default=None,
        alias="item-query",
        title="Query string",
        description="Query string for the items to search in the database that have a good match",
        min_length=3,
        max_length=50,
        pattern="^fixedquery$",
        deprecated=True,
    ),
):
    results = {"items": [{"item_id": "Foo"}, {"item_id": "Bar"}]}
    if q:
        results.update({"q": q})

from typing import Annotated, Union

from fastapi import FastAPI, Query

app = FastAPI()


@app.get("/items/")
async def read_items(q: Annotated[Union[str, None], Query(min_length=3)]):
    results = {"items": [{"item_id": "Foo"}, {"item_id": "Bar"}]}
    if q:
        results.update({"q": q})

from typing import Annotated

from fastapi import FastAPI, Query

app = FastAPI()


@app.get("/items/")
async def read_items(
    q: Annotated[str | None, Query(title="Query string", min_length=3)] = None,
):
    results = {"items": [{"item_id": "Foo"}, {"item_id": "Bar"}]}
    if q:
        results.update({"q": q})

async def read_items(
    q: Annotated[
        str | None,
        Query(
            title="Query string",
            description="Query string for the items to search in the database that have a good match",
            min_length=3,
        ),
    ] = None,
):
    results = {"items": [{"item_id": "Foo"}, {"item_id": "Bar"}]}
    if q:
        results.update({"q": q})

Validateurs et métadonnées génériques:

* `alias`
* `title`
* `description`
* `deprecated`

Validateurs spécifiques aux chaînes de caractères :

* `min_length`
* `max_length`
* `regex`

Parmi ces exemples, vous avez pu voir comment déclarer des validateurs pour les chaînes de caractères.

            {
                "type": "string_too_short",
                "loc": ["query", "q"],
                "msg": "String should have at least 3 characters",
                "input": "fa",
                "ctx": {"min_length": 3},
            }
        ]
    }


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

            {
                "type": "string_too_short",
                "loc": ["query", "q"],
                "msg": "String should have at least 3 characters",
                "input": "fa",
                "ctx": {"min_length": 3},
            }
        ]
    }


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

            {
                "type": "string_too_short",
                "loc": ["query", "q"],
                "msg": "String should have at least 3 characters",
                "input": "qu",
                "ctx": {"min_length": 3},
            }
        ]
    }


def test_query_params_str_validations_q_too_long(client: TestClient):
    response = client.get("/items/", params={"q": "q" * 51})
    assert response.status_code == 422

...使得参数可选，等同于：

```Python
q: str = None
```

但是 `Query` 显式地将其声明为查询参数。

然后，我们可以将更多的参数传递给 `Query`。在本例中，适用于字符串的 `max_length` 参数：

```Python
q: Union[str, None] = Query(default=None, max_length=50)
```

将会校验数据，在数据无效时展示清晰的错误信息，并在 OpenAPI 模式的*路径操作*中记录该参​​数。

## 添加更多校验

你还可以添加 `min_length` 参数：

{* ../../docs_src/query_params_str_validations/tutorial003.py hl[10] *}

## 添加正则表达式

            {
                "type": "string_too_short",
                "loc": ["query", "q"],
                "msg": "String should have at least 3 characters",
                "input": "fa",
                "ctx": {"min_length": 3},
            }
        ]
    }


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