Asynchronous Programming in Python

asyncio is the standard library's event loop and the base most async libraries target. uvloop is a drop-in replacement loop built on libuv that speeds up network-heavy asyncio code. Trio is an alternative async library built around structured concurrency, where tasks live inside nurseries and cannot outlive them; that idea influenced asyncio's TaskGroup. AnyIO provides a structured concurrency API that runs on top of either asyncio or Trio, and libraries such as Starlette and HTTPX use it. gevent takes a different route: greenlets and monkey-patching make ordinary blocking code cooperative without async and await. Twisted is the long-standing event-driven framework with implementations of many network protocols.

For CPU-bound work, the global interpreter lock limits threads in standard CPython builds, so concurrent.futures with a ProcessPoolExecutor or the multiprocessing module run code in separate processes. A useful rule: async or threads for I/O-bound work, processes for CPU-bound work. Free-threaded CPython builds, introduced as experimental in Python 3.13, remove the lock but require extensions that support them.