Dependencies - `Depends()` and `Security()`¶

`Depends()`¶

Dependencies are handled mainly with the special function Depends() that takes a callable.

Here is the reference for it and its parameters.

You can import it directly from fastapi:

from fastapi import Depends

fastapi.Depends ¶

Depends(dependency=None, *, use_cache=True, scope=None)

Declare a FastAPI dependency.

It takes a single "dependable" callable (like a function).

Don't call it directly, FastAPI will call it for you.

Read more about it in the FastAPI docs for Dependencies.

Example

from typing import Annotated

from fastapi import Depends, FastAPI

app = FastAPI()


async def common_parameters(q: str | None = None, skip: int = 0, limit: int = 100):
    return {"q": q, "skip": skip, "limit": limit}


@app.get("/items/")
async def read_items(commons: Annotated[dict, Depends(common_parameters)]):
    return commons

PARAMETER	DESCRIPTION
`dependency`	A "dependable" callable (like a function). Don't call it directly, FastAPI will call it for you, just pass the object directly. Read more about it in the FastAPI docs for Dependencies TYPE: `Optional[Callable[..., Any]]` DEFAULT: `None`
`use_cache`	By default, after a dependency is called the first time in a request, if the dependency is declared again for the rest of the request (for example if the dependency is needed by several dependencies), the value will be re-used for the rest of the request. Set `use_cache` to `False` to disable this behavior and ensure the dependency is called again (if declared more than once) in the same request. Read more about it in the FastAPI docs about sub-dependencies TYPE: `bool` DEFAULT: `True`
`scope`	Mainly for dependencies with `yield`, define when the dependency function should start (the code before `yield`) and when it should end (the code after `yield`). `"function"`: start the dependency before the path operation function that handles the request, end the dependency after the path operation function ends, but before the response is sent back to the client. So, the dependency function will be executed around the path operation function. `"request"`: start the dependency before the path operation function that handles the request (similar to when using `"function"`), but end after the response is sent back to the client. So, the dependency function will be executed around the request and response cycle. Read more about it in the FastAPI docs for FastAPI Dependencies with yield TYPE: `Union[Literal['function', 'request'], None]` DEFAULT: `None`

Source code in fastapi/param_functions.py

def Depends(  # noqa: N802
    dependency: Annotated[
        Optional[Callable[..., Any]],
        Doc(
            """
            A "dependable" callable (like a function).

            Don't call it directly, FastAPI will call it for you, just pass the object
            directly.

            Read more about it in the
            [FastAPI docs for Dependencies](https://fastapi.tiangolo.com/tutorial/dependencies/)
            """
        ),
    ] = None,
    *,
    use_cache: Annotated[
        bool,
        Doc(
            """
            By default, after a dependency is called the first time in a request, if
            the dependency is declared again for the rest of the request (for example
            if the dependency is needed by several dependencies), the value will be
            re-used for the rest of the request.

            Set `use_cache` to `False` to disable this behavior and ensure the
            dependency is called again (if declared more than once) in the same request.

            Read more about it in the
            [FastAPI docs about sub-dependencies](https://fastapi.tiangolo.com/tutorial/dependencies/sub-dependencies/#using-the-same-dependency-multiple-times)
            """
        ),
    ] = True,
    scope: Annotated[
        Union[Literal["function", "request"], None],
        Doc(
            """
            Mainly for dependencies with `yield`, define when the dependency function
            should start (the code before `yield`) and when it should end (the code
            after `yield`).

            * `"function"`: start the dependency before the *path operation function*
                that handles the request, end the dependency after the *path operation
                function* ends, but **before** the response is sent back to the client.
                So, the dependency function will be executed **around** the *path operation
                **function***.
            * `"request"`: start the dependency before the *path operation function*
                that handles the request (similar to when using `"function"`), but end
                **after** the response is sent back to the client. So, the dependency
                function will be executed **around** the **request** and response cycle.

            Read more about it in the
            [FastAPI docs for FastAPI Dependencies with yield](https://fastapi.tiangolo.com/tutorial/dependencies/dependencies-with-yield/#early-exit-and-scope)
            """
        ),
    ] = None,
) -> Any:
    """
    Declare a FastAPI dependency.

    It takes a single "dependable" callable (like a function).

    Don't call it directly, FastAPI will call it for you.

    Read more about it in the
    [FastAPI docs for Dependencies](https://fastapi.tiangolo.com/tutorial/dependencies/).

    **Example**

    ```python
    from typing import Annotated

    from fastapi import Depends, FastAPI

    app = FastAPI()


    async def common_parameters(q: str | None = None, skip: int = 0, limit: int = 100):
        return {"q": q, "skip": skip, "limit": limit}


    @app.get("/items/")
    async def read_items(commons: Annotated[dict, Depends(common_parameters)]):
        return commons
    ```
    """
    return params.Depends(dependency=dependency, use_cache=use_cache, scope=scope)

`Security()`¶

For many scenarios, you can handle security (authorization, authentication, etc.) with dependencies, using Depends().

But when you want to also declare OAuth2 scopes, you can use Security() instead of Depends().

You can import Security() directly from fastapi:

from fastapi import Security

fastapi.Security ¶

Security(dependency=None, *, scopes=None, use_cache=True)

Declare a FastAPI Security dependency.

The only difference with a regular dependency is that it can declare OAuth2 scopes that will be integrated with OpenAPI and the automatic UI docs (by default at /docs).

It takes a single "dependable" callable (like a function).

Don't call it directly, FastAPI will call it for you.

Read more about it in the FastAPI docs for Security and in the FastAPI docs for OAuth2 scopes.