FastAPI vs Flask: Which Python Framework Should You Choose?

I've shipped production apps with both Flask and FastAPI, and here's what I've learned: they're both great, but for different reasons.

Flask: The Reliable Classic

Flask has been around since 2010, and it's still popular for good reason.

What works:

• Dead simple to learn. Minimal boilerplate, just Python functions
• Massive ecosystem with extensions for everything
• Complete flexibility—build your app however you want
• Battle-tested in production by thousands of companies

The challenges:

• Manual data validation gets repetitive fast
• No automatic API documentation (need extra libraries)
• Async support exists but feels tacked on
• More code to achieve the same results as FastAPI

Quick example:

from flask import Flask, request

app = Flask(__name__)

@app.route('/users', methods=['POST'])
def create_user():
    data = request.get_json()
    # Manual validation needed
    if not data or 'email' not in data:
        return {'error': 'Invalid data'}, 400
    return {'id': 123}

FastAPI: The Modern Powerhouse

Launched in 2018, FastAPI takes advantage of modern Python features.

What works:

• Automatic validation using type hints—seriously reduces bugs
• Free interactive API docs at /docs (Swagger UI)
• Built for async from the ground up
• Noticeably faster performance (2-3x in my tests)
• Less code, more features

The challenges:

• Smaller ecosystem (though growing fast)
• Learning curve if your team doesn't know type hints or async
• The "magic" can be confusing when debugging

Same example in FastAPI:

from fastapi import FastAPI
from pydantic import BaseModel

app = FastAPI()

class User(BaseModel):
    email: str
    name: str

@app.post('/users')
def create_user(user: User):
    return {'id': 123}  # Validation automatic!

When I Use Flask

• Quick internal tools and admin panels
• Projects where Flask extensions solve my exact problem
• Teams new to Python web development
• "If it ain't broke" maintenance mode apps

When I Use FastAPI

• Public APIs where performance matters
• High-throughput microservices
• ML model deployment
• Any project with complex data validation
• Modern teams comfortable with type hints

Bottom Line

New API project? → Start with FastAPI. The productivity boost is real.

Simple CRUD app or internal tool? → Flask gets you there faster.

Already using Flask and it works? → Don't rewrite unless you're hitting actual limitations.

Both are excellent choices. FastAPI is my default now for APIs, but Flask still earns its place on plenty of my projects.

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What's your go-to? Let me know in the comments! 👇