replace_state_machine.py

class State:
    def __init__(self, state):
        self.state = state

    def transistion(self, inp):
        pass


class Start(State):
    def transistion(self, string, cursor):
        inp = string[cursor]
        if inp == '{':
            return OneOpenBracket(self.state), '', cursor+1
        return Character(self.state), inp, cursor+1


class Character(State):
    def transistion(self, string, cursor):
        inp = string[cursor]
        if inp == '{':
            return OneOpenBracket(self.state), '', cursor+1
        else:
            return Character(self.state), inp, cursor+1


class OneOpenBracket(State):
    def transistion(self, string, cursor):
        inp = string[cursor]
        if inp == '{':
            return Character(self.state), '{', cursor+1
        if inp in '0123456789':
            number = ''
            while cursor < len(string):
                inp = string[cursor]
                if inp in '0123456789':
                    number += inp
                else:
                    break
                cursor += 1
            if cursor < len(string) and string[cursor] == '}':
                return EndBracket(self.state), self.state['replacements'][int(number)], cursor
        else:
            return Error(self.state), '', cursor


class PlaceHolder(State):
    def transistion(self, string, cursor):
        number = ''
        while cursor < len(string):
            inp = string[cursor]
            if inp in '0123456789':
                number += inp
            else:
                break
            cursor += 1
        if cursor < len(string) and string[cursor] == '}':
            return EndBracket(self.state), self.state['replacements'][int(number)], cursor
        else:
            return Error(self.state), '', cursor


class EndBracket(State):
    def transistion(self, string, cursor):
        inp = string[cursor]
        if inp == '{':
            return OneOpenBracket(self.state), '', cursor+1
        else:
            return Character(self.state), '', cursor+1


class Error(State):
    def transistion(self, string, cursor):
        return self


def solution(formatString, replacements):
    state = Start({'replacements': replacements})
    ans = ''
    cursor = 0
    for letter in formatString:
        try:
            state, output, cursor = state.transistion(formatString, cursor)
        except Exception:
            return ''
        ans += output
    if isinstance(state, Error):
        return ''
    if isinstance(state, PlaceHolder):
        return ''
    if isinstance(state, OneOpenBracket):
        return ''
    return ans


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