How can I test async click commands from an async pytest function?

Question

I'm trying to test a click command that is async from pytest, but I am hitting the limits of my knowledge of asyncio (or approaching the problem with a wrong architecture)

On one side, I have a click command line, that creates a grpclib channel to hit a grpc api.

import asyncio
from grpclib import Channel
from functools import wraps

def async_cmd(func):
  @wraps(func)
  def wrapper(*args, **kwargs):
    return asyncio.run(func(*args, **kwargs))
  return wrapper

@click.command
@async_cmd
async def main():
  async with Channel('127.0.0.1', 1234) as channel:
    blah = await something(channel)
    do_stuff_with(blah)
  return 0

Now I'm trying to test things using pytest and pytest-asyncio:

from click.testing import CliRunner
from cli import main
from grpclib.testing import ChannelFor
import pytest

@pytest.mark.asyncio
async def test_main()
  async with ChannelFor([Service()]) as test_channel:
    # Plan is to eventually mock grpclib.Channel with test_channel here.
    runner = CliRunner()
    runner.invoke(main)

My issue is that the async_cmd around main expects to call asyncio.run. But by the time the test_main method is called, a loop is already running (launched by pytest).

What should I do?

• Should I modify my wrapper to join an existing loop (and how so?).
• Should I mock something somewhere?
• Should I just change my code do have my main just responsible for parsing the arguments and calling another function?

Answer 1

You are running your own event loop in the async_cmd decorator with this:

asyncio.run(func(*args, **kwargs))

Therefore, it is not apparent that you need to use @pytest.mark.asyncio, I suggest trying your testing without it.

If you need an async context manager for a Mock, you can init the context manager in a hook called via the mock as shown below in test_hook().

Test Code (for the test code)

import asyncio
import click
import functools as ft
import pytest
import time
from unittest import mock
from click.testing import CliRunner


class AsyncContext():

    def __init__(self, delay):
        self.delay = delay

    async def __aenter__(self):
        await asyncio.sleep(self.delay)
        return self.delay

    async def __aexit__(self, exc_type, exc, tb):
        await asyncio.sleep(self.delay)

TestAsyncContext = AsyncContext

def async_cmd(func):
    @ft.wraps(func)
    def wrapper(*args, **kwargs):
        return asyncio.run(func(*args, **kwargs))
    return wrapper


@click.command()
@async_cmd
async def cli():
    async with TestAsyncContext(0.5) as delay:
        await asyncio.sleep(delay)
    print('hello')


@pytest.mark.parametrize('use_mock, min_time, max_time',
                         ((True, 2.5, 3.5), (False, 1.0, 2.0)))
def test_async_cli(use_mock, min_time, max_time):
    def test_hook(delay):
        return AsyncContext(delay + 0.5)

    runner = CliRunner()
    start = time.time()
    if use_mock:
        with mock.patch('test_code.TestAsyncContext', test_hook):
            result = runner.invoke(cli)
    else:
        result = runner.invoke(cli)
    stop = time.time()
    assert result.exit_code == 0
    assert result.stdout == 'hello\n'
    assert min_time < stop - start < max_time

Test Results

============================= test session starts =============================
collecting ... collected 2 items

test_code.py::test_async_cli[True-2.5-3.5] 
test_code.py::test_async_cli[False-1.0-2.0] 

============================== 2 passed in 4.57s ==============================