#!/usr/bin/python

import sys

### how to create a function
def reverseFile(fname) :
    str = file(fname).read()
    str2 = ''
    for char in str :
        ### warning: this creates a new copy of str2 for each iteration, which
        ### is very inefficient. This is just means as an example of how to use
        ### files and for loops.
        str2 = char + str2

    open(fname, 'w').write(str2)
        
        
    if __name__ == "__main__" :
        if len(sys.argv) < 2 :
            print "Usage: reverser.py fname"
            sys.exit(0)
        else :
            reverseFile(sys.argv[1])