@pytest.fixture(name="client")
def get_client():
    from docs_src.query_params_str_validations.tutorial012_an_py39 import app

    client = TestClient(app)
    return client


@needs_py39
def test_default_query_values(client: TestClient):
    url = "/items/"
    response = client.get(url)
    assert response.status_code == 200, response.text
    assert response.json() == {"q": ["foo", "bar"]}


@needs_py39

                            "schema": IsDict(
                                {
                                    "anyOf": [{"type": "integer"}, {"type": "null"}],
                                    "default": 50,
                                    "title": "Standard Query Param",
                                }
                            )
                            | IsDict(

```

由于我们必须用 `Query(default=None)` 替换默认值 `None`，`Query` 的第一个参数同样也是用于定义默认值。

所以：

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

...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)
```

                        ),
                        "tax": {"title": "Tax", "type": "number", "default": 10.5},
                        "tags": {
                            "title": "Tags",
                            "type": "array",
                            "items": {"type": "string"},
                            "default": [],
                        },
                    },
                },

                            "required": False,
                            "schema": {
                                "title": "Skip",
                                "type": "integer",
                                "default": 0,
                            },
                            "name": "skip",
                            "in": "query",
                        },
                        {

First, of course, you can mix `Path`, `Query` and request body parameter declarations freely and **FastAPI** will know what to do.

And you can also declare body parameters as optional, by setting the default to `None`:

=== "Python 3.10+"

    ```Python hl_lines="18-20"
    {!> ../../../docs_src/body_multiple_params/tutorial001_an_py310.py!}
    ```

=== "Python 3.9+"

    ```Python hl_lines="18-20"

from typing import Union

from fastapi import FastAPI, Query

app = FastAPI()


@app.get("/items/")
async def read_items(q: Union[list[str], None] = Query(default=None)):
    query_items = {"q": q}

from fastapi import FastAPI, Query

app = FastAPI()


@app.get("/items/")
async def read_items(q: str | None = Query(default=None, max_length=50)):
    results = {"items": [{"item_id": "Foo"}, {"item_id": "Bar"}]}
    if q:
        results.update({"q": q})

from fastapi import FastAPI, Query

app = FastAPI()


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

from typing import Union

from fastapi import FastAPI, Query

app = FastAPI()


@app.get("/items/")
async def read_items(q: Union[str, None] = Query(default=None, max_length=50)):
    results = {"items": [{"item_id": "Foo"}, {"item_id": "Bar"}]}
    if q:
        results.update({"q": q})