```Python hl_lines="3"
import os

name = os.getenv("MY_NAME", "World")
print(f"Hello {name} from Python")
```

/// tip | Tipp

Das zweite Argument für <a href="https://docs.python.org/3.8/library/os.html#os.getenv" class="external-link" target="_blank">`os.getenv()`</a> ist der zurückzugebende Defaultwert.

Wenn nicht angegeben, ist er standardmäßig `None`. Hier übergeben wir `"World"` als Defaultwert.

///

### Es in der Dokumentation ansehen

Jetzt können Sie Ihre Anwendung mit Uvicorn starten und auf <a href="http://127.0.0.1:8000/docs" class="external-link" target="_blank">http://127.0.0.1:8000/docs</a> gehen.

Sie sehen Ihre Dokumentation, einschließlich eines Abschnitts „Callbacks“ für Ihre *Pfadoperation*, der zeigt, wie die *externe API* aussehen sollte:

 │                                                     │
 │  Serving at: http://127.0.0.1:8000                  │
 │                                                     │
 │  API docs: http://127.0.0.1:8000/docs               │
 │                                                     │
 │  Running in development mode, for production use:   │
 │                                                     │

 │                                                     │
 │  Serving at: http://127.0.0.1:8000                  │
 │                                                     │
 │  API docs: http://127.0.0.1:8000/docs               │
 │                                                     │
 │  Running in development mode, for production use:   │
 │                                                     │

# CORS (Cross-Origin Resource Sharing)

<a href="https://developer.mozilla.org/en-US/docs/Web/HTTP/CORS" class="external-link" target="_blank">CORS o "Cross-Origin Resource Sharing"</a> se refiere a situaciones en las que un frontend que se ejecuta en un navegador tiene código JavaScript que se comunica con un backend, y el backend está en un "origen" diferente al frontend.

## Origen

{* ../../docs_src/middleware/tutorial001.py hl[8:9,11,14] *}

/// tip

Keep in mind that custom proprietary headers can be added <a href="https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers" class="external-link" target="_blank">using the 'X-' prefix</a>.

///

### ✅ 🩺

🔜 👆 💪 ▶️ 👆 📱 ⏮️ Uvicorn &amp; 🚶 <a href="http://127.0.0.1:8000/docs" class="external-link" target="_blank">http://127.0.0.1:8000/docs</a>.

👆 🔜 👀 👆 🩺 ✅ "⏲" 📄 👆 *➡ 🛠️* 👈 🎦 ❔ *🔢 🛠️* 🔜 👀 💖:

///

/// note | 技術詳細

以下と一緒に使用できる関数なら何でも有効です:

* <a href="https://docs.python.org/3/library/contextlib.html#contextlib.contextmanager" class="external-link" target="_blank">`@contextlib.contextmanager`</a>または
* <a href="https://docs.python.org/3/library/contextlib.html#contextlib.asynccontextmanager" class="external-link" target="_blank">`@contextlib.asynccontextmanager`</a>

///

## Custom Headers { #custom-headers }

Keep in mind that custom proprietary headers can be added <a href="https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers" class="external-link" target="_blank">using the 'X-' prefix</a>.

Form kodlama türleri ve form alanları hakkında daha fazla bilgi edinmek istiyorsanız <a href="https://developer.mozilla.org/en-US/docs/Web/HTTP/Methods/POST" class="external-link" target="_blank"><abbr title="Mozilla Developer Network">MDN</abbr> web docs for <code>POST</code></a> sayfasını ziyaret edebilirsiniz.

///

/// warning | Uyarı