子依赖项

您可以创建具有子依赖项的依赖项。

它们可以按照您的需要任意嵌套。

FastAPI 将负责解析它们。

第一个依赖项（“可依赖项”）

您可以创建一个如下所示的第一层依赖项（“可依赖项”）：

Python 3.10+

from typing import Annotated

from fastapi import Cookie, Depends, FastAPI

app = FastAPI()


def query_extractor(q: str | None = None):
    return q


def query_or_cookie_extractor(
    q: Annotated[str, Depends(query_extractor)],
    last_query: Annotated[str | None, Cookie()] = None,
):
    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)],
):
    return {"q_or_cookie": query_or_default}

🤓 Other versions and variants

Python 3.9+Python 3.8+Python 3.10+ - non-AnnotatedPython 3.8+ - non-Annotated

from typing import Annotated, Union

from fastapi import Cookie, Depends, FastAPI

app = FastAPI()


def query_extractor(q: Union[str, None] = None):
    return q


def query_or_cookie_extractor(
    q: Annotated[str, Depends(query_extractor)],
    last_query: Annotated[Union[str, None], Cookie()] = None,
):
    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)],
):
    return {"q_or_cookie": query_or_default}

from typing import Union

from fastapi import Cookie, Depends, FastAPI
from typing_extensions import Annotated

app = FastAPI()


def query_extractor(q: Union[str, None] = None):
    return q


def query_or_cookie_extractor(
    q: Annotated[str, Depends(query_extractor)],
    last_query: Annotated[Union[str, None], Cookie()] = None,
):
    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)],
):
    return {"q_or_cookie": query_or_default}

Tip

Prefer to use the Annotated version if possible.

from fastapi import Cookie, Depends, FastAPI

app = FastAPI()


def query_extractor(q: str | None = None):
    return q


def query_or_cookie_extractor(
    q: str = Depends(query_extractor), last_query: str | None = Cookie(default=None)
):
    if not q:
        return last_query
    return q


@app.get("/items/")
async def read_query(query_or_default: str = Depends(query_or_cookie_extractor)):
    return {"q_or_cookie": query_or_default}

Tip

Prefer to use the Annotated version if possible.

from typing import Union

from fastapi import Cookie, Depends, FastAPI

app = FastAPI()


def query_extractor(q: Union[str, None] = None):
    return q


def query_or_cookie_extractor(
    q: str = Depends(query_extractor),
    last_query: Union[str, None] = Cookie(default=None),
):
    if not q:
        return last_query
    return q


@app.get("/items/")
async def read_query(query_or_default: str = Depends(query_or_cookie_extractor)):
    return {"q_or_cookie": query_or_default}

它声明了一个可选的查询参数 q，类型为 str，然后直接返回它。

这个依赖项非常简单（不太实用），但能帮助我们专注于理解子依赖项的工作原理。

第二个依赖项（“可依赖项”兼“依赖项”）

然后您可以创建另一个依赖函数（“可依赖项”），同时它自己也声明了一个依赖项（因此它也是一个“依赖项”）：

Python 3.10+

from typing import Annotated

from fastapi import Cookie, Depends, FastAPI

app = FastAPI()


def query_extractor(q: str | None = None):
    return q


def query_or_cookie_extractor(
    q: Annotated[str, Depends(query_extractor)],
    last_query: Annotated[str | None, Cookie()] = None,
):
    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)],
):
    return {"q_or_cookie": query_or_default}

🤓 Other versions and variants

Python 3.9+Python 3.8+Python 3.10+ - non-AnnotatedPython 3.8+ - non-Annotated

from typing import Annotated, Union

from fastapi import Cookie, Depends, FastAPI

app = FastAPI()


def query_extractor(q: Union[str, None] = None):
    return q


def query_or_cookie_extractor(
    q: Annotated[str, Depends(query_extractor)],
    last_query: Annotated[Union[str, None], Cookie()] = None,
):
    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)],
):
    return {"q_or_cookie": query_or_default}

from typing import Union

from fastapi import Cookie, Depends, FastAPI
from typing_extensions import Annotated

app = FastAPI()


def query_extractor(q: Union[str, None] = None):
    return q


def query_or_cookie_extractor(
    q: Annotated[str, Depends(query_extractor)],
    last_query: Annotated[Union[str, None], Cookie()] = None,
):
    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)],
):
    return {"q_or_cookie": query_or_default}

Tip

Prefer to use the Annotated version if possible.

from fastapi import Cookie, Depends, FastAPI

app = FastAPI()


def query_extractor(q: str | None = None):
    return q


def query_or_cookie_extractor(
    q: str = Depends(query_extractor), last_query: str | None = Cookie(default=None)
):
    if not q:
        return last_query
    return q


@app.get("/items/")
async def read_query(query_or_default: str = Depends(query_or_cookie_extractor)):
    return {"q_or_cookie": query_or_default}

Tip

Prefer to use the Annotated version if possible.

