plural3.py

import re

rules = \
  (
    (
     lambda word: re.search('[sxz]$', word),
     lambda word: re.sub('$', 'es', word)
     ),
    (
     lambda word: re.search('[^aeioudgkprt]h$', word),
     lambda word: re.sub('$', 'es', word)
    ),
    (
     lambda word: re.search('[^aeiou]y$', word),
     lambda word: re.sub('y$', 'ies', word)
    ),
    (
     lambda word: re.search('$', word),
     lambda word: re.sub('$', 's', word)
    )
  )
   
def plural(noun):
    for matchesRule, applyRule in rules:
        if matchesRule(noun):
            return applyRule(noun)

if __name__ == '__main__':
    import sys
    if sys.argv[1:]:
        print plural(sys.argv[1])
    else:
        print __doc__


This is the same set of rules as you defined in stage 2. The only difference is that instead of defining
named functions like match_sxz and apply_sxz, you have "inlined" those function definitions
directly into the rules list itself, using lambda functions.

Note that the plural function hasn't changed at all. It iterates through a set of rule functions, checksthe first rule, and if it returns a true value, calls the second rule and returns the value. Same as above,
word for word. The only difference is that the rule functions were defined inline, anonymously, using
lambda functions. But the plural function doesn't care how they were defined; it just gets a list of
rules and blindly works through them.

Defining the rule functions inline like this makes it very clear that you have some unnecessary
duplication here. You have four pairs of functions, and they all follow the same pattern. The match function is a single
call to re.search, and the apply function is a single call to re.sub. Let's factor out these similarities.