如果我在程序中输入5却没有其他字母或数字，为什么我的程序将s和c错位？

Question

This program is meant to ask you for a sentence and a number then it shifts the letters down the alphabet all by the inputted number and then lets you undo it by shift it by minus what you enter. For some reason when you enter 5 as your shift the letter s shift to different random letters and does not give you the correct word when you try and shift back and I have no idea why.

import sys
import time
letters = ["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"]
(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) = (1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14,
                                            15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26) 
def program():

    def encryption():

        def encryption1():
            global message
            global shift
            message = list ((input ("Please enter the sentence you would like to be %s\n>" % (EnDe1))).lower())
            print ("To %s your message please %s your private key number (from 1 - 10)" % (EnDe2, EnDe3))
            shift = int (input (">"))
            if EnDe == "b":
                shift = - (shift)
            if shift < 11 or shift > 0:
                for x in range(len(message)):        
                    if message[x] != " ":
                        if eval(message[x]) > 26 - shift:
                            message[x] = letters[eval(message[x]) + shift - 27]
                        else:
                            message[x] = letters[eval(message[x]) + shift - 1]
            else:
                shift = int (input ("only numbers from 1 to 10 are accepted, try again\n>"))
                encryption1()

        def choice():
            global EnDe
            global EnDe1
            global EnDe2
            global EnDe3
            EnDe = (input ("would you like to A)encrypt or B)decrypt\n>")).lower()
            if EnDe == "a":
                EnDe1 = "encrypted"
                EnDe2 = "encrypt"
                EnDe3 = "pick"
                encryption1()
            elif EnDe == "b":
                EnDe1 = "decrypted"
                EnDe2 = "decrypt"
                EnDe3 = "enter"
                encryption1()
            else:
                print ("please pick either 'A' or 'B' , ONLY!")
                time.sleep(2)
                choice()

        choice()              
        output = ''.join(message)
        print (output)
        retry = input ("would you like to Decrypt/Encrypt another message? (Y/N)\n>")
        retry = retry.lower()

        while retry != ("y" or "n"):
            retry = input ("please select either y or n\n>")
            retry = retry.lower()

        while retry == "y":
            program()
        else:
            sys.exit()
    encryption()     

Answer 1

The problem is that you define a global x variable, and also a local one. The local one shadows the global one and so the result of eval("x") is not anymore what you expected to have.

Solution: use a different variable for the for loop.

There is much that can be improved in your code. You can take advantage of the modulo operator and the ord function, avoiding the need for all those 26 letter names.

Here is how that for loop could look without all that:

if 0 < shift < 11:
    for i, ch in enumerate(message): 
        if ch != " ":
            message[i] = chr((ord(ch)-ord('a')+shift)%26+ord('a'))

Unrelated: note that retry != ("y" or "n") does not work like that. You should do retry not in "yn"