anyio

AnyIO has become my go-to foundation for async libraries because it abstracts away the differences between asyncio and Trio while providing a significantly better API than raw asyncio. The type hints are comprehensive and work beautifully with mypy and PyLance. Creating task groups is intuitive with `async with anyio.create_task_group()`, and the automatic exception propagation from child tasks is a game-changer compared to asyncio's fire-and-forget tasks.

The documentation is well-structured with clear examples for common patterns like timeouts, cancellation, and resource management. Error messages are actionable - when you forget to use `await` or misuse a context manager, the tracebacks actually help you fix the issue. The networking primitives (`create_tcp_listener`, streams) are much more ergonomic than asyncio's protocols.

One learning curve is understanding structured concurrency principles, but once you grasp task groups, your async code becomes more maintainable. Migration from raw asyncio is usually straightforward, though you'll need to refactor fire-and-forget tasks into properly structured groups.

AnyIO provides a clean abstraction over asyncio and Trio, making it practical to write backend-agnostic async code. The API design is intuitive with clear primitives for tasks, networking, and filesystem operations. I've used it extensively in libraries that need to support multiple async runtimes, and the abstraction holds up well in production.

From a security perspective, AnyIO follows secure defaults for TLS with proper certificate validation enabled by default and modern cipher suites. The networking APIs require explicit opt-in for insecure configurations, which is excellent. However, exception handling requires attention - cancelled tasks can leak context in tracebacks, and you need to be careful about wrapping operations to prevent information disclosure in logs.

One practical concern is the multi-layered exception hierarchy. When something fails in AnyIO's task groups or cancellation scopes, debugging can be challenging as exceptions get wrapped multiple times. Input validation for network operations is solid, but error messages sometimes expose internal socket details that you'll want to sanitize before showing to users.

AnyIO delivers on its promise of providing a unified async interface across asyncio and Trio. The API design is clean and intuitive once you understand the core concepts like task groups and cancellation scopes. The documentation is thorough with good examples for common patterns like spawning concurrent tasks, streaming data, and managing resources. Error messages are generally helpful, pointing you toward missing context managers when you forget them.

The learning curve is moderate if you're already comfortable with async/await, but steeper if you're new to structured concurrency. The task group pattern (similar to Trio's nurseries) takes some adjustment coming from raw asyncio, but it prevents common bugs like fire-and-forget tasks. Debugging can be tricky when wrapping existing asyncio code, as stack traces sometimes lose clarity across abstraction boundaries.

Community support is decent—GitHub issues get responses within days, and the maintainer is responsive. However, Stack Overflow content is limited, so you'll often need to dig into the official docs or examples. The typed API with good IDE support helps discovery significantly.