python regex re.sub with iterable substitution argument

re.itersub.py

import re

def sub_iter(reg: str, subs: Iterable, string: str) -> str:
    '''
    Perform an action equivalent to re.sub chronologically for every 
    match where the string substituted in is an equivalent item in an
    iterable.
    
    reg:      regular expression
    subs:     iterable of chronological sub items. (ideally a generator)
    string:   string to perform subs on. Not modified in place
    '''
    
    matches = re.finditer(reg, string)   #returns match object iterator
    new_s = list(string)
    shift = 0
    for match, sub in zip(matches, subs):
        start_index, stop_index = match.span()
        start = start_index + shift
        stop = stop_index + shift
        new_s[start:stop] = list(str(sub))

        old_len = (stop_index - start_index)
        new_len = len(str(sub))
        len_change = new_len - old_len
        shift += len_change
    return ''.join(new_s)
  
    
    
# example use - replace content between every parenthesis with 
# enumerated number of parenthesis
# () (replace this) not this () (    ) ---> (1) (2) not this (3) (4)
if __name__ == '__main__':
    import itertools
    
    string      = " test () () works () () "
    iterable    = itertools.count(start=1, step=1)
    regex       = r"(?<=\()(.*?)(?=\))"
    
    new_string = sub_iter(regex, iterable, string)
    print(new_string)