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17.4. plural.py, stage 3

17.4. plural.py, stage 3

Defining separate named functions for each match and apply rule isn't really necessary. You never call them directly; you define them in the rules list and call them through there. Let's streamline the rules definition by anonymizing those functions.

Example 17.8. 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)
   )                                           1

def plural(noun):                             
    for matchesRule, applyRule in rules:       2
        if matchesRule(noun):                 
            return applyRule(noun)            
1 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.
2 Note that the plural function hasn't changed at all. It iterates through a set of rule functions, checks the 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.

Now to add a new rule, all you need to do is define the functions directly in the rules list itself: one match rule, and one apply rule. But 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.

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