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Handling Cross-Origin Resource Sharing (CORS)

Cross-Origin Resource Sharing (CORS) is a mechanism that allows many resources (e.g., fonts, JavaScript, etc.) on a web page to be requested from another domain outside the domain from which the resource originated. In FastAPI, handling CORS means setting up our FastAPI application to correctly respond to CORS preflight requests and allow certain types of traffic from certain origins.

FastAPI and CORS​

FastAPI has a built-in middleware for managing CORS, which allows for easy configuration of CORS preflight responses.

Firstly, let's import the necessary modules and create an instance of FastAPI:

from fastapi import FastAPI

app = FastAPI()

Adding the Middleware​

To add a middleware, we use the add_middleware function of the FastAPI application instance. Let's add the CORS middleware:

from fastapi.middleware.cors import CORSMiddleware

app.add_middleware(
    CORSMiddleware,
    allow_origins=["*"],  # Allows all origins
    allow_credentials=True,
    allow_methods=["*"],  # Allows all methods
    allow_headers=["*"],  # Allows all headers
)

Here, the arguments to add_middleware are:

  • The middleware class - CORSMiddleware
  • allow_origins - a list of origins that are allowed to make requests to this server.
  • allow_credentials - indicates whether the response to the request can be exposed when the credentials flag is true.
  • allow_methods - a list of HTTP methods that are allowed to be executed by the server.
  • allow_headers - a list of headers that can be used during the actual request.

Fine-Tuning Middleware Configuration​

In real-world scenarios, allowing all origins, methods, and headers is not advisable due to security concerns. We can limit the allowed origins, methods, and headers as per our requirements:

app.add_middleware(
    CORSMiddleware,
    allow_origins=["http://localhost:3000", "http://localhost:8080"],
    allow_credentials=True,
    allow_methods=["GET", "POST"],
    allow_headers=["Content-Type"],
)

In this case, only requests from "http://localhost:3000" and "http://localhost:8080" using "GET" or "POST" methods with "Content-Type" headers will be accepted.

Conclusion​

Handling CORS in FastAPI is a straightforward process thanks to the built-in CORSMiddleware. By properly configuring CORS, you are effectively managing who can interact with your server and how they can interact with it, which is a key aspect of securing your FastAPI application.

In the next step of your learning journey, consider exploring more about other middleware in FastAPI, their purposes, and how they can be configured to meet the needs of your application.

Happy coding!