code: def assert_not_in(obj, seq, message=None, extra=None):
      
      # for very long strings, provide a truncated error
      if isinstance(seq, six.string_types) and obj in seq and len(seq) > 200:
          index = seq.find(obj)
          start_index = index - 50
          if start_index > 0:
              truncated = "(truncated) ..."
          else:
              truncated = ""
              start_index = 0
          end_index = index + len(obj) + 50
          truncated += seq[start_index:end_index]
          if end_index < len(seq):
              truncated += "... (truncated)"
          assert False, _assert_fail_message(message, obj, truncated, "is in", extra)
      assert obj not in seq, _assert_fail_message(message, obj, seq, "is in", extra)
docs: Raises an AssertionError if obj is in iter.