071b55cde1eb1ba89032b14adfb94e8d52f873/sdk/webpubsub/azure-messaging-webpubsubservice/swagger#settings)
+
+![img.png](rename_operation.png)
+
+
+
+## How to delete operation?
+
+Same with [How to rename operation](#How to rename operation). For example: [webpubsub delete operation](https://github.com/Azure/azure-sdk-for-python/tree/0d071b55cde1eb1ba89032b14adfb94e8d52f873/sdk/webpubsub/azure-messaging-webpubsubservice/swagger#settings)
+
+![](delete_operation.png)
+
+## How to rename operation parameter?
+
+Same with [How to rename operation](#How to rename operation). For example: [webpubsub rename parameter](https://github.com/Azure/azure-sdk-for-python/tree/0d071b55cde1eb1ba89032b14adfb94e8d52f873/sdk/webpubsub/azure-messaging-webpubsubservice/swagger#settings)
+
+![img.png](rename_parameter.png)
+
+## How to move operation parameter to client?
+
+Same with [How to rename operation](#How to rename operation). For example: [webpubsub move operation parameter to client](https://github.com/Azure/azure-sdk-for-python/tree/0d071b55cde1eb1ba89032b14adfb94e8d52f873/sdk/webpubsub/azure-messaging-webpubsubservice/swagger#userexistsimpl)
+
+![img.png](move_parameter_to_client.png)
+
+## How to move operation from operation groups to client?
+
+Same with [How to rename operation](#How to rename operation). For example: [webpubsub move operation to client](https://github.com/Azure/azure-sdk-for-python/tree/0d071b55cde1eb1ba89032b14adfb94e8d52f873/sdk/webpubsub/azure-messaging-webpubsubservice/swagger#settings)
+
+![](move_operation_to_client.png)
+
+## How to customize function?
+
+Write your function directly in your own client. For example: [webpubsub customize function](https://github.com/Azure/azure-sdk-for-python/blob/64ddee845ecd805ea39a581f60155735f84a7dcd/sdk/webpubsub/azure-messaging-webpubsubservice/azure/messaging/webpubsubservice/_patch.py#L326)
+
+Or
+
+Replace generated code with your handwritten code by `importlib`:
+
+```python
+def patch_sdk():
+    curr_package = importlib.import_module("azure.messaging.webpubsubservice")
+    curr_package.WebPubSubServiceClient.generated_function = handwritten_function
+```
+