N Queens Backtracking solution

nqueens.py

"""N-queens problem"""

def is_atk(x1, x2):
    "Check if 2 queens attack each other"
    r1, c1, r2, c2 = *x1, *x2
    return r1 == r2 or c1 == c2 or r1-c1 == r2-c2 or r1+c1 == r2+c2

def backtrack(n, r, Q):
    "Iterate through each column in a row and backtrack if the cell is not valid"
    if n == r: return True
    for c in range(n):
        s = True
        for q in range(r):
            if is_atk((r, c), Q[q]):
                s = False
                break
        if s:
            Q[r] = (r,c)
            if backtrack(n, r+1, Q): return True
    return False

def n_queens_solution(n):
    "Generate a solution to N-Queens"
    Q = [None]*n
    return Q if backtrack(n, 0, Q) else None

print(n_queens_solution(4))
print(n_queens_solution(8))