input: Write a python function to create a dictionary with unique characters from a string as keys and their first occurrence position as values.
  assert char_positions("Hello, World!") == {'H': 1, 'e': 2, 'l': 3, 'o': 4, ',': 5, ' ': 6, 'W': 7, 'r': 8, 'd': 9, '!': 10}
  assert char_positions("abcd") == {'a': 1, 'b': 2, 'c': 3, 'd': 4}
  assert char_positions("aabbcc") == {'a': 1, 'b': 3, 'c': 5}
output: def char_positions(s):
      result = {}
      for index, char in enumerate(s, 1):
          if char not in result:
              result[char] = index
      return result