FastAPI vs Flask vs Django - when to use which?

Difficulty: Beginner

Question

When would you choose FastAPI over Flask or Django? What are the trade-offs?

Answer

FastAPI - Best for: high-performance async APIs, microservices, ML model serving, projects where auto-documentation and type safety matter. - Pros: Fast, async-native, automatic validation, auto docs, modern Python patterns - Cons: Smaller ecosystem than Django, no built-in ORM or admin panel, relatively newer

Flask - Best for: small to medium APIs, rapid prototyping, projects needing maximum flexibility. - Pros: Minimal, flexible, large ecosystem of extensions - Cons: No async by default, no built-in validation, no auto docs, more boilerplate

Django - Best for: full-stack web apps, rapid development with admin panel, projects needing an ORM out of the box. - Pros: Batteries included (ORM, admin, auth, migrations), mature ecosystem - Cons: Heavy for pure APIs, sync by default (async support improving), opinionated structure

Code examples

Equivalent POST endpoint in all three frameworks

# FastAPI (auto-validates, auto-docs)
from fastapi import FastAPI
from pydantic import BaseModel
app = FastAPI()
class Item(BaseModel):
    name: str
    price: float
@app.post("/items")
def create(item: Item):
    return item

# Flask (manual validation)
from flask import Flask, request, jsonify
app = Flask(__name__)
@app.post("/items")
def create():
    data = request.get_json()
    # manual validation required
    return jsonify(data)

# Django REST Framework
from rest_framework.decorators import api_view
from rest_framework.response import Response
@api_view(['POST'])
def create(request):
    # uses DRF serializers for validation
    return Response(request.data)

Key points

• FastAPI is ideal when building APIs consumed by a frontend or mobile app
• Django is the go-to for projects that need an admin dashboard out of the box
• Flask and FastAPI can coexist in a microservices architecture

Concepts covered

framework comparison, async, ORM, use cases