How do I build a simple web API with Flask or FastAPI in Python?

· Category: Python Programming

Short answer

Flask and FastAPI are popular Python frameworks for building web APIs. Flask is minimal and extensible, while FastAPI offers automatic validation, serialization, and OpenAPI documentation based on type hints.

Steps

  1. Install the framework: pip install flask or pip install fastapi uvicorn.
  2. Define routes that return JSON responses.
  3. Run the development server.
# Flask
from flask import Flask, jsonify

app = Flask(__name__)

@app.route("/api/hello")
def hello():
    return jsonify({"message": "Hello, World!"})

if __name__ == "__main__":
    app.run(debug=True)

# FastAPI
from fastapi import FastAPI

app = FastAPI()

@app.get("/api/hello")
def hello():
    return {"message": "Hello, World!"}

Tips

  • FastAPI's dependency injection system simplifies authentication, database sessions, and validation.
  • Use pydantic models with FastAPI for request and response schemas.
  • Flask's ecosystem is mature; FastAPI excels in async and auto-documentation.

Common issues

  • Running Flask in production with app.run(debug=True) is insecure; use Gunicorn or uWSGI instead.
  • FastAPI async routes must use await for I/O; blocking calls freeze the event loop.
  • Forgetting to handle CORS can cause frontend integration issues.