code: def gap_proportion(sequences, gap_chars='-'):
      
      aln_len = None
      gaps = []
      for i, sequence in enumerate(sequences):
          if aln_len is None:
              aln_len = len(sequence)
              gaps = [0] * aln_len
          else:
              if not len(sequence) == aln_len:
                  raise ValueError(("Unexpected sequence length {0}. Is this "
                                    "an alignment?").format(len(sequence)))
  
          # Update any gap positions in gap list
          for j, char in enumerate(sequence.seq):
              if char in gap_chars:
                  gaps[j] += 1
  
      sequence_count = float(i + 1)
      gap_props = [i / sequence_count for i in gaps]
      return gap_props
docs: Generates a list with the proportion of gaps by index in a set of
      sequences.