from typing import Union

from fastapi import Cookie, Depends, FastAPI

app = FastAPI()


def query_extractor(q: Union[str, None] = None):
    return q


def query_or_cookie_extractor(
    q: str = Depends(query_extractor),
    last_query: Union[str, None] = Cookie(default=None),
):
    if not q:
        return last_query
    return q


@app.get("/items/")
async def read_query(query_or_default: str = Depends(query_or_cookie_extractor)):
    return {"q_or_cookie": query_or_default}

让我们重点关注声明的参数：

• 尽管这个函数本身是一个依赖项（“可依赖项”），但它也声明了另一个依赖项（它“依赖于”其他东西）。
  • 它依赖于 query_extractor，并将其返回值赋给参数 q。
• 它还声明了一个可选的 last_query cookie，类型为 str。
  • 如果用户没有提供查询参数 q，我们将使用上次使用的查询（之前已保存到 cookie 中）。

使用依赖项

然后我们可以这样使用依赖项：

Python 3.10+

from typing import Annotated

from fastapi import Cookie, Depends, FastAPI

app = FastAPI()


def query_extractor(q: str | None = None):
    return q


def query_or_cookie_extractor(
    q: Annotated[str, Depends(query_extractor)],
    last_query: Annotated[str | None, Cookie()] = None,
):
    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)],
):
    return {"q_or_cookie": query_or_default}

🤓 Other versions and variants

Python 3.9+Python 3.8+Python 3.10+ - non-AnnotatedPython 3.8+ - non-Annotated

from typing import Annotated, Union

from fastapi import Cookie, Depends, FastAPI

app = FastAPI()


def query_extractor(q: Union[str, None] = None):
    return q


def query_or_cookie_extractor(
    q: Annotated[str, Depends(query_extractor)],
    last_query: Annotated[Union[str, None], Cookie()] = None,
):
    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)],
):
    return {"q_or_cookie": query_or_default}

from typing import Union

from fastapi import Cookie, Depends, FastAPI
from typing_extensions import Annotated

app = FastAPI()


def query_extractor(q: Union[str, None] = None):
    return q


def query_or_cookie_extractor(
    q: Annotated[str, Depends(query_extractor)],
    last_query: Annotated[Union[str, None], Cookie()] = None,
):
    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)],
):
    return {"q_or_cookie": query_or_default}

Tip

Prefer to use the Annotated version if possible.

from fastapi import Cookie, Depends, FastAPI

app = FastAPI()


def query_extractor(q: str | None = None):
    return q


def query_or_cookie_extractor(
    q: str = Depends(query_extractor), last_query: str | None = Cookie(default=None)
):
    if not q:
        return last_query
    return q


@app.get("/items/")
async def read_query(query_or_default: str = Depends(query_or_cookie_extractor)):
    return {"q_or_cookie": query_or_default}

Tip

Prefer to use the Annotated version if possible.

from typing import Union

from fastapi import Cookie, Depends, FastAPI

app = FastAPI()


def query_extractor(q: Union[str, None] = None):
    return q


def query_or_cookie_extractor(
    q: str = Depends(query_extractor),
    last_query: Union[str, None] = Cookie(default=None),
):
    if not q:
        return last_query
    return q


@app.get("/items/")
async def read_query(query_or_default: str = Depends(query_or_cookie_extractor)):
    return {"q_or_cookie": query_or_default}

Info

请注意，在路径操作函数中我们只声明了一个依赖项：query_or_cookie_extractor。

但 FastAPI 会知道需要先解析 query_extractor，以便在调用 query_or_cookie_extractor 时将其结果传递给它。

graph TB

query_extractor(["query_extractor"])
query_or_cookie_extractor(["query_or_cookie_extractor"])

read_query["/items/"]

query_extractor --> query_or_cookie_extractor --> read_query

多次使用同一依赖项

如果同一个路径操作多次声明了某个依赖项，例如多个依赖项拥有共同的子依赖项，FastAPI 会知道在同一请求中该子依赖项只需调用一次。

它会将返回值保存在“缓存”中，并在同一请求中传递给所有需要该值的“依赖项”，而不是为同一请求多次调用该依赖项。

在高级场景中，如果您需要依赖项在同一请求的每个步骤中（可能多次）被调用，而不是使用“缓存”值，可以在使用 Depends 时设置参数 use_cache=False：

Python 3.8+Python 3.8+ non-Annotated

async def needy_dependency(fresh_value: Annotated[str, Depends(get_value, use_cache=False)]):
    return {"fresh_value": fresh_value}

Tip

如果可能，请优先使用 Annotated 版本。

async def needy_dependency(fresh_value: str = Depends(get_value, use_cache=False)):
    return {"fresh_value": fresh_value}

总结回顾

尽管这里使用了许多花哨的术语，但依赖注入系统其实非常简单。

就是一些与路径操作函数形式相似的函数。

但它仍然非常强大，允许您声明任意深度的嵌套依赖“图”（树结构）。

Tip

在这些简单示例中，这一切可能看起来并不那么实用。

但您将在安全相关章节中看到它的巨大作用。

同时您也会体会到它能为您节省多少代码量。