gistfile1.py

class Ceasar( str ):
	def __shiftLetter( self, letter, shift ):
		letter_code = ord( letter )
		shift = int( shift )
		
		if letter.islower():
			base = ord( 'a' )
			last = ord( 'z' )
		else:
			base = ord( 'A' )
			last = ord( 'Z' )

		if letter_code not in range( base, last+1 ):
			return letter;

		return chr((letter_code - base + (26 + (shift)) ) % 26 + base);

	def encrypt( self, shift, message=None ):
		message = message if message else self
		cipher_message = ''
	
		for letter in message:
			cipher_message += self.__shiftLetter( letter, shift )

		return __class__( cipher_message )

	def decrypt( self, shift, message=None ):
		return self.encrypt( shift*-1, message )

message = Ceasar('hello! world zoo?')
m = message.encrypt(3)
print( m, m.decrypt(3) )