I am trying to create a two player chess game. To do this I want to generate a list of legal moves. I am having trouble with the brevity of the code to generate the moves for the rooks and bishops.

The code I have pasted below works (i.e. shows legal bishop moves) but there is a lot of repetition, which is not ideal. To make this work for both black and white bishops I would need to add even more repetitive lines.

Is there a better way to do this? I do not want to use any 'chess modules' as this is a learning exercise for me; as you can tell, I am a beginner.

import numpy as np

# chess board is a numpy array
# white pieces are capitalised, black pieces are not
board = np.zeros((8,8),dtype=str)

# example piece positions
white_bishop = (4,5)
white_pawn = (6,7)
black_knight = (0,1)
board[white_bishop] = 'B'
board[white_pawn] = 'P'
board[black_knight] = 'n'

def bishop(insquare):
    '''
    Gets all legal moves for bishop on insquare
    output is a list (outsquares) of indices of possible squares
    '''
    out_squares = []
    y,x = insquare
   
    y1 = y-1 # these are steps in one of the four directions bishops can go
    x1 = x-1
    while (x1 > -1) and (y1 > -1): # edge of the board
        if not board[(y1,x1)].isupper(): # if not a white piece...
            out_squares.append((y1,x1)) # square is legal
            if board[(y1,x1)].islower(): # if black piece...
                y1 = -1 # look no further in this direction
        else: # if white piece...
            y1 = -1 # look no further in this direction
        y1 -= 1 # check next square
        x1 -= 1
    
    # check all other directions
    y1 = y+1
    x1 = x+1
    while (x1 < 8) and (y1 < 8):
        if not board[(y1,x1)].isupper():
            out_squares.append((y1,x1))
            if board[(y1,x1)].islower():
                y1 = 8
        else:
            y1 = 8
        y1 += 1
        x1 += 1
  
    y1 = y + 1
    x1 = x - 1
    while (x1 > -1) and (y1 < 8):
        if not board[(y1,x1)].isupper():
            out_squares.append((y1,x1))
            if board[(y1,x1)].islower():
                y1 = 8
        else:
            y1 = 8
        y1 += 1
        x1 -= 1
    
    y1 = y - 1
    x1 = x + 1
    while (x1 < 8) and (y1 > -1):
        if not board[(y1,x1)].isupper():
            out_squares.append((y1,x1))
            if board[(y1,x1)].islower():
                y1 = -1
        else:
            y1 = -1
        y1 = y1 - 1
        x1 = x1 + 1
    return out_squares

# Get all legal moves for the white bishop
print bishop((4,5))
# >>> [(3, 4), (2, 3), (1, 2), (5, 6), (5, 4), (6, 3), (7, 2), (3, 6), (2, 7)]