was_into_check behavior on multiple kings

was_into_check (L1995-L1997)

    def was_into_check(self) -> bool:
        king = self.king(not self.turn)
        return king is not None and self.is_attacked_by(self.turn, king)

Here it only picks one king.... but if the position loaded has multiple kings, it only return the bool of one king which depends on FEN parsing order, that means - a flipped position can bring different king of the corresponding side to move.

While it isn't too much - this is kind of UB for pseudo-legal FENs, and it affects Board.is_valid, Board.status and possibly its variants.

This reproduces the bug:

import chess

board = chess.Board(None)
board.set_piece_at(chess.A1, chess.Piece(chess.ROOK, chess.WHITE))
board.set_piece_at(chess.A8, chess.Piece(chess.KING, chess.BLACK)) # attacked
board.set_piece_at(chess.H8, chess.Piece(chess.KING, chess.BLACK)) # safe
board.turn = chess.WHITE

print(board)
print(board.was_into_check())

returns False, but should be True (because the king at A8 is in check)

I would be happy to submit a PR to fix this (by iterating over the kings bitboard), but I wanted to ask first: is this "single king" assumption kept intentionally for performance reasons?

Please assign this to me if you would like me to implement the fix.

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`was_into_check` behavior on multiple kings #1169

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was_into_check behavior on multiple kings #1169

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EiSiMo commented on Feb 3, 2026

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`was_into_check` behavior on multiple kings #1169