develop a code for cesar cipher

def cipher(text, key):
  result = ''
  for letter in text:
    if letter.isalpha():
      num = ord(letter)
      num += key
      if letter.isupper():
        if num > ord('Z'):
          num -= 26
        elif num < ord('A'):
          num += 26
      elif letter.islower():
        if num > ord('z'):
          num -= 26
        elif num < ord('a'):
          num += 26
      result += chr(num)
    else: 
      result += letter
  return result
    
text = 'ABCxyz'
key = 3

print('Plain Text: ' + text)
print('Shift pattern: ' + str(key))
print('Cipher: ' + cipher(text, key))