input: Write a function to count the occurrences of each character in a string.
  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("aabbcc") == {'a': 2, 'b': 2, 'c': 2}
  assert char_occurrences("") == {}
  assert char_occurrences("123321") == {'1': 2, '2': 2, '3': 2}
output: def char_occurrences(s):
      occurrences = {}
      for char in s:
          if char in occurrences:
              occurrences[char] += 1
          else:
              occurrences[char] = 1
      return occurrences