Hailstone sequence generator with a usage message

Here, we try to read the integer from the command line argument as a string at sys.argv[1]. If none was supplied by the user, this attempt will raise an IndexError (since sys.argv would be a list containing only the program name at sys.argv[0]); if the user supplied a command line argument but not one which can be cast to an integer, the line n = int(sys.argv[1]) will raise a ValueError. We also raise our own ValueError exception if the argument is an integer less than 1.

import sys

try:
    n = int(sys.argv[1])
    if n < 1:
        raise ValueError
except (IndexError, ValueError):
    print('Please enter an integer greater than 0 on the command line.\n'
          'For example, python {:s} 8'.format(sys.argv[0]))
    sys.exit(1)

while True:
    print(n, end=' ')
    if n == 1:
        break
    if n % 2:
        n = 3 * n + 1
    else:
        n = n // 2
print()