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Python Asyncio Event Loop Tasks and coroutines

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- Python - Asyncio

Event loops, tasks and coroutines

  • Asyncio starts an event loop.
    • The event loop contains within it a list of objects called Tasks. Each Task maintains a single stack, and its own execution pointer as well.
    • There are 2 types of task in the event loop: active task and paused task.
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  • At any one time the event loop can ONLY have ONE Task actually executing (the processor can still only do one thing at a time, after all), whilst the other tasks in the loop are all paused.
  • The currently executing task will continue to execute exactly as if it were executing a function in a normal (synchronous) Python program, right up until it gets to a point where it would have to wait for something to happen before it can continue.
    • Then, instead of waiting, the code in the Task yields control.
    • But this can only be done if the task is awaitable.
  • This means that it asks the event loop to pause the Task it is running in, and wake it up again at a future point once the thing it needs to wait for has happened.
  • The event loop can then select one of its other sleeping tasks to wake up and become the executing task instead.
    • Or if none of them are able to awaken (because they’re all waiting for things to happen) then it can wait.
  • This way the CPU’s time can be shared between different tasks, all of which are executing code capable of yielding like this when they would otherwise wait.
An event loop CANNOT forcibly interrupt a coroutine that is currently executing.
  • A coroutine that is executing will continue executing until it (coroutine) yields control.
  • The event loop serves to select which coroutine to schedule next, and keeps track of which coroutines are blocked and unable to execute until some IO has completed, but it only does these things when no coroutine is currently executing.
  • This is different from multithreading in python since the interpreter can interrupt executing threads which is why we need locks in threading and not in asyncio.
  • This approach works well for IO-bound code, where long pauses are expected to wait for something else (often another computer) to respond to a request.

Last updated: 2022-11-11