import re def build_match_and_apply_functions(pattern, search, replace): def matches_rule(word): return re.search(pattern, word) def apply_rule(word): return re.sub(search, replace, word) return (matches_rule, apply_rule) def rules(rules_filename): with open(rules_filename, encoding='utf-8') as pattern_file: for line in pattern_file: pattern, search, replace = line.split(None, 3) yield build_match_and_apply_functions(pattern, search, replace) def plural(noun, rules_filename='plural5-rules.txt'): for matches_rule, apply_rule in rules(rules_filename): if matches_rule(noun): return apply_rule(noun) raise ValueError('no matching rule for {0}'.format(noun)) print(plural('vacancy'))