FastAPI vs Flask: Which Python Web Framework?
FastAPI is an async, type-hint-driven framework with automatic docs and validation; Flask is the minimal, flexible micro-framework with a vast extension ecosystem.
FastAPI uses Python type hints to validate requests, generate OpenAPI docs, and run on ASGI with strong async performance. Flask is a lightweight WSGI micro-framework that stays out of your way and relies on extensions for structure. FastAPI optimizes for typed, modern APIs; Flask for simplicity and freedom.
| FastAPI | Flask | |
|---|---|---|
| Async | Native (ASGI) | Add-on (WSGI by default) |
| Validation / docs | Automatic (type hints) | Via extensions |
| Performance | High (async) | Good (sync) |
| Ecosystem | Growing | Very large |
| Best for | Typed modern APIs | Simple, flexible apps |
In CI
Both test with pytest and a test client; FastAPI gives typed request/response validation that catches more at the boundary. CI flow is identical - install, lint, test. Choose FastAPI for async APIs with auto-docs, Flask for minimal, extension-driven apps.
Speed it up
Cache the pip or poetry environment between runs. Both run on CI runners; faster managed runners shorten install and test steps.
The verdict
Building a modern, typed, async API with automatic docs: FastAPI. Wanting a minimal, flexible framework with a huge extension ecosystem: Flask. FastAPI for new APIs, Flask for simple or legacy-friendly apps.