count unique positions in a pgn file

getpositions.py

#!/usr/bin/env python3

import sys
import chess.pgn

positions = []

class UniqPositionsVisitor(chess.pgn.BaseVisitor):
    def visit_board(self, board):
        if board.fen() not in positions:
            positions.append(board.fen())

    def result(self):
        return True

for f in sys.argv[1:]:
    pgn = open(f)

    while True:
        game = chess.pgn.read_game(pgn, Visitor=UniqPositionsVisitor)
        if game == None:
            break
    pgn.close()

print(len(positions))