FastAPI document: Concurrency and async / await

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TL; DR

awaitを書く必要があるならasync def

awaitを書く必要がないならdef

ブロッキングIO

CPUバウンドならdefではなくasync def

If your application (somehow) doesn't have to communicate with anything else and wait for it to respond, use async def.

通信しないケース（＝IOがないので、それがブロッキングかブロッキングでないかはここでは関係ない）

Very Technical Details

If you are coming from another async framework that does not work in the way described above and you are used to defining trivial compute-only path operation functions with plain def for a tiny performance gain (about 100 nanoseconds), please note that in FastAPI the effect would be quite opposite.

In these cases, it's better to use async def unless your path operation functions use code that performs blocking I/O.

「ブロッキングIOするコードを使うpath operation関数でない限り、async defを使うのがよい」