[Tutor] os.popen() questions (original) (raw)

Brian van den Broek bvande at po-box.mcgill.ca
Sat Jul 24 19:43:02 CEST 2004

Magnus Lyckå said unto the world upon 24/07/2004 06:42:

At 19:19 2004-07-23 -0400, Brian van den Broek wrote:

import os theprogram = r'C:\clamav-devel\bin\clamscan.exe' os.chdir('C:/testdir') a = os.popen('dir .').readlines() # works fine os.system('start %s > testing.txt' %theprogram) b = os.popen(theprogram).readlines() print b # prints as [] print a

Are you sure that clamscan.exe writes to stdout? Maybe it actually writes to stderr? I don't think popen.readlines will catch that (but DOS ">" will). You might want to use popen3 or popen4 instead. -- Magnus Lycka (It's really Lyckå), magnus at thinkware.se Thinkware AB, Sweden, www.thinkware.se I code Python ~ The Agile Programming Language

Thanks Magnus (and Alan for a similar response),

that did it. The clamscan.exe program did indeed write to stderr rather than stdout. (The 50+ pdf doc with the program said so -- I don't know if I missed it, or, being new to this area, missed its significance :-[ )

I've a follow-up, if you don't mind.

I looked at Python in a Nutshell to see how to apply the advice I was given. There I found a brief discussion of the difference between the flavours of popen. Martelli says:

popen2 is simpler to use than popen3 when it's okay for cmd's standard error to go to the same destination as your own process's standard error, and popen4 is simpler when it's okay for cmd's standard error and output to be mixed with each other.

What determines whether these things are "okay"? Is it highly program dependant, or is there some generally applicable criteria for when this is fine to do?

Thanks,

Brian vdB

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