When should a FastAPI path operation use `def` or `async def`?

Chooses between synchronous and asynchronous FastAPI handlers based on the libraries and work they execute.

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TL;DR

I use async def when the handler calls awaitable I/O and use normal def when it must call blocking I/O through synchronous libraries. FastAPI calls async def directly on the event loop, while normal def path operations and dependencies run in a thread pool and are awaited. Putting a blocking database or HTTP call directly inside async def blocks the event loop. CPU-heavy work is not fixed by either choice and usually needs a separate process or bounded worker design.

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Understanding of event-loop blocking, FastAPI's thread-pool behavior, async I/O, CPU work, and capacity.

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The choice follows the actual call chain, not a belief that async syntax is automatically faster.
I use async def when the handler calls awaitable I/O and use normal def when it must call blocking I/O through synchronous libraries. FastAPI calls async def directly on the event loop, while normal def path operations and dependencies run in a thread pool and are awaited. Putting a blocking database or HTTP call directly inside async def blocks the event loop. CPU-heavy work is not fixed by either choice and usually needs a separate process or bounded worker design.
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Details

Using async def around blocking code and assuming it becomes non-blocking.
Calling def inherently slower or obsolete.
Using async syntax for CPU parallelism.
Ignoring thread-pool and downstream capacity.

When should a FastAPI path operation use `def` or `async def`?

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