input: Write a python function to count the occurrence of each character in a string.
  assert count_characters("Hello, World!") == {'H': 1, 'e': 1, 'l': 3, 'o': 2, ',': 1, ' ': 1, 'W': 1, 'r': 1, 'd': 1, '!': 1}
  assert count_characters("test") == {'t': 2, 'e': 1, 's': 1}
  assert count_characters("aabbcc") == {'a': 2, 'b': 2, 'c': 2}
output: def count_characters(s):
      counts = {}
      for char in s:
          if char in counts:
              counts[char] += 1
          else:
              counts[char] = 1
      return counts