cesar.py

word = input("Expression > ")
step = int(input("Decallage > "))

word = word[::-1] # Reverse letters order

def decryptLetter(letter, key):
	pos = ord(letter)
	upBound, downBound = 0, 0

	if pos >= 97 and pos <= 122:
		upBound = 122
		downBound = 97
	elif pos >= 65 and pos <= 90:
		upBound = 90
		downBound = 65
	else:
		return letter

	pos += key
	if pos > upBound:
		pos -= 26
	if pos < downBound:
		pos += 26
	return chr(pos)

print(''.join([decryptLetter(char, step) for char in word]))