Asyncio testing a function which uses loop.call_soon_threadsafe

Question

I have the following function:

def send_command(self, cmd):
    self.loop.call_soon_threadsafe(
        functools.partial(
            self._transport.write, str(cmd).encode() + b"\n"
        )
    )

The system under test (sut) is a class inheriting from asyncio.Protocol which sends some commands to a piece of hardware on a socket. I have to use threads since this is part of a GUI under wxPython. Finally, if I call self._transport.write the code works fine on Linux but crashes on Windows™.

When running the test:

@pytest.mark.asyncio
async def test_send_command(self):
    self.sut._transport = Mock()
    self.sut.send_command("ook eek")
    assert self.sut._transport.write.called is True

I get an assert error. The self.sut._transport.write is never called. If I call self._transport.write directly in the function, the code crashes on Windows™ but the test passes just fine.

What am I missing here?

Anyone?…

Surely, this is not such an edge case…

Answer 1

A work around…

After reading and experimenting with even loops, using this:

import asyncio
import selectors

selector = selectors.SelectSelector()
loop = asyncio.SelectorEventLoop(selector)
asyncio.set_event_loop(loop)

by passes the problem. Of course, it means using a sub-efficient loop on Windows. ☹ PHA!

Anyone with a better solution is welcome to some fake internet points.