On Sun, Dec 9, 2012 at 7:14 AM, Glyph <glyph@twistedmatrix.com> wrote:
On Dec 7, 2012, at 5:10 PM, anatoly techtonik <techtonik@gmail.com> wrote:

What about reading from other file descriptors?  subprocess.Popen allows arbitrary file descriptors to be used.  Is there any provision here for reading and writing non-blocking from or to those?

On Windows it is WriteFile/ReadFile and PeekNamedPipe. On Linux it is select. Of course a test is needed, but why it should not just work?

This is exactly why the provision needs to be made explicitly.

On Windows it is WriteFile and ReadFile and PeekNamedPipe - unless the handle is a socket in which case it needs to be WSARecv.  Or maybe it's some other weird thing - like, maybe a mailslot - and you need to call a different API.

IIRC on Windows there is no socket descriptor that can be used as a file descriptor. Seems reasonable to limit the implementation to standard file descriptors in this platform.

On *nix it really shouldn't be select.  select cannot wait upon a file descriptor whose value is greater than FD_SETSIZE, which means it sets a hard (and small) limit on the number of things that a process which wants to use this facility can be doing.

I didn't know that. Should a note be added to http://docs.python.org/2/library/select ?
I also thought that poll acts like, well, a polling function - eating 100% CPU while looping over inputs over and over checking if there is something to react to.
 
On the other hand, if you hard-code another arbitrary limit like this into the stdlib subprocess module, it will just be another great reason why Twisted's spawnProcess is the best and everyone should use it instead, so be my guest ;-).

spawnProcess requires a reactor. This PEP is an alternative for the proponents of green energy. =)