import fileinput


def replace_text_in_file_demo():
    """
    1. The original file is moved to a backup file
    2. The standard output is redirected to the original file within the loop
    3. Thus any print statements write back into the original file
    """
    for line in fileinput.input('foo.txt', inplace=True, backup='.bak'):
        print line.replace("foo", "bar"),