plural6.py

import re

def rules(language):
    for line in file('rules.%s' % language):
        pattern, search, replace = line.split()
        yield lambda word: re.search(pattern, word) and re.sub(search,         replace, word)

def plural(noun, language='en'):
    for rule in rules(language):
        result = rule(noun)
        if result: return result

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

for line in file(...) is a common idiom for reading lines from a file, one line at a time. It
works because file actually returns a generator whose next() method returns the next line of the
file.

No magic here. Remember that the lines of the rules file have three values separated by whitespace, so
line.split() returns a tuple of 3 values, and you assign those values to 3 local variables.
 
And then you yield. What do you yield? A function, built dynamically with lambda, that is actually a
closure (it uses the local variables pattern, search, and replace as constants). In other words,
rules is a generator that spits out rule functions.

Since rules is a generator, you can use it directly in a for loop. The first time through the for
loop, you will call the rules function, which will open the rules file, read the first line out of it,
dynamically build a function that matches and applies the first rule defined in the rules file, and yields
the dynamically built function. The second time through the for loop, you will pick up where you left
off in rules (which was in the middle of the for line in file(...) loop), read the second
line of the rules file, dynamically build another function that matches and applies the second rule
defined in the rules file, and yields it. And so forth.


In stage 5, you read the entire rules file and built a list of all the possible rules
before you even tried the first one. Now with generators, you can do everything lazily: you open the first and read the
first rule and create a function to try it, but if that works you don't ever read the rest of the file or create any other
functions.