Watch stdout and stderr of a subprocess simultaneously

Question

How can I watch standard output and standard error of a long-running subprocess simultaneously, processing each line as soon as it is generated by the subprocess?

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Answer 1

To accomplish this, you need a function that will take two async sequences and merge them, producing the results from either one or the other, as they become available. With such a function in stock, run could look like this:

async def run(cmd):
    p = await asyncio.create_subprocess_shell(cmd, stdout=PIPE, stderr=PIPE)
    async for f in merge(p.stdout, p.stderr):
        print(datetime.now(), f.decode().strip())

A function like merge does not (yet) exist in the standard library, but the aiostream external library provides one. You can also write your own using an async generator and asyncio.wait():

async def merge(*iterables):
    iter_next = {it.__aiter__(): None for it in iterables}
    while iter_next:
        for it, it_next in iter_next.items():
            if it_next is None:
                fut = asyncio.ensure_future(it.__anext__())
                fut._orig_iter = it
                iter_next[it] = fut
        done, _ = await asyncio.wait(iter_next.values(),
                                     return_when=asyncio.FIRST_COMPLETED)
        for fut in done:
            iter_next[fut._orig_iter] = None
            try:
                ret = fut.result()
            except StopAsyncIteration:
                del iter_next[fut._orig_iter]
                continue
            yield ret

The above run will still differ from your desired output in one detail: it will not distinguish between output and error lines. But this can be easily accomplished by decorating the lines with an indicator:

async def decorate_with(it, prefix):
    async for item in it:
        yield prefix, item

async def run(cmd):
    p = await asyncio.create_subprocess_shell(cmd, stdout=PIPE, stderr=PIPE)
    async for is_out, line in merge(decorate_with(p.stdout, True),
                                    decorate_with(p.stderr, False)):
        if is_out:
            print(datetime.now(), line.decode().strip())
        else:
            print(datetime.now(), "E:", line.decode().strip())