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I am writing a small chess GUI with Python chess library. Yesterday, I ran my code a lot of times without any problem. Then I installed some other Python libraries. Today I noticed a strange mistake: board.legel_moves generates wrong moves. I tried to reinstall Python 3.13.1 and the latest Python chess library, the mistake still exists.

The code is very short:

import chess

board = chess.Board()
starting_position=board.fen()

for move in board.legal_moves:
    print(starting_position)
    board.set_fen(starting_position)
    print(move)
    board.push(move)
    print(f' board pushed move {board.fen()}')
    print()

wrong results start from the fifth move h7h6 till the end. It should be h2h3. I just show the first six examples.

rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR w KQkq - 0 1
g1h3
 board pushed move rnbqkbnr/pppppppp/8/8/8/7N/PPPPPPPP/RNBQKB1R b KQkq - 1 1

rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR w KQkq - 0 1
g1f3
 board pushed move rnbqkbnr/pppppppp/8/8/8/5N2/PPPPPPPP/RNBQKB1R b KQkq - 1 1

rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR w KQkq - 0 1
b1c3
 board pushed move rnbqkbnr/pppppppp/8/8/8/2N5/PPPPPPPP/R1BQKBNR b KQkq - 1 1

rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR w KQkq - 0 1
b1a3
 board pushed move rnbqkbnr/pppppppp/8/8/8/N7/PPPPPPPP/R1BQKBNR b KQkq - 1 1

rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR w KQkq - 0 1
h7h6
 board pushed move rnbqkbnr/ppppppp1/7P/8/8/8/PPPPPPPP/RNBQKBNR b KQkq - 0 1

rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR w KQkq - 0 1
g7g6
 board pushed move rnbqkbnr/pppppp1p/6P1/8/8/8/PPPPPPPP/RNBQKBNR b KQkq - 0 1

If I comment out the board push move part board.push(move), everything is normal.

rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR w KQkq - 0 1
g1h3
 board pushed move rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR w KQkq - 0 1

rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR w KQkq - 0 1
g1f3
 board pushed move rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR w KQkq - 0 1

rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR w KQkq - 0 1
b1c3
 board pushed move rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR w KQkq - 0 1

rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR w KQkq - 0 1
b1a3
 board pushed move rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR w KQkq - 0 1

rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR w KQkq - 0 1
h2h3
 board pushed move rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR w KQkq - 0 1

rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR w KQkq - 0 1
g2g3
 board pushed move rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR w KQkq - 0 1

If I comment out board.set_fen(starting_position) and add board.pop() at the end, no mistake.

Can anyone give me some suggestions? Thanks

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3
  • I tried older version of Python chess: 1.10.1 or 1.9.4. Same problem still exists. Commented Dec 13, 2024 at 21:14
  • A guy on Github told me to copy board to bypass the problem. It works. Commented Dec 14, 2024 at 10:36
  • Hey welcome to StackOverflow ! If you found the answer to your problem may you post an answer to explain how you achieve it ? Commented Dec 15, 2024 at 11:20

1 Answer 1

Reset to default
1

I don't know why, but the push will affect the board. make a copy of the board, and no problem anymore.

import chess
import copy

board = chess.Board()
starting_position=board.fen()
new_board= copy.copy(board)

for move in board.legal_moves:
    print(starting_position)
    board=copy.copy(new_board)
    board.set_fen(starting_position)
    print(move)
    board.push(move)
    print(f' board pushed move {board.fen()}')
    print()
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