A "middleware" is a function that works with every **request** before it is processed by any specific *path operation*. And also with every **response** before returning it.

* It takes each **request** that comes to your application.
* It can then do something to that **request** or run any needed code.
* Then it passes the **request** to be processed by the rest of the application (by some *path operation*).

  private fun testConnectViaProxy(proxyConfig: ProxyConfig) {
    server.enqueue(
      MockResponse(body = "this response comes via a proxy"),
    )
    val url = "http://android.com/foo".toHttpUrl()
    val response = proxyConfig.connect(server, client, url).execute()
    assertContent("this response comes via a proxy", response)
    val request = server.takeRequest()
    assertThat(request.requestLine).isEqualTo(

    You could also use `from starlette.staticfiles import StaticFiles`.

    **FastAPI** provides the same `starlette.staticfiles` as `fastapi.staticfiles` just as a convenience for you, the developer. But it actually comes directly from Starlette.

### What is "Mounting"

"Mounting" means adding a complete "independent" application in a specific path, that then takes care of handling all the sub-paths.

!!! check
    Notice that the value your function received (and returned) is `3`, as a Python `int`, not a string `"3"`.

    So, with that type declaration, **FastAPI** gives you automatic request <abbr title="converting the string that comes from an HTTP request into Python data">"parsing"</abbr>.

## Data validation

!!! note "Technical Details"
    You could also use `from starlette.requests import Request`.

!!! note "Technical Details"
    You could also use `from starlette.testclient import TestClient`.

    **FastAPI** provides the same `starlette.testclient` as `fastapi.testclient` just as a convenience for you, the developer. But it comes directly from Starlette.

!!! tip

We are already halfway there.

We just need to add a *path operation* for the user/client to actually send the `username` and `password`.

     * we consider that it's signature will not be changed by the SUBSTITUTION_OVERRIDE, so the wrapper can be unwrapped.
     *
     * This have a few caveats when it comes to the inner classes. TODO Provide a reference to some more in-detail description of that.
     *
     * @receiver A declaration that needs to be unwrapped.

    call: Call,
    url: HttpUrl,
  ) {
  }

  /**
   * Invoked after proxy selection.
   *
   * Note that the list of proxies is never null, but it may be a list containing
   * only [Proxy.NO_PROXY]. This comes up in several situations:
   *
   * * If neither a proxy nor proxy selector is configured.
   * * If the proxy is configured explicitly as [Proxy.NO_PROXY].
   * * If the proxy selector returns only [Proxy.NO_PROXY].

<img src="/img/deployment/https/https07.svg">

The client (browser) will know that the response comes from the correct server because it is using the cryptography they agreed using the **HTTPS certificate** before.

### Multiple Applications