Python: re-creating async in 10 lines using generators

In this article, Lucas reimplements asynchronous programming in Python using generators as a learning exercise. Using generators and threads, Lucas mimics the way promises work in JavaScript: when you call a generator function (or when you create a generator expression), you don't get the results right away. You get an object (the generator) that knows how to compute the result.

This is similar to a promise (in JS and other languages). You get a promise that you'll get a result back. With the generator, you also have this “promise” that you'll get the result, you just need to call next on the generator.

The article is fairly short and straightforward and I just wanted to propose a different implementation that leans on generator functions instead of short-circuiting and generator expression “hacks”, so I rewore the final code in the article to both test my understanding and check if I could clean those “hacks”. On a second look, I realised I could write a decorator that turns any “normal” function into an “async” function of this generator-plus-threads flavour:

from threading import Thread

def into_async(callable):
    def async_gen_function(*args, **kwargs):
        return_value = []
        thread = Thread(target=lambda: return_value.append(callable(*args, **kwargs)))
        thread.start()

        def gen():
            thread.join()
            yield return_value[0]

        return gen()

    return async_gen_function

from threading import Thread

def into_async(callable):
    def async_gen_function(*args, **kwargs):
        return_value = []
        thread = Thread(target=lambda: return_value.append(callable(*args, **kwargs)))
        thread.start()

        def gen():
            thread.join()
            yield return_value[0]

        return gen()

    return async_gen_function

@into_async
def async_lines(path):
    print("Reading file", path)
    with open(path) as f:
        return_value = list(f)
    print("Finished reading", path)

    return return_value

# NOTE: See the original link for these two files.
# (long1 has ints from range(50_000_000) and long2 has
# ints from range(0, 100_000_000, 2).)
async_lines1 = async_lines('long1.txt')
async_lines2 = async_lines('long2.txt')

def result_gen():
    lines1 = next(async_lines1)
    lines2 = next(async_lines2)
    yield int(lines1[-2]) + int(lines2[-2])

result = result_gen()

print("Result generator created.")
print(next(result))