Shift Cypher encryption and decryption

ShiftCypher.py

import string

REPLACE_SPACE = None

def map_alphabet():
	alphabet = string.lowercase
	map = dict()
	map[" "] = REPLACE_SPACE  # if spaces are found in code
	for idx in range(0, 26):
		map[alphabet[idx]] = idx
	#a = 0, b = 1......z = 25
	return map
	
	
def encrypt(message, key):
# key is an integer 0-26 that offsets the message before modulation
	enum_after_mod = []
	encrypted_message = ""
	char_mapping = map_alphabet()
	for char in message:
		try:
			enum_after_mod.append((char_mapping[char] + key) % 26)
		except TypeError:
			enum_after_mod.append(REPLACE_SPACE)
	for enumerations in enum_after_mod:
		for key in char_mapping.keys():
			if enumerations  == char_mapping[key]:
				encrypted_message += key
	return encrypted_message
	
def decrypt(encrypted_message, key):
	enum_after_mod = []
	decrypted_message = ""
	char_mapping = map_alphabet()
	for char in encrypted_message:
		try:
			enum_after_mod.append((char_mapping[char] - key) % 26)
		except TypeError:
			enum_after_mod.append(REPLACE_SPACE)
	for enumerations in enum_after_mod:
		for key in char_mapping.keys():
			if enumerations == char_mapping[key]:
				decrypted_message += key
	return decrypted_message

if __name__ == "__main__":
	msg = raw_input("Message to encrypt: ").lower()
	k = int(raw_input("Cypher key: "))
	encrypted_msg = encrypt(msg, k)
	#print encrypted_msg
	decrypted_msg = decrypt(encrypted_msg, k)
	#print decrypted_msg