First, let's create a text file that contains the rules you want. No fancy data structures, just space- (or tab-)delimited strings in three columns. You'll call it rules.en; “en” stands for English. These are the rules for pluralizing English nouns. You could add other rule files for other languages later.

[sxz]$                  $               es
[^aeioudgkprt]h$        $               es
[^aeiou]y$              y$              ies
$                       $               s

Now let's see how you can use this rules file.

Example 17.16. plural5.py

import re
import string                                                                     

def buildRule((pattern, search, replace)):                                        
    return lambda word: re.search(pattern, word) and re.sub(search, replace, word) 

def plural(noun, language='en'):                             
    lines = file('rules.%s' % language).readlines()          
    patterns = map(string.split, lines)                      
    rules = map(buildRule, patterns)                         
    for rule in rules:                                      
        result = rule(noun)                                  
        if result: return result                            

	You're still using the closures technique here (building a function dynamically that uses variables defined outside the function), but now you've combined the separate match and apply functions into one. (The reason for this change will become clear in the next section.) This will let you accomplish the same thing as having two functions, but you'll need to call it differently, as you'll see in a minute.
	Our plural function now takes an optional second parameter, language, which defaults to en.
	You use the language parameter to construct a filename, then open the file and read the contents into a list. If language is en, then you'll open the rules.en file, read the entire thing, break it up by carriage returns, and return a list. Each line of the file will be one element in the list.
	As you saw, each line in the file really has three values, but they're separated by whitespace (tabs or spaces, it makes no difference). Mapping the string.split function onto this list will create a new list where each element is a tuple of three strings. So a line like [sxz]$ $ es will be broken up into the tuple ('[sxz]$', '$', 'es'). This means that patterns will end up as a list of tuples, just like you hard-coded it in stage 4.
	If patterns is a list of tuples, then rules will be a list of the functions created dynamically by each call to buildRule. Calling buildRule(('[sxz]$', '$', 'es')) returns a function that takes a single parameter, word. When this returned function is called, it will execute re.search('[sxz]$', word) and re.sub('$', 'es', word).
	Because you're now building a combined match-and-apply function, you need to call it differently. Just call the function, and if it returns something, then that's the plural; if it returns nothing (None), then the rule didn't match and you need to try another rule.

So the improvement here is that you've completely separated the pluralization rules into an external file. Not only can the file be maintained separately from the code, but you've set up a naming scheme where the same plural function can use different rule files, based on the language parameter.

The downside here is that you're reading that file every time you call the plural function. I thought I could get through this entire book without using the phrase “left as an exercise for the reader”, but here you go: building a caching mechanism for the language-specific rule files that auto-refreshes itself if the rule files change between calls is left as an exercise for the reader. Have fun.