Moving from Flask to FastAPI: A Comprehensive Comparison and Migration Guide

This article, translated from "Moving from Flask to FastAPI" by Amal Shaji, introduces FastAPI as a lightweight, asynchronous ASGI framework and compares it with the more established Flask micro‑framework.

FastAPI vs Flask

FastAPI focuses on three main concerns: speed, developer experience, and open standards. It combines Starlette, Pydantic, OpenAPI, and JSON Schema, offering automatic data validation, type‑hinted editors, and performance comparable to Go or Node.js.

Flask has a larger ecosystem and community support, making it the winner for many plug‑in use‑cases.

Installation

<code>pip install flask
# or
poetry add flask
pipenv install flask
conda install flask</code>

<code>pip install fastapi uvicorn
# or
poetry add fastapi uvicorn
pipenv install fastapi uvicorn
conda install fastapi uvicorn -c conda-forge</code>

FastAPI requires an ASGI server such as Uvicorn or Daphne.

"Hello World" Applications

Flask example:

<code># flask_code.py
from flask import Flask
app = Flask(__name__)

@app.route("/")
def home():
    return {"Hello": "World"}

if __name__ == "__main__":
    app.run()</code>

FastAPI example:

<code># fastapi_code.py
import uvicorn
from fastapi import FastAPI
app = FastAPI()

@app.get("/")
def home():
    return {"Hello": "World"}

if __name__ == "__main__":
    uvicorn.run("fastapi_code:app")</code>

FastAPI supports hot‑reload via reload=True passed to uvicorn.run() or by running uvicorn run fastapi_code:app --reload .

Configuration

Both frameworks support environment variables, config files, instance folders, and class‑based settings. See the official docs for details:

Flask config: https://flask.palletsprojects.com/en/2.0.x/config/

FastAPI settings: https://fastapi.tiangolo.com/advanced/settings/

Routing, Templates, and Views

HTTP methods:

<code># Flask
from flask import request
@app.route("/", methods=["GET", "POST"])
def home():
    if request.method == "POST":
        return {"Hello": "POST"}
    return {"Hello": "GET"}</code>

<code># FastAPI
@app.get("/")
def home():
    return {"Hello": "GET"}

@app.post("/")
def home_post():
    return {"Hello": "POST"}</code>

URL parameters:

<code># Flask
@app.route("/employee/<int:id>")
def employee(id):
    return {"id": id}</code>

<code># FastAPI
@app.get("/employee/{id}")
def employee(id: int):
    return {"id": id}</code>

Query parameters work similarly, with FastAPI using function arguments and type hints.

Static Files

Flask serves from a static folder by default. FastAPI requires explicit mounting:

<code>from fastapi.staticfiles import StaticFiles
app.mount("/static", StaticFiles(directory="static"), name="static")</code>

Asynchronous Tasks

FastAPI natively supports async endpoints and background tasks:

<code>from fastapi import BackgroundTasks

@app.post("/upload/{filename}")
async def upload_and_process(filename: str, background_tasks: BackgroundTasks):
    background_tasks.add_task(process_file, filename)
    return {"message": "processing file"}</code>

Flask 2.0 also allows async routes, but full async support often requires external tools like Celery.

Dependency Injection

FastAPI provides a powerful Depends system; Flask requires third‑party extensions such as flask‑injector .

Data Validation and Serialization

FastAPI uses Pydantic models for request validation and response serialization, while Flask relies on external libraries (e.g., Flask‑Pydantic, Marshmallow).

Middleware

Both frameworks allow custom middleware; FastAPI uses the @app.middleware("http") decorator.

Modularization

Flask uses Blueprints; FastAPI uses APIRouter to split applications into reusable modules.

Other Features

Automatic documentation: FastAPI provides OpenAPI, Swagger UI, and ReDoc out of the box; Flask needs extensions.

Admin interfaces: Flask‑Admin for CRUD; FastAPI has FastAPI‑Admin and SQLAlchemy‑Admin.

Authentication: FastAPI includes fastapi.security for HTTP Basic, OAuth2, JWT, etc.; Flask relies on third‑party extensions.

CORS: FastAPI has built‑in CORSMiddleware ; Flask uses flask‑cors .

Testing

Both frameworks support test clients. Flask uses app.test_client() ; FastAPI uses TestClient from fastapi.testclient .

Deployment

Flask is typically run behind a WSGI server such as Gunicorn. FastAPI runs with Uvicorn and can be combined with Gunicorn workers for concurrency.

Docker

Simple Dockerfiles are provided for both Flask (using Gunicorn) and FastAPI (using Uvicorn).

Conclusion

While Flask remains popular for its simplicity and mature ecosystem, FastAPI offers modern features—async support, automatic validation, and built‑in documentation—that make it an attractive choice for new API projects, especially those involving machine‑learning models.

References

FastAPI: https://fastapi.tiangolo.com/

Flask: https://flask.palletsprojects.com/en/2.0.x/

Additional resources and tutorials are listed at the end of the original article.