'内存错误'和缩短代码

Question

我正在编写一个python Vigenere密码，我遇到了一个错误“内存错误”。我不确定它是什么以及它为什么会发生，并且解密代码与加密代码相同（它确实可以加密但不能解密）。

playing = True
string = ""
Alphabet = ('z','a','b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j', 'k', 'l', 'm', 'n', 'o', 'p', 'q', 'r', 's', 't', 'u', 'v', 'w', 'x', 'y', 'z')

def EncryptVigenere():
        plainText = input("Please enter the plain text: ")
        key = input("Please enter the key: ")
        keyList = []
        keyLength = 0
        num = 0
        while keyLength < len(plainText):
            try:
                char = key[num]
            except IndexError:
                num = 0
                char = key[num]
            keyList.append(str(char))
            keyLength += 1
            num+= 1
            CipherText = [] 
            IndexValue = 0
            keyIncrement = 0
        for plainTextChar in plainText:
             IndexValue = Alphabet.index(keyList[keyIncrement]) + Alphabet.index(plainTextChar)
             while IndexValue > 26:
                 IndexValue = IndexValue - 26
             CipherText.append(Alphabet[IndexValue])
             keyIncrement = keyIncrement + 1
             print (''.join(CipherText))

        finish = input('Would you like to go again Y or N')
        if finish == 'n' or finish == 'N':
            retry = input ("Would you like to go again? Y or N: ")
            if retry == 'N' or retry == 'n':
                print ("Please exit the window")
                import time
                time.sleep(1)
                import sys
                sys.exit()


def DecryptVigenere():             
            plainText = input("Please enter the plain text: ")
            key = input("Please enter the key: ")
            keyList = []
            keyLength = 0
            num = 0
            while keyLength < len(plainText):
                try:
                    char = key[num]
                except IndexError:
                    num = 0
                    char = key[num]
                keyList.append(str(char))
                keyLength -= 1
                num -= 1
                CipherText = [] 
                IndexValue = 0
                keyIncrement = 0
            for plainTextChar in plainText:
                 IndexValue = Alphabet.index(keyList[keyIncrement]) + Alphabet.index(plainTextChar)
                 while IndexValue > 26:
                     IndexValue = IndexValue - 26
                 CipherText.append(Alphabet[IndexValue])
                 keyIncrement = keyIncrement - 1
                 print (''.join(CipherText))

                 finish = input('Would you like to go again Y or N')
                 if finish == 'n' or finish == 'N':
                     retry = input ("Would you like to go again? Y or N: ")
                     if retry == 'N' or retry == 'n':
                         print ("Please exit the window")
                         import time
                         time.sleep(1)
                         import sys
                         sys.exit()


while playing == True:
    string = ""
    eord = input('Type "d" to "decrypt" and "e" to "encrypt": ')

    if eord == 'e':
        texte = input ("Type your word to encrypt: ")
        key1 = int(input("Choose a key between 1-26: "))
        for letter in texte:
            number = (ord(letter)) + (key1)
            letter=(chr(number))
            string = (str(string)) + (str(letter))
        print (string)
        keyword = input ("Type 'encrypt' code further or 'decrypt' further: ")

        if keyword == 'encrypt' or keyword == 'e':
            EncryptVigenere()

        elif keyword == 'decrypt' or keyword == 'd':
            DecryptVigenere()   


    elif eord == 'd':
        texd = input ("Type your word to decrypt: ")
        key2 = int(input("Choose a key between 1-16: "))        
        for letter in texd:
            number = (ord(letter)) - (key2)
            letter=(chr(number))
            string = (str(string)) + (str(letter))
        print (string)
        keyword = input ("Type 'encrypt' code further or 'decrypt' further: ")

        if keyword == 'decrypt' or keyword == 'd':
            DecryptVigenere()

        elif keyword == 'encrypt' or keyword == 'e':
            EncryptVigenere()    

以下是运行代码后出现的错误：

Type "d" to "decrypt" and "e" to "encrypt": e
Type your word to encrypt: hello
Choose a key between 1-26: 3
khoor
Type 'encrypt' code further or 'decrypt' further: e
Please enter the plain text: darrien
Please enter the key: brit
f
fs
fsa
fsal
fsalk
fsalkw
fsalkww
Would you like to go again Y or Ny
Type "d" to "decrypt" and "e" to "encrypt": d
Type your word to decrypt: khoor
Choose a key between 1-16: 3
hello
Type 'encrypt' code further or 'decrypt' further: d
Please enter the plain text: fsalkww
Please enter the key: brit
Traceback (most recent call last):
  File "D:\##Python Cipher##2.py", line 111, in <module>
    DecryptVigenere()
  File "D:\##Python Cipher##2.py", line 54, in DecryptVigenere
    keyList.append(str(char))
MemoryError

有谁知道导致错误的原因是什么？ 还有办法缩短代码吗？ 感谢。