Using Python-Constraint to solve the eight queens puzzle

queens-solver.py

#!/usr/bin/env python2

from constraint import *

problem = Problem()
cols = range(1,9)	# these are the variables
rows = range(1,9)	# these are the domains
problem.addVariables(cols, rows)	# adding multiple variables at once

# that each queen has to be in a separate column is 
# implied through the loop and added constraints
for col1 in cols:
    for col2 in cols:
        if col1 < col2:
            problem.addConstraint(lambda row1, row2, col1=col1, col2=col2:
                abs(row1-row2) != abs(col1-col2) and	# this is the diagonal check
                row1 != row2, (col1, col2))				# this is the horizontal check

solution = problem.getSolution()
print solution

n = int(sys.argv[1])
for i in range(n):
    p.addVariable(i, range(n))

for i in range(n-1):
    for j in range(i+1, n):
        p.addConstraint(lambda qi, qj, i=i, j=j:
                        qi != qj and
                        abs(qi - qj) != j - i,
                         (i, j))