AnyIO provides backend-agnostic structured concurrency for Python, supporting both asyncio and Trio. Master vocabulary for task groups, ExceptionGroup semantics, cancel scopes, move_on_after vs fail_after, shield behavior, worker thread offloading, and the anyio.run() entry point.
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A developer opens an anyio.create_task_group() and spawns two tasks. One task raises an exception. What happens to the other task?
AnyIO's task groups implement structured concurrency: when any task raises an exception, all sibling tasks are immediately cancelled. The task group then raises an ExceptionGroup (Python 3.11+) or BaseExceptionGroup containing all exceptions (including from the cancellation). This ensures no tasks are orphaned.
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What is the key difference between anyio.move_on_after() and anyio.fail_after()?
move_on_after(n) creates a cancel scope that silently exits after n seconds — code after the block continues executing. fail_after(n) raises TimeoutError if the block doesn't complete in time. Check scope.cancelled_caught after move_on_after to detect whether a timeout occurred.
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An engineer uses async with anyio.from_thread.run_sync_in_worker_thread(func). What problem does this solve?
run_sync_in_worker_thread (or the similar anyio.to_thread.run_sync) offloads blocking I/O or CPU-bound synchronous code to a thread pool, preventing it from blocking the async event loop. This is the correct pattern for integrating legacy synchronous libraries with async code.
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AnyIO's CancelScope has a shield attribute. When set to True, what behavior does it provide?
Setting shield=True on a CancelScope makes it immune to cancellation signals from enclosing scopes. Code inside a shielded scope will run to completion even if an outer cancel scope is cancelled. This is useful for cleanup operations that must not be interrupted, like flushing buffers or releasing external resources.
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A developer wants to run the same async code on both asyncio and Trio backends without changes. Which AnyIO function enables this at the entry point?
anyio.run(coroutine_function, backend='trio') (or backend='asyncio') is AnyIO's backend-agnostic entry point. It starts the specified backend's event loop and runs the coroutine. By parameterizing the backend, the same application code can be tested against both asyncio and Trio without modification.