使用Python约束解析器求解N个皇后

Question

Below is a solution for the N-queens problem using the Python-Constraint Resolver from Labix . Could someone explain to me, or refer me to any web page where it explains the meaning of the last 3 lines of this code?

Moreover, how could I use the AllDifferentConstraint constraint to make the below code shorter?

from constraint import *

problem = Problem()
size = 8
cols = range(size)
rows = range(size)
problem.addVariables(cols, rows)
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
                                    row1 != row2, (col1, col2))

Answer 1

problem.addConstraint(lambda row1, row2, col1=col1, col2=col2:
                        abs(row1-row2) != abs(col1-col2) and
                        row1 != row2, (col1, col2))

This is roughly equivalent to this:

def constraintFunction (col1, col2):
    def innerFunction (row1, row2):
        return abs(row1 - row2) != abs(col1 - col2) and row1 != row2
    return innerFunction

problem.addConstraint(constraintFunction(col1, col2), (col1, col2))

And that last line is equivalent to this:

func = constraintFunction(col1, col2)
problem.addConstraint(func, (col1, col2))