Tests basic assumptions about running event loops.

get_event_loop_tests.py

"""
Tests basic assumptions about running event loops.

Gets and compares the result of `get_event_loop` with `get_running_loop` before
and after setting it to None.

A more strict situation than in the documentation holds (for Python 3.7.3),
given default event loop policy:
`get_event_loop` returns the same result after setting the event loop to None,
when ran from a coroutine.

Outside coroutines it raises an RuntimeError
"""
import asyncio


async def test_async() -> None:
    loop_1 = asyncio.get_event_loop()

    asyncio.set_event_loop(None)

    running_loop = asyncio.get_running_loop()
    assert loop_1 == running_loop

    asyncio.set_event_loop(None)
    loop_2 = asyncio.get_event_loop()

    assert loop_1 == loop_2 == running_loop
    print("Done, all passed.")

    # restore
    asyncio.set_event_loop(loop_1)


def test_non_async() -> None:
    loop_1 = asyncio.get_event_loop()

    assert loop_1 is not None
    print("There is a default event loop")

    print("And there is no running loop:")
    try:
        asyncio.get_running_loop()
        print("Unreachable")
    except RuntimeError as err:
        print("No running loop outside coroutine, exception:", err)

    asyncio.set_event_loop(None)
    print("After `set_event_loop(None)`, getting the loop raises a "
          "RuntimeError when not in a coroutine")
    try:
        asyncio.get_event_loop()
        print("Unreachable")
    except RuntimeError as err:
        print("RuntimeError:", err)

    # restore
    asyncio.set_event_loop(loop_1)


LOOP = asyncio.get_event_loop()
LOOP.run_until_complete(test_async())

test_non_async()