プログラマ-起動パスワード

import string

def solution(s, n):
    answer = []
    lower = [x for x in string.ascii_lowercase]
    upper = [x for x in string.ascii_uppercase]

    for a in range(len(s)):
        if s[a] == ' ':
            answer.append(' ')
            pass
        else:
            if s[a].isupper(): # 대문자일때
                if upper.index(s[a])+n >= len(upper):
                    temp_index = upper.index(s[a])+n - len(upper)
                else:
                    temp_index = upper.index(s[a])+n
                answer.append(upper[temp_index])
            else : #소문자일때
                if lower.index(s[a])+n >= len(lower):
                    temp_index = lower.index(s[a])+n - len(lower)
                else:
                    temp_index = lower.index(s[a])+n
                answer.append(lower[temp_index])

    return ''.join(answer)