docs: add pre/post processing docs for langchain python

docs/en/samples/pre_post_processing/python.md

@@ -0,0 +1,31 @@
+---
+title: "(Python) Pre and post processing"
+type: docs
+weight: 4
+description: >
+  How to add pre and post processing to your Python toolbox applications.
+---
+
+## Prerequisites
+
+This tutorial assumes that you have set up a basic toolbox application as described in the [local quickstart](../../getting-started/local_quickstart).
+
+This guide demonstrates how to implement these patterns in your Toolbox applications.
+
+## Python
+
+{{< tabpane persist=header >}}
+{{% tab header="ADK" text=true %}}
+Coming soon.
+{{% /tab %}}
+{{% tab header="Langchain" text=true %}}
+The following example demonstrates how to use `ToolboxClient` with LangChain's middleware to implement pre and post processing for tool calls.
+
+```py
+{{< include "python/langchain/agent.py" >}}
+```
+
+For more information, see the [LangChain Middleware documentation](https://docs.langchain.com/oss/python/langchain/middleware/custom#wrap-style-hooks).
+You can also add model-level (`wrap_model`) and agent-level (`before_agent`, `after_agent`) hooks to intercept messages at different stages of the execution loop. See the [LangChain Middleware documentation](https://docs.langchain.com/oss/python/langchain/middleware/custom#wrap-style-hooks) for details on these additional hook types.
+{{% /tab %}}
+{{< /tabpane >}}