How to add a custom decorator to a FastAPI route?

Question

I want to add an auth_required decorator to my endpoints. ( Please consider that this question is about decorators, not middleware )

So a simple decorator looks like this:

def auth_required(func):
    def wrapper(*args, **kwargs):
        if user_ctx.get() is None:
            raise HTTPException(...)
        return func(*args, **kwargs)
    return wrapper

So there are 2 usages:

@auth_required
@router.post(...)

or

@router.post(...)
@auth_required

The first way doesn't work because router.post creates a router that saved into self.routes of APIRouter object. The second way doesn't work because it fails to verify pydantic object. For any request model, it says missing args, missing kwargs .

So my question is - how can I add any decorators to FastAPI endpoints? Should I get into router.routes and modify the existing endpoint? Or use some functools.wraps like functions?

Answer 1

How can I add any decorators to FastAPI endpoints?

As you said, you need to use @functools.wraps(...) --(PyDoc) decorator as,

from fastapi import FastAPI
from pydantic import BaseModel


class SampleModel(BaseModel):
    name: str
    age: int


app = FastAPI()





@app.post("/")

async def root(payload: SampleModel):
    return {"message": "Hello World", "payload": payload}

The main caveat of this method is that you can't access the request object in the wrapper and I assume it is your primary intention.

If you need to access the request, you must add the argument to the router function as,

@app.post("/")

async def root( payload: SampleModel):
    return {"message": "Hello World", "payload": payload}

---

I am not sure what's wrong with the FastAPI middleware, after all, the @app.middleware(...) is also a decorator.

Answer 2

Simply use the dependencies inside of the path operation decorator:

from fastapi import Depends, FastAPI, Header, HTTPException

app = FastAPI()


async def verify_token(x_token: str = Header()):
    if x_token != "fake-super-secret-token":
        raise HTTPException(status_code=400, detail="X-Token header invalid")


async def verify_key(x_key: str = Header()):
    if x_key != "fake-super-secret-key":
        raise HTTPException(status_code=400, detail="X-Key header invalid")
    return x_key


@app.get("/items/", dependencies=[Depends(verify_token), Depends(verify_key)])
async def read_items():
    return [{"item": "Foo"}, {"item": "Bar"}]

Answer 3

Here is how you can use a decorator that adds extra parameters to the route handler:

from fastapi import FastAPI, Request
from pydantic import BaseModel


class SampleModel(BaseModel):
    name: str
    age: int


app = FastAPI()

def do_something_with_request_object(request: Request):
    print(request)

def auth_required(handler):
    async def wrapper(request: Request, *args, **kwargs):
        do_something_with_request_object(request)
        return await handler(*args, **kwargs)

    # Fix signature of wrapper
    import inspect
    wrapper.__signature__ = inspect.Signature(
        parameters = [
            # Use all parameters from handler
            *inspect.signature(handler).parameters.values(),

            # Skip *args and **kwargs from wrapper parameters:
            *filter(
                lambda p: p.kind not in (inspect.Parameter.VAR_POSITIONAL, inspect.Parameter.VAR_KEYWORD),
                inspect.signature(wrapper).parameters.values()
            )
        ],
        return_annotation = inspect.signature(handler).return_annotation,
    )

    return wrapper


@app.post("/")
@auth_required # Custom decorator
async def root(payload: SampleModel):
    return {"message": f"Hello {payload.name}, {payload.age} years old!"}