How do Django, Flask, and FastAPI compare, and when would you choose each?

Detailed Answer

Django: batteries included

# models.py
from django.db import models

class Article(models.Model):
    title = models.CharField(max_length=200)
    body = models.TextField()
    published_at = models.DateTimeField(auto_now_add=True)

Django ships an ORM, a migration system, an admin panel generated automatically from your models, authentication/authorization, a templating engine, and form handling — all designed to work together out of the box, following the "convention over configuration" philosophy. Best fit: content-driven sites, internal tools, and applications where you want a mature, opinionated full-stack framework so you're not assembling and gluing together a dozen separate pieces yourself.

Flask: minimal and unopinionated

from flask import Flask, jsonify

app = Flask(__name__)

@app.route("/users/<int:user_id>")
def get_user(user_id):
    return jsonify({"id": user_id, "name": "Ada"})

Flask provides routing and request/response handling and essentially nothing else by default — you choose your own ORM (SQLAlchemy is common), your own auth solution, your own validation library. This flexibility is the point: Flask fits well for small services, APIs with unconventional requirements, or teams that want full control over which pieces go into their stack rather than accepting Django's defaults.

FastAPI: async-first, type-hint-driven

from fastapi import FastAPI
from pydantic import BaseModel

app = FastAPI()

class User(BaseModel):
    name: str
    age: int

@app.post("/users")
async def create_user(user: User):   # request body validated automatically from the type hint
    return {"id": 1, **user.model_dump()}

FastAPI uses Python type hints (via Pydantic) to automatically validate incoming request data, serialize responses, and generate interactive OpenAPI/Swagger documentation — all with minimal boilerplate compared to manually validating input in Flask/Django views. Being ASGI-native, it handles high-concurrency async workloads (calling other services, databases) efficiently without extra configuration.

Comparison at a glance

	Django	Flask	FastAPI
Philosophy	batteries-included	minimal, unopinionated	modern, type-hint-driven
Interface	WSGI (ASGI for async views since 3.1)	WSGI	ASGI (async-first)
ORM	built-in	bring your own (commonly SQLAlchemy)	bring your own
Auto validation/docs	forms + DRF (for APIs)	manual / extensions	built-in (Pydantic + OpenAPI)
Best for	full-stack apps, admin-heavy tools	small services, custom stacks	high-performance JSON APIs
Learning curve	steeper upfront, faster after	gentle, scales with complexity	gentle, strong typing payoff

Practical decision guide

Building a content-heavy site or internal admin-driven tool quickly, with a database and want conventions already decided → Django.
Building a small service or need full control over exactly which libraries make up the stack → Flask.
Building a new JSON API, especially one needing high concurrency, automatic validation, and generated docs → FastAPI.

Interview-ready summary: Django trades flexibility for productivity via a complete, opinionated stack (ORM, admin, auth) best suited to full-stack, database-driven apps. Flask trades built-in features for flexibility, suiting small or custom-architected services. FastAPI is the modern default for high-performance async JSON APIs, using type hints for automatic validation and documentation generation.