I just submitted a bug at SF entitled 'os.popen() negative error code IOError'. However, not knowing SF too well, I've messed up the formatting of the test code, so here it is. When a negative return code is received by the os.popen() family, an IOError is raised when the last pipe from the process is closed. The following code demonstrates the problem: import sys import os import traceback import sys import os import traceback if __name__ == '__main__': if len(sys.argv) == 1: try: r = os.popen('%s %s %s' % (sys.executable, sys.argv[0], -1,)) r.close() except IOError: traceback.print_exc() try: w, r = os.popen2('%s %s %s' % (sys.executable, sys.argv[0], -1,)) w.close() r.close() except IOError: traceback.print_exc() try: w, r, e = os.popen3('%s %s %s' % (sys.executable, sys.argv[0], -1,)) w.close() r.close() e.close() except IOError: traceback.print_exc() else: sys.exit(int(sys.argv[1])) ---------- Run ---------- Traceback (most recent call last): File "Q:\Viper\src\webvis\tests\test.py", line 11, in ? r.close() IOError: (0, 'Error') Traceback (most recent call last): File "Q:\Viper\src\webvis\tests\test.py", line 18, in ? r.close() IOError: (0, 'Error') Traceback (most recent call last): File "Q:\Viper\src\webvis\tests\test.py", line 26, in ? e.close() IOError: (0, 'Error') Tim Delaney