Convert the old style string formatting of Python into the new style one.

convert_string_template.py

import re


def convert_string_template(string):
    index = -1
    def repl(matched):
        nonlocal index
        keyword = matched.group(1)
        if keyword:
            return "{%s}" % keyword.strip('()')
        else:
            index += 1
            return "{%d}" % index
    return re.sub(r'(?:%(\(\w+\))?[diouxXeEfFgGcrs])', repl, string)


assert convert_string_template("%s %d %(a)s %(b)d %d") == '{0} {1} {a} {b} {2}'