List index out of range error when attempting to make a basic shift code

[djwilson0495]

I'm trying to make a basic shift, my code is as follows:

alphabet = ['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 shift():
    message = input("Please enter a message:")
    number = int(input('Please enter the number that you would like to shift by (1-26):'))
    new_message = ""
    for letter in message:
        letter = letter.lower()
        if letter.isalpha():
            new_position = alphabet.index(letter) + number
            if new_position > 26:
                new_position = new_position - 27
            new_letter = alphabet[new_position]
        elif letter.isnumeric():
            letter = letter
            print(letter)
        elif letter == " " or letter == "," or letter == "." or letter == ";" or letter == ":":
            letter = letter
            print(letter)
        else:
            print("Error in message please try a different message")
        print(new_message)
        print()


def decode():
    message = input("Please enter a message to decode:")
    number = int(input('Please enter the number shift key (between 1 and 26):'))
    new_message = ""
    for letter in message:
        letter = letter.lower()
        #y = alphabet.index(x)
        #y = y + number
        if letter.isalpha():
            new_position = alphabet.index(letter) - number
            if new_position < 0:
                new_position = new_position + 27
            new_letter = alphabet[new_position]
            new_message = new_message + new_letter
        elif letter.isnumeric():
            letter = letter
            print(letter)
        elif letter == " " or letter == "," or letter == "." or letter == ";" or letter == ":":
            letter = letter
            print(letter)
        else:
            print("Error in message please try a different message")
    print(new_message)
    print()


def main():
    repeat = True
    while repeat == True:
        print("1) Make a code")
        print("2) Decode a message")
        print("3) Quit")
        print()
        choice = int(input("Enter your selection:"))
        if choice == 1:
            shift()
        elif choice == 2:
            decode()
        elif choice == 3:
            repeat = False
        else:
            print("Incorrect choice, please try again")
            choice = int(input("Enter your selection:"))

main()

However I get the following error:

Error:
  File "c:/Users/djwil/Documents/python/learning python/Chapter 19 - Chunky Challenges/Shift Code.py", line 70, in <module>
    main()
  File "c:/Users/djwil/Documents/python/learning python/Chapter 19 - Chunky Challenges/Shift Code.py", line 63, in main
    decode()
  File "c:/Users/djwil/Documents/python/learning python/Chapter 19 - Chunky Challenges/Shift Code.py", line 38, in decode
    new_letter = alphabet[new_position]
IndexError: list index out of range

However I thought that the lines of code:

if new_position < 0:
                new_position = new_position + 27

and:

            if new_position > 26:
                new_position = new_position - 27

should be preventing this from happening. Can anyone help?

[deanhystad]

There are only 26 letters in alphabet, not 27. The maximum value for new_position is 25, not 26.

[bowlofred]

Rather than keeping track with another number (so you have something to change in multiple places), you can use the mod operator and the actual array.

Instead of

            if new_position > 26:
                new_position = new_position - 27

Consider

            new_position %= len(alphabet)

[BitPythoner]

if z is in the message, then the program will attempt to shift z to the next available value in the list, this does not exist. Maybe add an if statement (psuedocode):

if z is in the message:
    turn z into a

[deanhystad]

Consider

            new_position %= len(alphabet)

Works for positive and negative values. No need for an if.

x = -2 % 26
y = 28 % 26
print(x, y)

Output:
24 2