//
  // Validation requirements:
  //  1. certificate must contain one or more PEM blocks.
  //  2. All PEM blocks must have the "CERTIFICATE" label, contain no headers, and the encoded data
  //   must be a BER-encoded ASN.1 Certificate structure as described in section 4 of RFC5280.
  //  3. Non-PEM content may appear before or after the "CERTIFICATE" PEM blocks and is unvalidated,

  // The data must consist only of PEM certificate blocks that parse as valid
  // X.509 certificates.  Each certificate must include a basic constraints
  // extension with the CA bit set.  The API server will reject objects that
  // contain duplicate certificates, or that use PEM block headers.
  //
  // Users of ClusterTrustBundles, including Kubelet, are free to reorder and
  // deduplicate certificate blocks in this file according to their own logic,

import java.net.SocketTimeoutException
import java.util.ArrayDeque
import java.util.concurrent.locks.Condition
import java.util.concurrent.locks.ReentrantLock
import okhttp3.Headers
import okhttp3.internal.EMPTY_HEADERS
import okhttp3.internal.assertNotHeld
import okhttp3.internal.connection.Locks.withLock
import okhttp3.internal.http2.flowcontrol.WindowCounter
import okhttp3.internal.toHeaderList

```Python
async with lifespan(app):
    await do_stuff()
```

Wenn Sie wie oben einen Kontextmanager oder einen asynchronen Kontextmanager erstellen, führt dieser vor dem Betreten des `with`-Blocks den Code vor dem `yield` aus, und nach dem Verlassen des `with`-Blocks wird er den Code nach dem `yield` ausführen.

In unserem obigen Codebeispiel verwenden wir ihn nicht direkt, sondern übergeben ihn an FastAPI, damit es ihn verwenden kann.

Wir können denselben Ansatz auch verwenden, um in einem Exceptionhandler auf den Requestbody zuzugreifen.

Alles, was wir tun müssen, ist, den Request innerhalb eines `try`/`except`-Blocks zu handhaben:

```Python hl_lines="13  15"
{!../../../docs_src/custom_request_and_route/tutorial002.py!}
```

                return false
            }

            return true
        }

        private inline fun withElement(element: FirElement, block: () -> Unit) {
            stack.addLast(element)
            try {
                block()
            } finally {
                stack.removeLast()
            }
        }
    }

It is also built to work as a future reference.

So you can come back and see exactly what you need.

## Run the code

All the code blocks can be copied and used directly (they are actually tested Python files).

To run any of the examples, copy the code to a file `main.py`, and start `fastapi dev` with:

<div class="termy">

```console

    **FastAPI** uses them internally to achieve this.

## Dependencies with `yield` and `HTTPException`

You saw that you can use dependencies with `yield` and have `try` blocks that catch exceptions.

The same way, you could raise an `HTTPException` or similar in the exit code, after the `yield`.

!!! tip

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import okhttp3.internal.http2.Http2Stream
import okhttp3.internal.http2.Http2Writer

/**
 * Centralisation of central locks according to docs/contribute/concurrency.md
 */
internal object Locks {
  inline fun <T> Dispatcher.withLock(action: () -> T): T {
    contract { callsInPlace(action, InvocationKind.EXACTLY_ONCE) }
    return lock.withLock(action)
  }

  inline fun <T> RealConnection.withLock(action: () -> T): T {