Python etc / asyncio: `get_event_loop`

asyncio: get_event_loop

In asyncio, the main thread loop is automatically created for you once you call asyncio.get_event_loop(). That doesn't happen in any other thread:

In : asyncio.get_event_loop()
Out: <_UnixSelectorEventLoop running=False closed=False debug=False>

In : Thread(target=asyncio.get_event_loop).start()
Exception in thread Thread-385:
RuntimeError: There is no current event loop in thread 'Thread-385'.

The get_event_loop() method returns the loop bound to the current thread. You can use set_event_loop(loop) to bind the loop to the current thread after creating it with loop = asyncio.new_event_loop().

You can run any loop, even if another one is bound to the thread. That's why in Python 3.6 get_event_loop() works differently within a coroutine. It returns not the loop bound to the thread, but the currently running loop. It's important when a coroutine tries to interact with its loop:

import asyncio
import sys

async def sleep(t, indication_t):
    async def indication():
        while True:
            print('.', end='')
            await asyncio.sleep(indication_t)

    loop = asyncio.get_event_loop()  # <-- here
    task = loop.create_task(indication())
    await asyncio.sleep(t)

loop = asyncio.get_event_loop()
loop.run_until_complete(sleep(5, 0.5))

Even though you can run any loop you can never run two loops in the single thread. Though it's technically possible, it's explicitly forbidden by asyncio:

In : async def run_another():
...:     loop = asyncio.new_event_loop()
...:     loop.run_forever()
In : loop = asyncio.get_event_loop()
In : loop.run_until_complete(run_another())
RuntimeError: Cannot run the event loop while another loop is running