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Recipe 9.11. Driving an External Process with popen
Credit: Sébastien Keim, Tino Lange, Noah
Spurrier
Problem
You want to drive an external
process that accepts commands from its standard input, and you
don't care about the responses (if any) that the
external process may emit on its standard output.
Solution
If you need to drive only the other process' input
and don't care about its output, the simple
os.popen function is enough. For example, here is
a way to do animated graphics by driving the free program
gnuplot via os.popen:
import os
f = os.popen('gnuplot', 'w')
print >>f, "set yrange[-300:+300]"
for n in range(300):
print >>f, "plot %i*cos(x)+%i*log(x+10)" % (n, 150-n)
f.flush( )
f.close( )
Discussion
When you want to use Python as a glue language, sometimes (in
particularly easy cases) the simple function popen
(from the standard library module os) may be all
you need. Specifically, os.popen may suffice when
you need to drive an external program that accepts commands on its
standard input, as long as you can ignore any response that the
program might be making on its standard output (and also error
messages that the program might be sending to its standard error). A
good example is given by the free plotting program
gnuplot. (os.popen may also
suffice when you need to obtain the output from a program that does
not need to read its standard input.)The statement f = os.popen('gnuplot', 'w') creates
a file-like object connected to the standard input of the program it
launches, namely 'gnuplot'. (To try this recipe,
you have to have gnuplot installed on your
PATH, but since gnuplot is
freely available and widely ported software, that should not be a
problem!) Whatever we write to f, the
external process receives on its standard input, just as would happen
if we used that same program interactively. For more of the same,
check out http://sourceforge.net/projects/gnuplot-py/:
it's a rich and interesting Python interface to
gnuplot implemented entirely on the basis of the
simple idea shown in this recipe!When your needs are more sophisticated than
os.popen can accommodate, you may want to look at
os.popen2 and other such higher-numbered functions
in module os, or, in Python 2.4, the new standard
library module subprocess. However, in many cases,
you're likely to be disappointed: as soon as you get
beyond the basics, driving (from your own programs) other external
programs that were designed to be used interactively can become more
than a little frustrating. Fortunately, a solution is at hand:
it's pexpect, a third-party
Python module that you can find at http://pexpect.sourceforge.net/.
pexpect is designed specifically for the task of
driving other programs, and it lets you check on the other
program's responses as well as sending commands to
the other program's standard input. Still, while
pexpect will most definitely offer you all the
power you need, os.popen will probably suffice
when you don't need anything fancy!
See Also
Module os (specifically
os.popen) in the Library
Reference and Python in a
Nutshell; gnuplot is at http://www.gnuplot.info/;
gnuplot.py is at http://sourceforge.net/projects/gnuplot-py/;
pexpect is at http://pexpect.sourceforge.net/.
