plural-rules.txt

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

plural2.py

#!/usr/bin/env python3
# plural2.py

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)


rules = []

'''
This block only populates the rules list above.
It does not do any work, it just prepares the logic for doing the work.
'''
with open('plural-rules.txt', encoding='ascii') as pattern_file:
    for line in pattern_file:
        pattern, search, replace = line.split(None, 3) 
        rules.append(build_match_and_apply_functions(pattern, search, replace))

'''
This is the method that does the work.
Defining it isn't enough though, you have to call it somewhere.
'''
def plural(noun):
    for matches_rule, apply_rule in rules:
        if matches_rule(noun):
            return apply_rule(noun)

'''
Testing __name__ is important because you only want to do this if
this script was called directly on the command line.
If you were to import this py file as a library like so:
import plural
You would want it to provide a plural method for you to use. You
would not want it to look at sys.argv[1] which probably would not
exist and would raise an exception.
'''
if __name__ == '__main__':
    import sys
    if sys.argv[1:]:
        print(plural(sys.argv[1]))
    else:
        print(__doc__)

transcript of run

$ python3 plural2.py city
cities