#---------- Typical usage of sys.stderr and sys.stdout----------#
% cat cap_file.py
#!/usr/bin/env python
import sys, string
if len(sys.argv) < 2:
    sys.stderr.write("No filename specified\n")
else:
    fname = sys.argv[1]
    try:
        input = open(fname).read()
        sys.stdout.write(string.upper(input))
    except:
        sys.stderr.write("Could not read '%s'\n" % fname)
% ./cap_file.py this > CAPS
% ./cap_file.py nosuchfile > CAPS
Could not read 'nosuchfile'
% ./cap_file.py > CAPS
No filename specified