```Python
q: str | None = Query(default=None, max_length=50)
```

Esto validará los datos, mostrará un error claro cuando los datos no sean válidos, y documentará el parámetro en el esquema del *path operation* de OpenAPI.

### `Query` como valor por defecto o en `Annotated` { #query-as-the-default-value-or-in-annotated }

Ten en cuenta que cuando uses `Query` dentro de `Annotated` no puedes usar el parámetro `default` para `Query`.

我们打算添加约束条件：即使 `q` 是可选的，但只要提供了该参数，则该参数值**不能超过50个字符的长度**。

### 导入 `Query`

为此，首先从 `fastapi` 导入 `Query`：

{* ../../docs_src/query_params_str_validations/tutorial002.py hl[1] *}

## 使用 `Query` 作为默认值

现在，将 `Query` 用作查询参数的默认值，并将它的 `max_length` 参数设置为 50：

{* ../../docs_src/query_params_str_validations/tutorial002.py hl[9] *}

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

                    },
                    "summary": "Get Query",
                    "operationId": "get_query_query_get",
                    "parameters": [
                        {
                            "required": True,
                            "schema": {"title": "Query"},
                            "name": "query",
                            "in": "query",
                        }
                    ],

    response = client.get("/query/int?query=42")
    assert response.status_code == 200
    assert response.json() == "foo bar 42"


def test_query_int_query_42_5():
    response = client.get("/query/int?query=42.5")
    assert response.status_code == 422
    assert response.json() == {
        "detail": [
            {
                "type": "int_parsing",
                "loc": ["query", "query"],

                            "required": True,
                            "schema": {"title": "Level1", "type": "string"},
                            "name": "level1",
                            "in": "query",
                        }
                    ],
                    "responses": {
                        "200": {
                            "description": "Successful Response",

		xhttp.AmzSignature,
	)

	// Add missing query parameters if any provided in the request URL
	for k, v := range req.Form {
		if !defaultSigParams.Contains(k) {
			query[k] = v
		}
	}

	// Get the encoded query.
	encodedQuery := query.Encode()

	// Verify if date query is same.
	if req.Form.Get(xhttp.AmzDate) != query.Get(xhttp.AmzDate) {
		return ErrSignatureDoesNotMatch
	}

import pytest
from dirty_equals import IsOneOf
from fastapi import FastAPI, Query
from fastapi.testclient import TestClient
from pydantic import BaseModel, Field

app = FastAPI()

# =====================================================================================
# Without aliases


@app.get("/required-list-str")
async def read_required_list_str(p: Annotated[list[str], Query()]):
    return {"p": p}

from typing import Union

from fastapi import Body, FastAPI, Query
from fastapi.testclient import TestClient

app = FastAPI()


@app.get("/query")
def read_query(q: Union[str, None]):
    return q


@app.get("/explicit-query")
def read_explicit_query(q: Union[str, None] = Query()):
    return q


@app.post("/body-embed")
def send_body_embed(b: Union[str, None] = Body(embed=True)):
    return b

{* ../../docs_src/path_params_numeric_validations/tutorial006_an_py39.py hl[13] *}

## Resumen { #recap }

Con `Query`, `Path` (y otros que aún no has visto) puedes declarar metadatos y validaciones de string de las mismas maneras que con [Parámetros de Query y Validaciones de String](query-params-str-validations.md){.internal-link target=_blank}.

Y también puedes declarar validaciones numéricas:

* `gt`: `g`reater `t`han

{* ../../docs_src/path_params_numeric_validations/tutorial006_an_py39.py hl[13] *}

## Zusammenfassung { #recap }

Mit `Query`, `Path` (und anderen, die Sie noch nicht gesehen haben) können Sie Metadaten und Stringvalidierungen auf die gleichen Weisen deklarieren wie in [Query-Parameter und Stringvalidierungen](query-params-str-validations.md){.internal-link target=_blank} beschrieben.

Und Sie können auch Zahlenvalidierungen deklarieren: