run a subprocess with pipes connected to both input and output?

Use the standard popen2 module. For example:

	import popen2
	fromchild, tochild = popen2.popen2("command")
	output = fromchild.readline()
Warning: in general, it is unwise to do this, because you can easily cause a deadlock where your process is blocked waiting for output from the child, while the child is blocked waiting for input from you. This can be caused because the parent expects the child to output more text than it does, or it can be caused by data being stuck in stdio buffers due to lack of flushing. The Python parent can of course explicitly flush the data it sends to the child before it reads any output, but if the child is a naive C program it can easily have been written to never explicitly flush its output, even if it is interactive, since flushing is normally automatic.

Note that a deadlock is also possible if you use popen3 to read stdout and stderr. If one of the two is too large for the internal buffer (increasing the buffersize does not help) and you read() the other one first, there is a deadlock, too.

Note on a bug in popen2: unless your program calls wait() or waitpid(), finished child processes are never removed, and eventually calls to popen2 will fail because of a limit on the number of child processes. Calling os.waitpid with the os.WNOHANG option can prevent this; a good place to insert such a call would be before calling popen2 again.

Another way to produce a deadlock: Call a wait() and there is still more output from the program than what fits into the internal buffers.

In many cases, all you really need is to run some data through a command and get the result back. Unless the data is infinite in size, the easiest (and often the most efficient!) way to do this is to write it to a temporary file and run the command with that temporary file as input. The standard module tempfile exports a function mktemp() which generates unique temporary file names.

 import tempfile
 import os
 class Popen3:
    This is a deadlock-save version of popen, that returns
    an object with errorlevel, out (a string) and err (a string).
    (capturestderr may not work under windows.)
    Example: print Popen3('grep spam','\n\nhere spam\n\n').out
    def __init__(self,command,input=None,capturestderr=None):
        command="( %s ) > %s" % (command,outfile)
        if input:
            command=command+" <"+infile
        if capturestderr:
            command=command+" 2>"+errfile
        self.errorlevel=os.system(command) >> 8
        if input:
        if capturestderr:
Note that many interactive programs (e.g. vi) don't work well with pipes substituted for standard input and output. You will have to use pseudo ttys ("ptys") instead of pipes. There is some undocumented code to use these in the library module -- I'm afraid you're on your own here.

A different answer is a Python interface to Don Libes' "expect" library. A Python extension that interfaces to expect is called "expy" and available from

A pure Python solution that works like expect is pexpect of Noah Spurrier. A beta version is available from