FastAPI - Mounting Flask App

FastAPI is a Python framework for building APIs, while Flask is a lightweight framework for developing web applications. Since both provide different features, they can be used together in a single application by mounting a Flask app within a FastAPI app.

Syntax:

• from fastapi.middleware.wsgi import WSGIMiddleware
• main_app.mount("/flask", WSGIMiddleware(flask_app))

Here,

• main_app: It is the main app on which you want to mount the Flask app.
• flask_app: It is the Flask app that has to be mounted on the main app.

Mounting Flask App

In this example, a FastAPI app (app) and a Flask app (flaskapp) are created, and the Flask app is mounted onto the FastAPI app using WSGIMiddleware.

main.py: This code combines FastAPI and Flask and defines two routes:

1. The FastAPI route at "/app" responds with "Main app called!" when accessed.
2. The Flask route at "/app2" responds with "Flask app called!" when accessed.

from fastapi import FastAPI
from flask import Flask
from fastapi.middleware.wsgi import WSGIMiddleware

app = FastAPI()
flaskapp = Flask(__name__)

@app.get("/app")
def main_app():
    return "Main app called!"

@flaskapp.route("/app2")
def flask_app():
    return "Flask app called!"

app.mount("/flask", WSGIMiddleware(flaskapp))

For calling the value returned from main app function, we use the following code in test file

Test 1: This code sends a GET request to http://127.0.0.1:8000/app using the requests library and parses the response using .json(). It returns the server’s response in JSON.

import requests
print(requests.get("http://127.0.0.1:8000/app").json())

Output:

Browser Output:

For calling the value returned from sub-app function, we use the following code in test file:

Test 1: This code sends a GET request to http://127.0.0.1:8000/flask/app2 using the requests library and prints the response as text. It returns the output from the Flask app.

import requests
print(requests.get("http://127.0.0.1:8000/flask/app2").text)

Output

Output