input: Write a function to count the occurrence of each character in a given input string. If the string is empty, return an empty dictionary.
  def char_occurrences(a_string):
   char_count = {}
   for char in a_string:
   if char in char_count:
   char_count[char] += 1
   else:
   char_count[char] = 1
   return char_count
  
  assert char_occurrences("Hello, World!") == {'H': 1, 'e': 1, 'l': 3, 'o': 2, ',': 1, ' ': 1, 'W': 1, 'r': 1, 'd': 1, '!': 1}
  assert char_occurrences("") == {}
  assert char_occurrences("aabbcc") == {'a': 2, 'b': 2, 'c': 2}
  assert char_occurrences("1234567890") == {'1': 1, '2': 1, '3': 1, '4': 1, '5': 1, '6': 1, '7': 1, '8': 1, '9': 1, '0': 1}
  assert char_occurrences("!@#$%^&*()") == {'!': 1, '@': 1, '#': 1, '$': 1, '%': 1, '^': 1, '&': 1, '*': 1, '(': 1, ')': 1}
output: def char_occurrences(a_string):
      char_count = {}
      for char in a_string:
          if char in char_count:
              char_count[char] += 1
          else:
              char_count[char] = 1
      return char_count