Flask vs Django vs FastAPI: Which Python Web Framework?

If you're building a web backend in Python, you'll choose between three main frameworks. They can all build almost anything; they differ in how much they give you out of the box.

• Django — batteries included. Admin panel, ORM, auth, forms, migrations.
• Flask — minimal. Routing and templates; you choose the rest.
• FastAPI — modern APIs. Type hints, automatic validation and docs, async.

(New to Python? What is Python?)

The same endpoint in each

Return a user by ID as JSON:

Flask

from flask import Flask, jsonify

app = Flask(__name__)

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

FastAPI

from fastapi import FastAPI
from pydantic import BaseModel

app = FastAPI()

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

@app.get("/users/{user_id}")
def get_user(user_id: int) -> User:
    return User(id=user_id, name="Ada")

Django (a view plus a URL entry, in a project with an app already set up)

# views.py
from django.http import JsonResponse

def get_user(request, user_id):
    return JsonResponse({"id": user_id, "name": "Ada"})

# urls.py
path("users/<int:user_id>", views.get_user)

Notice FastAPI: the type hints aren't decoration. Passing /users/abc automatically returns a clear 422 validation error, and interactive API docs appear at /docs with no extra code.

Side by side

	Django	Flask	FastAPI
Philosophy	Everything included	Small core, add extensions	Typed APIs first
Database / ORM	Built in (Django ORM + migrations)	Add SQLAlchemy	Add SQLAlchemy / SQLModel
Admin panel	Built in	Extension	No
Auth	Built in	Extension	Build or add a library
Validation	Forms / DRF serializers	Manual or extension	Pydantic, automatic
API docs	Via add-ons	Via add-ons	Automatic (OpenAPI)
Async	Supported, partial	Limited	Native
HTML pages	Excellent (templates)	Good (Jinja)	Possible, not its focus
Learning curve	Steeper, more concepts	Gentle	Gentle if you know type hints

Choose Django when…

• You're building a full web application with users, admin, content and forms.
• You want proven conventions and don't want to choose libraries.
• The admin panel alone would save you weeks (internal tools, content management).

Add Django REST Framework (or Django Ninja) for APIs.

Choose Flask when…

• The app is small or unusual and you want full control.
• You're learning how web backends work — there's little magic.
• You're adding a few endpoints around existing Python code.

Choose FastAPI when…

• You're building an API — for a React frontend, a mobile app, or other services.
• You're serving AI/ML models or calling LLM APIs, where async and streaming help.
• You like types and want validation and documentation for free.

FastAPI has become the default for new Python APIs, especially in AI projects. (Deploying a FastAPI app)

What AI tools generate

All three are well known to coding agents. FastAPI and Django are the most common in AI-generated Python backends. Whichever you choose, write it in your project's CLAUDE.md so the agent doesn't mix patterns.

The summary

• Django: the full kit, best for complete web apps with admin and auth.
• Flask: minimal and flexible, best for small or custom apps.
• FastAPI: typed, async APIs with automatic validation and docs.
• Building an API for a JS frontend or AI features? FastAPI. A content-heavy app with an admin? Django.

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Related: What Is Python? · Python vs JavaScript · Python Virtual Environments · What Is a Framework?