[Python-Dev] subprocess crossplatformness and async communication (original) (raw)

Nick Craig-Wood nick at craig-wood.com
Mon Jan 26 13:15:30 CET 2009

On Sat, Jan 24, 2009 at 07:58:40AM -0800, Guido van Rossum wrote:

I'm confused. The subprocess already allows reading/writing its stdin/stdout/stderr, and AFAIK it's a platform-neutral API. I'm sure there's something missing, but your post doesn't make it clear what exactly, and the recipe you reference is too large to digest easily. Can you explain what it is that the current subprocess does't have beyond saying "async communication" (which could mean many things to many people)?

The main problem with subprocess is that it doesn't work if you want to have a conversation / interact with your child process.

subprocess works very well indeed for this case :-

run child send stuff to stdin child reads stdin and writes stdout child exits read stuff from stdout

But for the conversational case (eg using it to do a remote login) it doesn't work at all :-

run child send stuff to stdin child reads stdin and writes stdout read stuff from stdout send stuff to stdin child reads stdin and writes stdout read stuff from stdout send stuff to stdin child reads stdin and writes stdout read stuff from stdout child exits

In subprocess "read stuff from stdout" means read stdout until the other end closes it, not read what is available and return it, so it blocks on reading the first reply and never returns.

Hence Anatoly's request for "async communication" and the existence of that recipe.

http://code.activestate.com/recipes/440554/

I've spend quite a lot of time explaning this to people on comp.lang.python. I usually suggest either the recipe as suggested by Anatoly or if on unix the pexpect module. There are other solutions I know of, eg in twisted and wxPython.

I heard rumours of a pexpect port to Windows but I don't know how far that has progressed.

A cross platform async subprocess would indeed be a boon!

-- Nick Craig-Wood <nick at craig-wood.com> -- http://www.craig-wood.com/nick

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