cat.py

#!/usr/bin/env python

from itertools import imap, chain
from sys import stdin, stdout

def iterfile(filename):
    fo = stdin if filename == "-" else open(filename)
    return iter(fo)

def main(argv=None):
    import sys
    from argparse import ArgumentParser

    argv = argv or sys.argv

    parser = ArgumentParser(
        description="Concatenate FILE(s),to standard output.")
    parser.add_argument("files", nargs="*", help="FILE(s) to print",
                        metavar="FILE")
    parser.add_argument("-n", "--number", help="number all output lines",
                        action="store_true", default=False)
    args = parser.parse_args(argv[1:])

    template = "{0:3d} {1}" if args.number else "{1}"
    for lnum, line in enumerate(chain(*imap(iterfile, args.files or ["-"])), 1):
        stdout.write(template.format(lnum, line))

if __name__ == "__main__":
    main()