app = FastAPI()


@app.get("/")
def read_root():
    return {"Hello": "World"}


@app.get("/items/{item_id}")
def read_item(item_id: int, q: Union[str, None] = None):
    return {"item_id": item_id, "q": q}
```

<details markdown="1">
<summary>Ya da <code>async def</code>...</summary>

Eğer kodunuzda `async` / `await` varsa, `async def` kullanalım:

```Python hl_lines="9  14"

        ("/users", 200, {"q": None, "skip": 0, "limit": 100}),
    ],
)
def test_get(path, expected_status, expected_response):
    response = client.get(path)
    assert response.status_code == expected_status
    assert response.json() == expected_response


def test_openapi_schema():
    response = client.get("/openapi.json")
    assert response.status_code == 200, response.text

  @Test fun byteCount() {
    assertThat(EMPTY_HEADERS.byteCount()).isEqualTo(0L)
    assertThat(
      Headers.Builder()
        .add("abc", "def")
        .build()
        .byteCount(),
    ).isEqualTo(10L)
    assertThat(
      Headers.Builder()
        .add("abc", "def")
        .add("ghi", "jkl")
        .build()
        .byteCount(),
    ).isEqualTo(20L)
  }

  @Test fun addDate() {

        try {
            handshake();
        } catch (IOException ex) { }
        return connection.getHeaderFieldInt(header, def);
    }

    public long getHeaderFieldDate(String header, long def) {
        try {
            handshake();
        } catch (IOException ex) { }
        return connection.getHeaderFieldDate(header, def);
    }

app = FastAPI()


@app.get("/")
def read_root():
    return {"Hello": "World"}


@app.get("/items/{item_id}")
def read_item(item_id: int, q: Union[str, None] = None):
    return {"item_id": item_id, "q": q}
```

<details markdown="1">
<summary>Or use <code>async def</code>...</summary>

If your code uses `async` / `await`, use `async def`:

```Python hl_lines="9  14"

from typing import Union


def process_item(item: Union[int, str]):

import pytest

from docs_src.async_tests.test_main import test_root


@pytest.mark.anyio
async def test_async_testing():

def process_items(items_t: tuple[int, int, str], items_s: set[bytes]):

async def dep0(response: Response):
    response.headers["x-level0"] = "True"


async def dep1(response: Response):
    response.headers["x-level1"] = "True"


async def dep2(response: Response):
    response.headers["x-level2"] = "True"


async def dep3(response: Response):
    response.headers["x-level3"] = "True"


async def dep4(response: Response):

from fastapi import FastAPI

app = FastAPI()


@app.get("/items/", operation_id="some_specific_id_you_define")
async def read_items():