For a demo in class.

mangle.py

import sys
import re

# Note: In a language without types, it is good practice to
# annotate your functions with a type contract. For example,
# "find_matches" takes two strings and has no return value. 
# Writing a purpose statement indicates you know what you are trying
# to do, and makes it easier for other people to read your code.

# CONTRACT
# find_matches : string string -> void
# PURPOSE
# Consumes two strings, and has no return value. 
# Takes a regular expression pattern and a filename.
# find_matches goes through every line of the file, looking for
# matches, and prints out lines matching the pattern provided.
def find_matches (pattern, file):
  f = open (file, 'r')
  for line in f:
    if re.match(pattern, line):
      print line
  f.close()

# CONTRACT
# main : void -> void
# PURPOSE
# Run when the script is executed
def main():
  if len(sys.argv) < 3:
    print "I need a file to mangle!"
    sys.exit(-1)

  pat  = sys.argv[1]
  file = sys.argv[2]

  print "Searching in the file %s for the word %s" % (file, pat)
  find_matches(pat, file)

# This is the "Python-y" way to indicate which function
# to run first.
if __name__ == "__main__":
    main()