A simple numeric cipher created with Python

cipher.py

# create alphabet and digits dictionary

import string
alphabet_master = string.ascii_lowercase + string.digits + ' '
alphabet_master = {a:(alphabet_master.index(a) + 1) for a in alphabet_master}

# the cipher

def cipher(direction,key,message):
		
	# make a copy of the alphabet

	alphabet = alphabet_master.copy()
	
	# update alphabet per key
	
	for k,v in alphabet.items():
	
		# add the key to the current value

		alphabet[k] += key
		
		# loop around if key > len(alphabet_master)
		
		if alphabet[k] > len(alphabet_master):
		
			alphabet[k] = alphabet[k] - len(alphabet_master)

	# encode
	
	if direction == 'encode':
				
		# format message: remove all but alphanumeric and space
		
		import re
		message = re.sub('[^\w\s]+','',message).lower()
		
		# encode message
		
		encoded = []
		for a in list(message):
		
			encoded.append(str(alphabet[a]))
		
		# return a string of the encoded message

		return '/'.join(encoded)
				
	# decode
	
	if direction == 'decode':
	
		# swap k,v in alphabet
		
		alphabet_swapped = {}
		for k,v in alphabet.items():
			
			alphabet_swapped[v] = k
						
		# format message, split into list
		
		message = message.split('/')
		
		# decode message
		
		decoded = []
		for n in message:
			
			decoded.append(alphabet_swapped[int(n)])

		# return a string of the decoded message

		return ''.join(decoded)

# use the cipher

def go():
	
	# instructions
	
	print("\n"*20)
	print("Use this script to encode and decode messages with a simple numeric cipher.")

	# get direction

	while True:

		direction = input("\nDo you want to encode or decode? (E/D) > ")
		
		if not direction.lower() in ('e','d','encode','decode'):

			print("Sorry, that's not a valid entry.")
			continue
			
		else:
		
			if direction in ('e','encode'):
			
				direction = 'encode'
				
			else:
			
				direction = 'decode'
		
			break

	# get the message

	while True:

		message = input("\nWhat is your secret message? > ")
		
		if not len(message) > 0:

			print("Sorry, that's not a valid message.")
			continue
			
		else:
		
			break

	# get the key

	while True:

		key = input("\nEnter a whole number to use as the cipher key > ")
		
		try:
		
			int(key)
			
		except:

			print("Sorry, that's not a valid whole number.")
			continue
		
		key = int(key)
		break
		
	# do it
	
	if direction == 'encode':
	
		print(f"\nEncoding message \"{message}\" using key \"{key}\"")
		
	else:
	
		print(f"\nDecoding message \"{message}\" using key \"{key}\"")
	
	print("\nHere's your message:")
	print(cipher(direction,key,message))
	print("\n"*3)
	
go()