replace_lookahead.py

def solution(template, replacements):
    ans = ''
    i = 0

    error = False
    while i < len(template):
        print(i)
        if template[i] == '{':
            if (i+1) < len(template) and template[i+1] == '{':
                ans += '{'
                i += 2
            else:
                lookahead = i+1
                number = ''
                while lookahead < len(template) and template[lookahead] != '}':
                    char = template[lookahead]
                    error =  char not in set('0123456789')
                    if error:
                        break
                    number += template[lookahead]
                    lookahead += 1

                if template[lookahead] == '}':
                    if int(number) < len(replacements):
                        ans += replacements[int(number)]
                    else:
                        error = True
                
                i = lookahead +1

        else:
            ans += template[i]
            i += 1
        
        if error:
            break

    return '' if error else ans