nqueens.py

def is_safe(board, i, j):
    n = len(board)
    j_left = j
    j_right = j
    while i >= 0:
        if (j_left >= 0 and board[i][j_left] == 1) or board[i][j] == 1 or (j_right < n and board[i][j_right] == 1):
            return False
        i -= 1
        j_left -= 1
        j_right += 1
    return True


def rec(board, i):
    n = len(board)
    if i == n:
        return 1
    else:
        nb_solutions = 0
        for j in range(n):
            if is_safe(board, i, j):
                board[i][j] = 1
                nb_solutions += rec(board, i+1)
                board[i][j] = 0
        return nb_solutions
    
    
def n_queens(n):
    board = [[0]*n for _ in range(n)]
    return rec(board, 0)


print(n_queens(8)) # Output: 92

Alternatively, if add memoization of placed queens, is_safe can be more readable

Additionally, we can get solution from it. Just print
placed_queens when hit base case (i==n)

def is_safe(board, i, j, placed_queens):
    return not any(i == i_ or j == j_ or abs(i - i_) == abs(j - j_) for i_, j_ in placed_queens)


def rec(board, i, placed_queens = None):
    n = len(board)
    if i == n:
        return 1
    
    if placed_queens is None:
    	placed_queens = []

    nb_solutions = 0
    for j in range(n):
        if is_safe(board, i, j, placed_queens):
            board[i][j] = 1
            placed_queens.append((i, j))
            nb_solutions += rec(board, i+1, placed_queens[:])
            placed_queens.pop()
            board[i][j] = 0
    return nb_solutions
    
    
def n_queens(n):
    board = [[0]*n for _ in range(n)]
    return rec(board, 0)


print(n_queens(8))