Advanced Dependencies

Warning

This is, more or less, an "advanced" chapter.

If you are just starting with FastAPI you might want to skip this chapter and come back to it later.

Parameterized dependencies

All the dependencies we have seen are a fixed function or class.

But there could be cases where you want to be able to set parameters on the dependency, without having to declare many different functions or classes.

Let's imagine that we want to have a dependency that checks if the query parameter q contains some fixed content.

But we want to be able to parameterize that fixed content.

A "callable" instance

In Python there's a way to make an instance of a class a "callable".

Not the class itself (which is already a callable), but an instance of that class.

To do that, we declare a method __call__:

from fastapi import Depends, FastAPI

app = FastAPI()


class FixedContentQueryChecker:
    def __init__(self, fixed_content: str):
        self.fixed_content = fixed_content

    def __call__(self, q: str = ""):
        if q:
            return self.fixed_content in q
        return False


checker = FixedContentQueryChecker("bar")


@app.get("/query-checker/")
async def read_query_check(fixed_content_included: bool = Depends(checker)):
    return {"fixed_content_in_query": fixed_content_included}

In this case, this __call__ is what FastAPI will use to check for additional parameters and sub-dependencies, and this is what will be called to pass a value to the parameter in your path operation function later.

Parameterize the instance

And now, we can use __init__ to declare the parameters of the instance that we can use to "parameterize" the dependency:

from fastapi import Depends, FastAPI

app = FastAPI()


class FixedContentQueryChecker:
    def __init__(self, fixed_content: str):
        self.fixed_content = fixed_content

    def __call__(self, q: str = ""):
        if q:
            return self.fixed_content in q
        return False


checker = FixedContentQueryChecker("bar")


@app.get("/query-checker/")
async def read_query_check(fixed_content_included: bool = Depends(checker)):
    return {"fixed_content_in_query": fixed_content_included}

In this case, FastAPI won't ever touch or care about __init__, we will use it directly in our code.

Create an instance

We could create an instance of this class with:

from fastapi import Depends, FastAPI

app = FastAPI()


class FixedContentQueryChecker:
    def __init__(self, fixed_content: str):
        self.fixed_content = fixed_content

    def __call__(self, q: str = ""):
        if q:
            return self.fixed_content in q
        return False


checker = FixedContentQueryChecker("bar")


@app.get("/query-checker/")
async def read_query_check(fixed_content_included: bool = Depends(checker)):
    return {"fixed_content_in_query": fixed_content_included}

And that way we are able to "parameterize" our dependency, that now has "bar" inside of it, as the attribute checker.fixed_content.

Use the instance as a dependency

Then, we could use this checker in a Depends(checker), instead of Depends(FixedContentQueryChecker), because the dependency is the instance, checker, not the class itself.

And when solving the dependency, FastAPI will call this checker like:

checker(q="somequery")

...and pass whatever that returns as the value of the dependency in our path operation function as the parameter fixed_content_included:

from fastapi import Depends, FastAPI

app = FastAPI()


class FixedContentQueryChecker:
    def __init__(self, fixed_content: str):
        self.fixed_content = fixed_content

    def __call__(self, q: str = ""):
        if q:
            return self.fixed_content in q
        return False


checker = FixedContentQueryChecker("bar")


@app.get("/query-checker/")
async def read_query_check(fixed_content_included: bool = Depends(checker)):
    return {"fixed_content_in_query": fixed_content_included}

Tip

All this might seem contrived. And it might not be very clear how is it useful yet.

These examples are intentionally simple, but show how it all works.

In the chapters about security, you will be using utility functions that are implemented in this same way.

If you understood all this, you already know how those utility tools for security work underneath.