python chess board.attackers how to use

Question

I'm playing around with some python chess code, its for fun.
However i have difficulty in using the board.attackers function.
I'm not sure what it accepts
This code below makes use of the chess lib described here:
http://python-chess.readthedocs.io/en/latest/core.html

in the docs example code exists where they put in chess.F3 though i need some conversion from move to chess.xx i guess ?.

def staticAnalysis(board, move, my_color):
  score = random.random()
  board.push(move)
 
  for (piece, value) in [(chess.PAWN, 1), 
                       (chess.BISHOP, 4), 
                       (chess.KING, 0), 
                       (chess.QUEEN, 10), 
                       (chess.KNIGHT, 5),
                       (chess.ROOK, 3)]:
    score += len(board.pieces(piece, my_color)) * value
    score -= len(board.pieces(piece, not my_color)) * value
     
  score += 100 if board.is_checkmate() else 0
  score += 10 if board.is_capture(move)  else 0

  # to get proposed destination of the move, removes piece, thus Ph1 becomes h1
  square =  str(move)[-2:]  

  # but its not accepted ?.. what can be used to do achieve this ?.
  myAttackers = board.attackers(not my_color,  square)
  score +=len(attackers)*-2

return score

attacker is a new variable for the board.attacker as in the manual https://python-chess.readthedocs.io/en/latest/core.html? I think the problem is in my square its not the proper input — Peter, Oct 13 '22 at 23:30

score 0 · Answer 1 · answered Oct 13 '22 at 23:21

0

For us to have a better idea, you should define your functions and variables. A bit of googling found the docs here: https://python-chess.readthedocs.io/en/latest/

Based on this snippet in the docs attackers = board.attackers(chess.WHITE, chess.F3), it looks like you need to pass the color and square not as a string but as the chess. function.

answered Oct 13 '22 at 23:21

Bryce Draper

87
6

I red that but how to convert a proposed move as in the function(board, move, my color) into the format chess.F3, ..its a different format. ( i tried square function to get the final position but its not compatible with ches.xx – Peter Oct 13 '22 at 23:47

score 0 · Answer 2 · answered Oct 14 '22 at 00:04

0

Ok its more simple then i thought..

print(chess.B2)
#equals
print ((ord("B")-ord("A"))+(2-1)*8)

answered Oct 14 '22 at 00:04

Peter

2,043
1
21
45

score 0 · Accepted Answer · answered Dec 14 '22 at 18:16

There are built-in methods that take care of converting between a square's str name and int value already:

>>> chess.square_name(21)
'f3'
>>> chess.parse_square('f3')
21

However, your solution using ord() is technically more efficient as it runs in O(1) whereas chess.parse_square calls SQUARES.index(...) and runs in O(n). But no need to reinvent the wheel here.

For more context, chess.Square is simply an alias for a Python int. Here is the actual source code where it's defined:

Square = int
SQUARES = [
    A1, B1, C1, D1, E1, F1, G1, H1,
    A2, B2, C2, D2, E2, F2, G2, H2,
    A3, B3, C3, D3, E3, F3, G3, H3,
    A4, B4, C4, D4, E4, F4, G4, H4,
    A5, B5, C5, D5, E5, F5, G5, H5,
    A6, B6, C6, D6, E6, F6, G6, H6,
    A7, B7, C7, D7, E7, F7, G7, H7,
    A8, B8, C8, D8, E8, F8, G8, H8,
] = range(64)

This is shorthand for chess.A1=0, chess.B1=1, ... , chess.H8=63.

In your code, you are providing attackers() with a string (because you define square = str(move)[-2:] ), not a chess.Square (an int), which is causing your error.

You can replace the line where you define square with this, so its type is actually chess.Square (here I know the type of move is chess.Move since you use board.push(move) earlier):

square = move.to_square  # a ``chess.Square``

On a side note, I noticed your piece values look a bit off. Maybe you wanted them like this:

[(chess.PAWN, 1),
(chess.BISHOP, 3),
(chess.KING, 0),
(chess.QUEEN, 9),
(chess.KNIGHT, 3),
(chess.ROOK, 5)]

values of knight and bishop, are well complex to say at least. Especially towards end games, I prefer knights above bishops, and if one has only one bishop left it depends on pawn position (blocking /defending). Most people in my chess club though agree a knight is slightly higher, although left with 2 bishops for an end game is a different story. — Peter, Dec 16 '22 at 13:11
@Peter Above are just the widely-accepted values ([ex1](https://www.chess.com/terms/chess-piece-value), [ex2](https://en.wikipedia.org/wiki/Chess_piece_relative_value)). Some places I've heard call a knight 3 and a bishop 3.25, with other heuristics in place ex. if it's an open endgame with BB vs. NN—but it's futile to use human reasoning to beat the computer-verified values. At least a rook should certainly not be 3 if a bishop is 5. At least my opinion as a 2000-rated on lichess. — Jackson H, Dec 17 '22 at 01:33
it's not only human reasoning, its common knowledge, but most engines dont go that deep into this matter. — Peter, Dec 17 '22 at 01:44

python chess board.attackers how to use

3 Answers3