replace_regex.py

import re


def solution(formatString, replacements):
    formatString = re.sub(r'([^{]){(\d+)}}', r'\g<1>{\g<2>}}}', formatString)
    formatString = re.sub(r'{{(\d+)}([^}])', r'{{\g<1>}}\g<2>', formatString)

    try:
        return formatString.format(*replacements)
    except ValueError:
        return ''

if __name__ == '__main__':
    print(solution("A {{{0}} {2}} with {0} {1}.", ["fun", "activities", "party"]))