EveryThirdLine.py

#!/usr/bin/python

def extract_third_lines():
    try:
        filename = raw_input("Enter name of file to read: ")
        with open(filename) as f:
            lines = f.readlines()
            my_str = ""
            for line in lines[2::3]:
                my_str += line

        return my_str # extracts string made from every 3rd line
    except IOError:
        return "Error: can't find file or read data."
    else:
        f.close()

def main():
    print extract_third_lines()

main()