How to write a custom FastAPI middleware class

Question

I have read FastAPI's documentation about middlewares (specifically, the middleware tutorial, the CORS middleware section and the advanced middleware guide), but couldn't find a concrete example of how to write a middleware class which you can add using the add_middleware function (in contrast to a basic middleware function added using a decorator) there nor on this site.

The reason I prefer to use add_middleware over the app based decorator, is that I want to write a middleware in a shared library that will be used by several different projects, and therefore I can't tie it to a specific FastAPI instance.

So my question is: how do you do it?

Answer 1

Since FastAPI is actually Starlette underneath, you could use BaseHTTPMiddleware, which allows one to implement a middleware class (you may want to have a look at this post as well). Below are given two variants of the same approach on how to do that, where the add_middleware() function is used to add the middleware class (Option 2 might be easier for most users). Please note that it is currently not possible to use BackgroundTasks (if that's a requirement for your task) with BaseHTTPMiddleware—check #1438 and #1640 for more details and any recent updates. Alternatives can be found in this answer and this answer.

Option 1

middlewares.py

from fastapi import Request

class MyMiddleware:
    def __init__(self, some_attribute: str):
        self.some_attribute = some_attribute

    async def __call__(self, request: Request, call_next):
        # do something with the request object
        content_type = request.headers.get('Content-Type')
        print(content_type)
        
        # process the request and get the response    
        response = await call_next(request)
        
        return response

app.py

from fastapi import FastAPI
from middlewares import MyMiddleware
from starlette.middleware.base import BaseHTTPMiddleware

app = FastAPI()
my_middleware = MyMiddleware(some_attribute="some_attribute_here_if_needed")
app.add_middleware(BaseHTTPMiddleware, dispatch=my_middleware)

Option 2

middlewares.py

from fastapi import Request
from starlette.middleware.base import BaseHTTPMiddleware

class MyMiddleware(BaseHTTPMiddleware):
    async def dispatch(self, request: Request, call_next):
        # do something with the request object, for example
        content_type = request.headers.get('Content-Type')
        print(content_type)
        
        # process the request and get the response    
        response = await call_next(request)
        
        return response

app.py

from fastapi import FastAPI
from middlewares import MyMiddleware

app = FastAPI()
app.add_middleware(MyMiddleware)

If you want to provide configuration options to the middleware class you should override the __init__ method, ensuring that the first argument is app, and any remaining arguments are optional keyword arguments. Example:

middlewares.py

from fastapi import Request
from starlette.middleware.base import BaseHTTPMiddleware

class MyMiddleware(BaseHTTPMiddleware):
    def __init__(self, app, some_attribute: str):
        super().__init__(app)
        self.some_attribute = some_attribute

    async def dispatch(self, request: Request, call_next):
        # do something with the request object, for example:
        content_type = request.headers.get('Content-Type')
        print(content_type)
        
        # process the request and get the response    
        response = await call_next(request)
        
        # add new header to the response
        response.headers['Custom'] = self.some_attribute
        
        return response

app.py

from fastapi import FastAPI
from middlewares import MyMiddleware

app = FastAPI()
app.add_middleware(MyMiddleware, some_attribute="some_attribute_here_if_needed")