from typing import Optional


def say_hi(name: Optional[str] = None):
    if name is not None:
        print(f"Hey {name}!")
    else:

from typing import Union


def say_hi(name: Union[str, None] = None):
    if name is not None:
        print(f"Hey {name}!")
    else:

/// tip

The next sections are **not necessarily "advanced"**.

And it's possible that for your use case, the solution is in one of them.

///

## Read the Tutorial first { #read-the-tutorial-first }

You could still use most of the features in **FastAPI** with the knowledge from the main [Tutorial - User Guide](../tutorial/index.md){.internal-link target=_blank}.

from docs_src.custom_response.tutorial009c_py39 import app

client = TestClient(app)


def test_get():
    response = client.get("/")

app = FastAPI()

app.add_middleware(
    TrustedHostMiddleware, allowed_hosts=["example.com", "*.example.com"]
)


@app.get("/")
async def main():

例如，你可以创建一个名为 `main.py` 的文件，其中包含以下内容：

```Python hl_lines="3"
import os

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

/// tip

第二个参数是 <a href="https://docs.python.org/zh-cn/3.8/library/os.html#os.getenv" class="external-link" target="_blank">`os.getenv()`</a> 的默认返回值。

如果没有提供，默认值为 `None`，这里我们提供 `"World"` 作为默认值。

///

然后你可以调用这个 Python 程序：

//// tab | Linux, macOS, Windows Bash

                        logger.warn("Could not close a process input stream.", e);
                    }
                });
                CommonPoolUtil.execute(() -> {
                    try {
                        CloseableUtil.closeQuietly(p.getErrorStream());
                    } catch (final Exception e) {
                        logger.warn("Could not close a process error stream.", e);
                    }

                        if (!file.exists()) {
                            if (!file.mkdirs()) {
                                throw new CrawlerSystemException("Could not create " + file.getAbsolutePath());
                            }
                            break;
                        }
                        if (file.isDirectory()) {
                            break;

    expected.addAll(index, elements);
    expectContents(expected);
  }

  /*
   * TODO: if we're testing a list, we could check indexOf(). (Doing it in
   * AbstractListTester isn't enough because many tests that run on lists don't
   * extends AbstractListTester.) We could also iterate over all elements to
   * verify absence
   */
  protected void expectMissing(E... elements) {
    for (E element : elements) {

     * @param repositories
     * @param session
     * @return PluginDescriptor The component descriptor for the Maven plugin.
     * @throws PluginNotFoundException The plugin could not be found in any repositories.
     * @throws PluginResolutionException The plugin could be found but could not be resolved.
     * @throws InvalidPluginDescriptorException
     */
    @Override
    public PluginDescriptor loadPlugin(