[Python-Dev] PEP 3145 (With Contents)

Glyph glyph at twistedmatrix.com
Sun Dec 9 05:14:47 CET 2012

On Dec 7, 2012, at 5:10 PM, anatoly techtonik <techtonik at 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.

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.

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 ;-).

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