Cieser cipher in python

Cipher.py

def start():
   
   normaltext = input('enter your word/phrase:  ')#gahter the string
   normaltext1 = normaltext.lower()#convert to lowercase
   Alphabets = "abcdefghijklmnopqrstuvwxyz"
   shift = int(input('by how much do you want to shift'))#how much the user wants to shift by
   cipher =' '
   TypeInput = input("choose type + or  -")#shifting left or right (names for this i forgot)
   if TypeInput == "+":#condition "+"
      for d in normaltext1:
         if d in Alphabets:
            cipher += Alphabets [(Alphabets . index (d)+shift )%(len(Alphabets))]#runs through Alphabets and searches for the index based on shift
   else:
       for d in normaltext1:
          if d in Alphabets:
             cipher += Alphabets [(Alphabets . index (d)-shift )%(len(Alphabets))]#same as line 12 but reversed
   

   print('Your cipher code is:  '  +  cipher)#print the damn thing
while True:
   start()