Use it with

linecat.py

#!/usr/bin/env python
"""
Use it as::

    ls -l | python linecat.py
    
or::

    python linecat.py < file.txt
    
"""
import fileinput

input = fileinput.input()

for i, line in enumerate(input, 1):
    print "{:>3} | {}".format(i, line.rstrip('\n'))