Need feedback on subprocess-using function

Nobody nobody at
Mon Oct 5 03:46:40 CEST 2009

On Sat, 03 Oct 2009 13:21:00 +0000, gb345 wrote:

> I'm relatively new to Python, and I'm trying to get the hang of
> using Python's subprocess module.  As an exercise, I wrote the Tac
> class below, which can prints output to a file "in reverse order",
> by piping it through the Unix tac utility.  (The idea is to delegate
> the problem of managing the memory for an arbitrarily large task
> to tac.)

>         self.pipe = subprocess.Popen(['tac'], stdout=out,
>                                      stdin=subprocess.PIPE,
>                                      stderr=subprocess.PIPE)

> This works OK, as far as I can tell, but I'm not sure that I've
> dotted all the i's and crossed all the t's...  E.g., I had to add
> the line "p.stdin.close()" to the close method when I when I ran
> into sporadic deadlock at the statement.  Are there
> other similar problems lurking in this code? 

Yep. If the process writes more than a buffer-full of data to stderr, it
will deadlock. tac will block trying to write to stderr, and won't be
reading its stdin, so your program will block trying to write to tac.

This is why the POSIX popen() call only lets you attach a pipe to stdin or
stdout but not both.

If you want a "double-ended" slave process, you need to use polling or
non-blocking I/O or asynchronous I/O or multiple threads. I'm not aware of
any single solution which works on all platforms.

The easiest way around this problem is to redirect stderr to a temporary
file and read in the file's contents in the close() method.

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