How to mock async functions with unittest

Question

I want to test a function like this, with pytest:

def asdf():
    a = first_function()
    b = second_function()

where a has a block like this:

result_queue_first = queue.Queue()
result_queue_second = queue.Queue()
first_thread = threading.Thread(target=myfunction1, args=(result_queue_first))
first_thread.start()
second_thread = threading.Thread(target=myfunction2, args=(result_queue_second))
second_thread.start()
first_thread.join()
result_first = result_queue_first.get()
second_thread.join()
result_second = result_queue_second.get()

and b has inside a function that calculates something and assigns it to a variable

When I try to mock asdf using @patch

@patch(path.to.function.asdf)
def test_function(example):
    example.return_value = 'value'
    <sentences of the test>

I get this error

TypeError: 'coroutine' object does not support item assignment

The problem has to be with the threading part, but this works consistently fine when testing it locally. What can be happening?