Caesar Shift in Python (Brilliant)

alphabet = "abcdefghijklmnopqrstuvwxyz"

# convert between letters and numbers up to 26
def number_to_letter(i):
   return alphabet[i%26] # %26 does the wrap-around

def letter_to_number(l):
   return alphabet.find(l) # index in the alphabet

# How to encode a single character (letter or not)
def caesar_shift_single_character(l, amount):
   i = letter_to_number(l)
   if i == -1: # character not found in alphabet
       return "" # remove it, it's space or punctuation
   else:
       return number_to_letter(i + amount) # Caesar shift

# How to encode a full text
def caesar_shift(text, amount):
   shifted_text = ""
   for char in text.lower(): # also convert uppercase letters to lowercase
       shifted_text += caesar_shift_single_character(char, amount)
   return shifted_text

### MAIN PROGRAM ###

message = """
Once upon a midnight dreary, while I pondered, weak and weary,
Over many a quaint and curious volume of forgotten lore—
While I nodded, nearly napping, suddenly there came a tapping,
As of some one gently rapping, rapping at my chamber door—
"'Tis some visitor," I muttered, "tapping at my chamber door—
Only this and nothing more."
"""

code = caesar_shift(message, 2)
print(code)