Even loop creation in Asyncio of Python

Question

In asyncio library of Python, which method(s) create the event loop? Is it asyncio.run() / asyncio.get_event_loop() ? Also which method(s) start running the event loop? I tried searching; but it seems there are not very clear answers.

Answer 1

TL;DR

For high-level API, it's creating automatically in asyncio.run() by asyncio.new_event_loop() or by a loop_factory function if provided. In low-level API, it's asyncio.new_event_loop(). asyncio.get_event_loop() returning a running event loop or, if didn't run, policy's event loop (asyncio.get_event_loop_policy().get_event_loop()). It doesn't create a new event loop.

Structure of asyncio

In asyncio, we have a high-level and low-level APIs. High-level APIs fit for situations when we need to run a simple application with 3 or 5 asynchronous functions. In turn, Low-level APIs needs to control a process of creating event loops, managing Future's instance properties (like name, running status etc.), setting of policies and so on.

Creation of a new event loop

• asyncio.run() by asyncio.new_event_loop() or by a loop_factory function if provided
• asyncio.new_event_loop() in low-level API

How they're working?

asyncio.run() makes four things:

1. Creating a new event loop by functions above
2. Setting a new event loop in current thread by asyncio.set_event_loop()
3. Putting an asynchronous function to the event loop by asyncio.loop.create_task() where loop is asyncio.SelectorEventLoop or asyncio.ProactorEventLoop, depends from the system
4. Starting an event loop by asyncio.run_until_complete()

asyncio.new_event_loop() makes first clause, so you need to manually set a new event loop like in second clause and run

Starting an event loop

How mentioned above, asyncio.run() set a new event loop automatically. For low-level APIs, we can use:

• asyncio.loop.run_until_complete()
• asyncio.loop.run_forever()

How it's clearly from names, first will be running to the last finished task, and second will be running forever

Examples

In all examples, code will be like this:

import asyncio
...
...
...
if __name__ == "__main__":
    main()

where dots are rest of our code

Here's equivalent examples on the high-level API:

async def get_data():
    # Emulating a fetch from a remote server
    data = [
        {"name": "Jane", "age": 19},
        {"name": "John", "age": 24}
    ]
    await asyncio.sleep(1)
    return data


def main():
    print(asyncio.run(get_data()))

and low-level API:

def init_loop():
    loop = asyncio.new_event_loop()
    asyncio.set_event_loop(loop)
    return loop


async def get_data():
    # Emulating a fetch from a remote server
    data = [
        {"name": "Jane", "age": 19},
        {"name": "John", "age": 24}
    ]
    await asyncio.sleep(1)
    return data


def main():
    loop = init_loop()
    task = loop.create_task(get_data())
    loop.run_until_complete(task)
    print(task.result())

Result:

[{'name': 'Jane', 'age': 19}, {'name': 'John', 'age': 24}]