count_letters.py

'''
Use a dictionary to count the frequency of letters in the input string.
Only letters should be counted, not blank spaces, numbers, or punctuation.
Upper case should be considered the same as lower case
'''

def count_letters(text):
  result = {}
  # Go through each letter in the text
  for letter in text:
    # Check if the letter needs to be counted or not
    if letter is None or letter.isdigit() or letter == '+':
      continue
    # skip unnecessary characters
    elif letter == '=' or letter == ' ' or letter == '!' or letter == '.':
      continue
    # count number of letter
    else:
      letter = letter.lower()
      result[letter]=result.get(letter,0)+1

  return result

print(count_letters("AaBbCc"))
# Should be {'a': 2, 'b': 2, 'c': 2}
print(count_letters("Math is fun! 2+2=4"))
# Should be {'m': 1, 'a': 1, 't': 1, 'h': 1, 'i': 1, 's': 1, 'f': 1, 'u': 1, 'n': 1}
print(count_letters("This is a sentence."))
# Should be {'t': 2, 'h': 1, 'i': 2, 's': 3, 'a': 1, 'e': 3, 'n': 2, 'c': 1}