isatty.py

#!/usr/bin/env python 
#
# Short Python example of why `ls | wc -l` works by @iiSeymour
#
# try:
#     python isatty.py
# and:
#     python isatty.py | cat
#
# Explanation http://www.ginac.de/~kreckel/fileno/

import os
import sys

output = ['I', 'am', 'writing', 'to', 'a']

if os.isatty(sys.stdout.fileno()):
    output.append('terminal')
    sep = ' '
else:
    output.append('pipe')
    sep = '\n'

print sep.join(output)