# Simple Substitution Cipher
# https://www.nostarch.com/crackingcodes/ (BSD Licensed)

import pyperclip, sys, random


LETTERS = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ'

def main():
    myMessage = '''If a man is offered a fact which goes against his
instincts, he will scrutinize it closely, and unless the evidence
is overwhelming, he will refuse to believe it. If, on the other
hand, he is offered something which affords a reason for acting
in accordance to his instincts, he will accept it even on the
slightest evidence. The origin of myths is explained in this way.
-Bertrand Russell'''
    myKey = 'LFWOAYUISVKMNXPBDCRJTQEGHZ'
    myMode = 'encrypt' # Set to 'encrypt' or 'decrypt'.

    if keyIsValid(myKey):
        sys.exit('There is an error in the key or symbol set.')
    if myMode == 'encrypt':
        translated = encryptMessage(myKey, myMessage)
    elif myMode == 'decrypt':
        translated = decryptMessage(myKey, myMessage)
    print('Using key %s' % (myKey))
    print('The %sed message is:' % (myMode))
    print(translated)
    pyperclip.copy(translated)
    print()
    print('This message has been copied to the clipboard.')


def keyIsValid(key):
    keyList = list(key)
    lettersList = list(LETTERS)
    keyList.sort()
    lettersList.sort()

    return keyList == lettersList


def encryptMessage(key, message):
    return translateMessage(key, message, 'encrypt')


def decryptMessage(key, message):
    return translateMessage(key, message, 'decrypt')


def translateMessage(key, message, mode):
    translated = ''
    CharsA = LETTERS
    charsB = key
    if mode == 'decrypt':
        # For decrypting, we can use the same code as encrypting. We
        # just need to swap where the key and LETTERS strings are used.
        charsA, charsB = charsB, charsA

    # Loop through each symbol in the message:
    for symbol in message:
        if symbol.upper() in charsA:
            # Encrypt/decrypt the symbol:
            symIndex = charsA.find(symbol.upper())
            if symbol.isupper():
                translated += charsB[symIndex].upper()
            else:
                translated += charsB[symIndex].lower()
        else:
            # Symbol is not in LETTERS; just add it:
            translated += symbol

    return translated


def getRandomKey():
    key = list(LETTERS)
    random.shuffle(key)
    return ''.join(key)


if __name__ == '__main__':
    main()
# Simple Substitution Cipher
# https://www.nostarch.com/crackingcodes/ (BSD Licensed)

from books.CrackingCodesWithPython.pyperclip import copy
import sys
import random

LETTERS = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ'


def main():
    myMessage = '''If a man is offered a fact which goes against his
instincts, he will scrutinize it closely, and unless the evidence
is overwhelming, he will refuse to believe it. If, on the other
hand, he is offered something which affords a reason for acting
in accordance to his instincts, he will accept it even on the
slightest evidence. The origin of myths is explained in this way.
-Bertrand Russell'''
    myKey = 'LFWOAYUISVKMNXPBDCRJTQEGHZ'
    myMode = 'encrypt' # Set to 'encrypt' or 'decrypt'.

    if keyIsValid(myKey):
        sys.exit('There is an error in the key or symbol set.')
    if myMode == 'encrypt':
        translated = encryptMessage(myKey, myMessage)
    elif myMode == 'decrypt':
        translated = decryptMessage(myKey, myMessage)
    print('Using key %s' % myKey)
    print('The %sed message is:' % myMode)
    print(translated)
    copy(translated)
    print()
    print('This message has been copied to the clipboard.')


def keyIsValid(key):
    keyList = list(key)
    lettersList = list(LETTERS)
    keyList.sort()
    lettersList.sort()

    return keyList == lettersList


def encryptMessage(key, message):
    return translateMessage(key, message, 'encrypt')


def decryptMessage(key, message):
    return translateMessage(key, message, 'decrypt')


def translateMessage(key, message, mode):
    translated = ''
    charsA = LETTERS
    charsB = key
    if mode == 'decrypt':
        # For decrypting, we can use the same code as encrypting. We
        # just need to swap where the key and LETTERS strings are used.
        charsA, charsB = charsB, charsA

    # Loop through each symbol in the message:
    for symbol in message:
        if symbol.upper() in charsA:
            # Encrypt/decrypt the symbol:
            symIndex = charsA.find(symbol.upper())
            if symbol.isupper():
                translated += charsB[symIndex].upper()
            else:
                translated += charsB[symIndex].lower()
        else:
            # Symbol is not in LETTERS; just add it:
            translated += symbol

    return translated


def getRandomKey():
    key = list(LETTERS)
    random.shuffle(key)
    return ''.join(key)


if __name__ == '__main__':
    main()

Update simpleSubCipher: fixed imports, some PEP8 warnings, and typo