Flask vs FastAPI

Flask and FastAPI are both Python microframeworks, but they represent different generations of Python web development. Flask is the synchronous, flexible veteran. FastAPI is the async, type-safe newcomer.

The Async Advantage

Flask runs on WSGI, which processes one request per thread. Under high concurrency, this means you need many threads or processes, each consuming significant memory.

FastAPI runs on ASGI (via Starlette), which uses Python's asyncio to handle thousands of concurrent connections on a single process. For I/O-bound APIs that call databases, external APIs, or caches, this translates to much better throughput per dollar of compute.

Validation and Documentation

FastAPI's killer feature is its use of Python type hints for automatic request validation and OpenAPI documentation. Define a Pydantic model, use it as a function parameter, and FastAPI automatically validates incoming JSON, generates Swagger docs, and provides editor autocomplete.

Flask requires manually adding validation (Marshmallow, Cerberus) and documentation (Flask-RESTX, Flasgger) as separate steps, which often fall out of sync with the actual code.

Migration Path

Flask applications can incrementally migrate to FastAPI. Both frameworks support similar routing patterns, and Flask's synchronous handlers work as-is in FastAPI (FastAPI runs sync functions in a thread pool). This makes gradual migration practical.

System Design Considerations

For system design interviews, FastAPI is the modern default for Python APIs. Mention Flask when discussing legacy systems or when the application needs server-side HTML rendering.

Explore more in our comparison guides and prepare with interview questions.