Gyaan

FastAPI vs Flask vs Django

intermediate fastapi flask django comparison

This is the classic “tell me the difference between…” interview question for Python backend roles. The short answer: Django is a full-stack batteries-included framework, Flask is a minimal sync micro-framework, and FastAPI is a modern async API framework with type-driven validation.

Let’s break down the actual differences.

Feature
FastAPI
Flask
Django
Style
Async API
Sync micro
Full-stack
Server
ASGI (uvicorn)
WSGI (gunicorn)
WSGI / ASGI
Validation
Pydantic (built-in)
Manual / Marshmallow
DRF serializers
ORM
None (bring SQLAlchemy)
None
Django ORM
Admin panel
No
No
Yes (free)
Auto docs
Yes (OpenAPI)
No
DRF only
Performance
High
Medium
Medium

Async-first vs sync

Flask and Django were built in a sync world. Each request occupies a worker thread or process from start to finish. If our handler hits a slow database query, that worker just sits there waiting.

FastAPI is async by default. We write async def and use await for I/O. One worker can handle hundreds of concurrent requests because while one waits on the DB, another runs.

# FastAPI - async handler
@app.get("/orders/{user_id}")
async def get_orders(user_id: int):
    user = await db.fetch_user(user_id)
    orders = await db.fetch_orders(user_id)
    return {"user": user, "orders": orders}
# Flask - sync handler
@app.route("/orders/<int:user_id>")
def get_orders(user_id):
    user = db.fetch_user(user_id)
    orders = db.fetch_orders(user_id)
    return {"user": user, "orders": orders}

Django supports async views since 3.1, but a lot of the ecosystem (ORM, middleware) is still sync-leaning.

Validation

In Flask we pull values off request.json and validate them ourselves (or pull in Marshmallow). In Django REST Framework we write a Serializer class. In FastAPI it’s just a type hint.

# FastAPI
@app.post("/users/")
async def create_user(name: str, age: int):
    return {"name": name, "age": age}

That age: int automatically rejects "twenty" with a 422 response. No extra code.

Batteries vs choose-your-own

Django gives us ORM, admin, auth, sessions, migrations, templates — all in one box. Great for content sites, CRUD apps, internal tools.

FastAPI gives us routing, validation, docs. We bring our own ORM (usually SQLAlchemy or Tortoise), auth library, etc. More flexible, more decisions to make.

Flask sits in between but leans toward minimal — bring everything yourself.

Which to pick?

  • Django — content site, lots of CRUD, want admin panel for free, team likes monoliths.
  • Flask — small sync API, lots of legacy code, simple internal tools.
  • FastAPI — modern API, async I/O, ML model serving, microservices, want OpenAPI for free.

The interview answer

“FastAPI is async-first with built-in Pydantic validation and auto OpenAPI docs, Flask is a minimal sync micro-framework, and Django is a batteries-included full-stack framework with ORM and admin. For new high-throughput APIs I’d reach for FastAPI.” That covers 90% of the question.

References