子依赖项¶

您可以创建具有**子依赖项**的依赖项。

它们可以根据需要**深入**。

**FastAPI** 将负责解决它们。

第一个依赖项“dependable”¶

您可以创建一个第一个依赖项（“dependable”），例如

Python 3.10+Python 3.9+Python 3.8+Python 3.10 非注解Python 3.8 非注解

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}

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}

提示

如果可能，最好使用Annotated版本。

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}

提示

如果可能，最好使用Annotated版本。

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}

它将一个可选的查询参数q声明为str，然后只返回它。

这非常简单（不是很有用），但将帮助我们专注于子依赖项的工作原理。

第二个依赖项，“dependable”和“dependant”¶

然后，您可以创建另一个依赖项函数（一个“dependable”），它同时声明了自己的依赖项（因此它也是一个“dependant”）

Python 3.10+Python 3.9+Python 3.8+Python 3.10 非注解Python 3.8 非注解

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}

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}

提示

如果可能，最好使用Annotated版本。

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}

提示

如果可能，最好使用Annotated版本。

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}

让我们关注声明的参数

即使此函数本身是依赖项（“dependable”），它也声明了另一个依赖项（它“依赖”于其他内容）。
- 它依赖于query_extractor，并将从中返回的值分配给参数q。
它还声明了一个可选的last_query cookie，作为str。
- 如果用户没有提供任何查询q，我们将使用上次使用的查询，我们之前将其保存到 cookie 中。

使用依赖项¶

然后，我们可以使用依赖项

Python 3.10+Python 3.9+Python 3.8+Python 3.10 非注解Python 3.8 非注解

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}

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}

提示

如果可能，最好使用Annotated版本。

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}

提示

如果可能，最好使用Annotated版本。

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}

信息

请注意，我们仅在路径操作函数中声明了一个依赖项，即query_or_cookie_extractor。

但是**FastAPI**将知道它必须首先解决query_extractor，以便在调用query_or_cookie_extractor时将该结果传递给它。

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

多次使用相同的依赖项¶

如果您的某个依赖项为同一路径操作声明多次，例如，多个依赖项具有一个共同的子依赖项，**FastAPI**将知道每个请求仅调用该子依赖项一次。

它将把返回值保存在“缓存”中，并将其传递给该特定请求中需要它的所有“dependant”，而不是对同一请求多次调用依赖项。

在您知道需要在同一请求中的每个步骤（可能多次）调用依赖项（而不是使用“缓存”值）的高级场景中，您可以在使用Depends时设置参数use_cache=False

Python 3.8+Python 3.8+ 非注解

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

提示

如果可能，最好使用Annotated版本。

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

回顾¶

除了此处使用的所有花哨的词语外，**依赖项注入**系统非常简单。

就像路径操作函数一样的函数。

但即便如此，它仍然非常强大，允许你声明任意深度嵌套的依赖“图”（树）。

提示

通过这些简单的例子，你可能无法体会到它的全部用处。

但在关于**安全**的章节中，你将看到它的实用性。

你还会看到它能为你节省多少代码。