8.13. AsyncIO Event Loop

  • Async code can only run inside an event loop.

  • The event loop is the driver code that manages the cooperative multitasking.

  • You can create multiple threads and run different event loops in each of them.

  • Python will create a default event loop only in Main Thread

  • Python will not create an event loop automatically for you on any other than main thread by default, this is to prevent from having multiple event lops created explicitly

  • Event loop can execute only one callback (coroutine) at a time

  • Some callbacks (coroutines) can schedule themselves once again (trampoline)

  • Reactors

  • Proactors

For example, Django uses the main thread to wait for incoming requests, so we can't run an asyncio event loop there, but we can start a separate worker thread for our event loop 3.

An event loop runs in a thread (typically the main thread) and executes all callbacks and Tasks in its thread. While a Task is running in the event loop, no other Tasks can run in the same thread. When a Task executes an await expression, the running Task gets suspended, and the event loop executes the next Task. 4


Figure 8.15. Source: Michael Kennedy 1


Figure 8.16. Source: Michael Kennedy 1


Figure 8.17. Source: Michael Kennedy 1


Figure 8.18. Source: Michael Kennedy 1

8.13.1. Selectors


Figure 8.19. Source: Langa, Ł. import asyncio: Learn Python's AsyncIO 2


Figure 8.20. Source: Langa, Ł. import asyncio: Learn Python's AsyncIO 2

8.13.2. Loop

8.13.3. UVLoop

  • The ultimate loop implementation for UNIXes (run this on production)

$ pip install uvloop
... import uvloop
... uvloop.install()
... loop = asyncio.new_event_loop()
... loop
<uvloop.Loop running=False closed=False debug=False>

8.13.4. References


Kennedy, M. Demystifying Python's Async and Await Keywords. Publisher: JetBrainsTV. Year: 2019. Retrieved: 2022-03-10. URL: https://www.youtube.com/watch?v=F19R_M4Nay4


Langa, Ł. import asyncio: Learn Python's AsyncIO. Year: 2020. Retrieved: 2022-03-10. URL: https://www.youtube.com/playlist?list=PLhNSoGM2ik6SIkVGXWBwerucXjgP1rHmB


Poirier, D. Asyncio (superseded by async page). Year: 2021. Retrieved: 2022-03-17. URL: https://cheat.readthedocs.io/en/latest/python/asyncio.html


Python core developers. Developing with asyncio. Concurrency and Multithreading. Python documentation. Year: 2022. Retrieved: 2022-03-17. URL: https://docs.python.org/3/library/asyncio-dev.html#concurrency-and-multithreading