Flask vs FastAPI: differences for building Python web APIs

Both are small Python frameworks for HTTP APIs. Flask is a mature WSGI micro-framework: routing and request handling, with validation, serialization and API documentation added through extensions. FastAPI is an ASGI framework built on Starlette and Pydantic: it is async-first, validates requests from type hints, and generates OpenAPI documentation automatically.

Criterion by criterion

The same endpoint in Flask, tested

python -c '
from flask import Flask, jsonify, request
app = Flask(__name__)

@app.post("/items")
def create_item():
    data = request.get_json(silent=True) or {}
    if not isinstance(data.get("price"), (int, float)):
        return jsonify(error="price must be a number"), 422
    return jsonify(name=data.get("name"), price=data["price"]), 201

client = app.test_client()
for body in ({"name": "pen", "price": 1.5}, {"name": "pen", "price": "free"}):
    r = client.post("/items", json=body)
    print(r.status_code, r.get_json())'

201 {'name': 'pen', 'price': 1.5}
422 {'error': 'price must be a number'}

The same endpoint in FastAPI

from fastapi import FastAPI
from pydantic import BaseModel

app = FastAPI()

class Item(BaseModel):
    name: str
    price: float

@app.post("/items", status_code=201)
def create_item(item: Item):
    return item

The validation that Flask does by hand comes from the Item model: an invalid price gets a 422 response with a detailed error, and the endpoint appears in /docs.

Which one

Choose FastAPI for new JSON APIs with many endpoints, async database or HTTP calls, and a need for accurate API docs. Choose Flask when you render HTML, rely on its extensions, or already run WSGI infrastructure.