Transforma palavras no plural para palavras no singular.

singularize.py

import re

def rule(match, replace):
    return (re.compile("^([a-zA-z]*){}$".format(match)), re.compile("{}$".format(match)), replace)

def singularize(word):
    rules = [        
        rule("as", "a"),
        rule("es", "e"),
        rule("is", "i"),
        rule("os", "o"),
        rule("us", "u"),
        
        rule("res", "r"),
        rule("zes", "z"),
        
        rule("is" , "il"),
        
        rule("ais", "al"),
        rule("eis", "el"),
        rule("ois", "ol"),
        rule("uis", "ul"),
        
        rule("ns", "m"),
        
        rule("oes", "ao"),
        rule("aes", "ao"),
        rule("aos", "ao")   
    ]
    
    rules.reverse()
    
    for r in rules:
        if r[0].match(word):
            return r[1].sub(r[2], word)

    return word