# Lifespan Events { #lifespan-events }

You can define logic (code) that should be executed before the application **starts up**. This means that this code will be executed **once**, **before** the application **starts receiving requests**.

The same way, you can define logic (code) that should be executed when the application is **shutting down**. In this case, this code will be executed **once**, **after** having handled possibly **many requests**.

You can define background tasks to be run *after* returning a response.

This is useful for operations that need to happen after a request, but that the client doesn't really have to be waiting for the operation to complete before receiving the response.

This includes, for example:

* Email notifications sent after performing an action:

{* ../../docs_src/path_params/tutorial003_py39.py hl[6,11] *}

Otherwise, the path for `/users/{user_id}` would match also for `/users/me`, "thinking" that it's receiving a parameter `user_id` with a value of `"me"`.

Similarly, you cannot redefine a path operation:

{* ../../docs_src/path_params/tutorial003b_py39.py hl[6,11] *}

* `upgrade`: Upgrades

 *  New: `ConnectionListener` publishes events for connects, disconnects, and use of pooled
    connections.

 *  Fix: Immediately update the connection's flow control window instead of waiting for the
    receiving stream to process it.

    This change may increase OkHttp's memory use for applications that make many concurrent HTTP
    calls and that can receive data faster than they can process it. Previously, OkHttp limited

- For the `InPlacePodVerticalScaling` feature, the API server will no longer set the resize status to `Proposed` upon receiving a resize request. ([#130574](https://github.com/kubernetes/kubernetes/pull/130574), [@natasha41575](https://github.com/natasha41575)) [SIG Apps, Node and Testing]

If you depended on exceptions not being received by dependencies with `yield`, and receiving an exception breaks the code after `yield`, you can use a block with `try` and `finally`:

```Python
async def do_something():
    try:
        yield something
    finally:
        some_cleanup()

<p>
A channel provides a mechanism for
<a href="#Go_statements">concurrently executing functions</a>
to communicate by
<a href="#Send_statements">sending</a> and
<a href="#Receive_operator">receiving</a>
values of a specified element type.
The <a href="#Representation_of_values">value</a> of an uninitialized channel is <code>nil</code>.
</p>

<pre class="ebnf">