[Python-Dev] Draft PEP to make file objects support non-blocking mode.

Greg Ewing greg.ewing at canterbury.ac.nz
Tue Mar 22 01:49:23 CET 2005

Donovan Baarda wrote:

> Consider the following. This is pretty much the only way you can use
> popen2 reliably without knowing specific behaviours of the executed
> command;
 > ...
>   fcntl.fcntl(child_in, fcntl.F_SETFL, flags | os.O_NONBLOCK) # \
> ...			      # /
>   fcntl.fcntl(child_out, fcntl.F_SETFL, flags | os.O_NONBLOCK)# \

I still don't believe you need to make these non-blocking.
When select() returns a fd for reading/writing, it's telling
you that the next os.read/os.write call on it will not block.
Making the fd non-blocking as well is unnecessary and perhaps
even undesirable.

> For 1) and 2), note that popen2 returns file objects, but as they cannot
> be reliably used as file objects, we ignore them and grab their
> fileno(). Why does popen2 return file objects if they cannot reliably be
> used?

I would go along with giving file objects alternative read/write
methods which behave more like os.read/os.write, maybe called
something like readsome() and writesome(). That would eliminate
the need to extract and manipulate the fds, and might make it
possible to do some of this stuff in a more platform-independent

Greg Ewing, Computer Science Dept, +--------------------------------------+
University of Canterbury,	   | A citizen of NewZealandCorp, a	  |
Christchurch, New Zealand	   | wholly-owned subsidiary of USA Inc.  |
greg.ewing at canterbury.ac.nz	   +--------------------------------------+

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