Codecademy Python3 Code Challenges 1: Count Letters

letters = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz"
# Write your unique_english_letters function here:
def unique_english_letters(word):
  unique_letters = []

  for letter in word:
    if letter in letters and letter not in unique_letters:
      unique_letters.append(letter)
  return len(unique_letters)

# Uncomment these function calls to test your function:
print(unique_english_letters("mississippi"))
# should print 4
print(unique_english_letters("Apple"))
# should print 4