replace_lookahead_refactor.py

def solution(template, replacements):
    ans = ''
    i = 0
    iterator = TemplateIterator(template)
    error = False
    while template.hasnext():
        if is_literal_brace(template):
            ans += '{'
            template.advance(2)
        elif is_in_brace(template):
            number, error = template.advance_past('}')
            replacement, error = get_replacement(replacements, number)
            if error:
                return ''
            ans += replacement
        else:
            ans += template.current()
            template.advance(1)

    return '' if error else ans