Before Let's Encrypt, these **HTTPS certificates** were sold by trusted third parties.

The process to acquire one of these certificates used to be cumbersome, require quite some paperwork and the certificates were quite expensive.

But then **<a href="https://letsencrypt.org/" class="external-link" target="_blank">Let's Encrypt</a>** was created.

We create an async function `lifespan()` with `yield` like this:

```Python hl_lines="16  19"
{!../../../docs_src/events/tutorial003.py!}
```

                Each key is the original dependency callable, and the value is the
                actual dependency that should be called.

                This is for testing, to replace expensive dependencies with testing
                versions.

                Read more about it in the

```Python hl_lines="2-5  15  21"
{!../../../docs_src/handling_errors/tutorial006.py!}
```