我不断收到TypeError：+不支持的操作数类型：“ int”和“ str”

Question

我一直遇到相同的错误。错误不断出现，不用做什么。我正在尝试制作一个密码系统。代码运行，但错误不断出现。我一遍又一遍。

 message = raw_input("text: ")
 crypt = ' '
 letters = 'abcdefghijklmnopqrstuvwxyz0123456789;:[]'

  key = len(letters)/2 
  for char in message:
if char in letters:
    ans = letters.find(char)

    ans = ans + key

    if ans >= len(letters):
        ans = ans - len(letters)

    elif ans < 0:
        ans = ans + len(letters)

    crypt = crypt + letters[ans]

    else:
    crypt = crypt + char
    print("Encryption: ").format(crypt)    
    decrypt = crypt 

    word = "Not found"

    for ans in range(len(letters)):
    dec = ''

    for Al in crypt:
    if Al in letters:
        dec = letters.find(Al)
        dec = dec - key
        if dec < 0:
            dec = len(letters) + dec

            dec = dec + letters [dec]   
        else:
            dec = dec , Al       

    print ('Answers {} {}'.format(key, dec)) 

print("decryption: ")