Trouble with popen2
Rembrandt Q Einstein
hercules.rockefeller at springfield.??.us
Mon Sep 27 21:22:25 CEST 2004
Rembrandt Q Einstein wrote:
> I am running an external command and I need to know a) when it is done
> and b) what it wrote to both stdout and stderr. After a little
> searching, I found the popen2 module and used the Popen3 class. I'm
> having trouble with it hanging, though.
> Here is a very well put (by someone else) posting that describes some
> I came to a similar conclusion as that poster and his workaround
> (independently discovered by me) does do the job he requires. However,
> I need to also read stderr, so I made this sample writer and runner:
> import sys
> for line in range(0, int(sys.argv)):
> print "X" * 120
> print >>sys.stderr, "hi"
> import popen2
> f = popen2.Popen3("./writer.py 50000", True)
> outs = 
> errs = 
> while (f.poll() == -1):
> errs += f.childerr.readlines()
> outs += f.fromchild.readlines()
> This hangs in the childerr.readlines(). Is it blocking? If so, why? In
> any case, how can I be sure to read ALL data from BOTH stderr and stdout
> and not be in danger of hanging?
> PS: I just found that if I swap the order of the readlines() statements,
> it works. But I don't want to use that until I understand why. I
> suspect it's a race condition and I don't want to rely on that.
It's possible that when I have child.readlines() first, it consumes all
50000 output lines and then err.readlines() grabs the "hi" and it just
exists. When I have them the other way, err.readlines() blocks (I
thought readlines() was non-blocking, though) and it just hangs.
My real external program isn't nice enough to order the output like
that, so here's a more realistic writer:
for i in range(0, int(sys.argv)):
if math.fmod(i, 100) == 0:
print >>sys.stderr, "hi"
print >>sys.stdout, "X" * 120
The challenge is to write a program that will run this one in a
subprocess and read all of both stderr and stdout.
More information about the Python-list