Create a new response field. Raises if type_ is invalid.
    """
    class_validators = class_validators or {}
    if PYDANTIC_V2:
        field_info = field_info or FieldInfo(
            annotation=type_, default=default, alias=alias
        )
    else:
        field_info = field_info or FieldInfo()
    kwargs = {"name": name, "field_info": field_info}
    if PYDANTIC_V2:
        kwargs.update({"mode": mode})
    else:

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, min_length=3, max_length=50),
):
    results = {"items": [{"item_id": "Foo"}, {"item_id": "Bar"}]}
    if q:
        results.update({"q": q})

    for key, value in build_config.items():
        context.config.build_config[key] = value
    # Get main dependencies
    dependencies: List[str] = metadata.get("dependencies", [])
    # Add optional dependencies to the default dependencies for this (non-slim) package
    for include_optional in include_optional_dependencies:
        optional_dependencies_group = optional_dependencies.get(include_optional, [])

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

The main thing you need to run a **FastAPI** application (or any other ASGI application) in a remote server machine is an ASGI server program like **Uvicorn**, this is the one that comes by default in the `fastapi` command.

There are several alternatives, including:

* <a href="https://www.uvicorn.org/" class="external-link" target="_blank">Uvicorn</a>: a high performance ASGI server.

```

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)
```

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, title="Query string", min_length=3),
):
    results = {"items": [{"item_id": "Foo"}, {"item_id": "Bar"}]}
    if q:
        results.update({"q": q})

@app.put("/items/{item_id}")
async def read_items(
    item_id: UUID,
    start_datetime: datetime = Body(),
    end_datetime: datetime = Body(),
    process_after: timedelta = Body(),
    repeat_at: Union[time, None] = Body(default=None),
):
    start_process = start_datetime + process_after
    duration = end_datetime - start_process
    return {
        "item_id": item_id,
        "start_datetime": start_datetime,

For example, even though `users` would go after `items` in alphabetical order, it is shown before them, because we added their metadata as the first dictionary in the list.

## OpenAPI URL

By default, the OpenAPI schema is served at `/openapi.json`.

But you can configure it with the parameter `openapi_url`.

For example, to set it to be served at `/api/v1/openapi.json`:

```Python hl_lines="3"

):
    if not q:
        return last_query
    return q


@app.get("/items/")
async def read_query(
    query_or_default: Annotated[str, Depends(query_or_cookie_extractor)],
):