python-constraint sudoku solver

sudoku.py

from constraint import *
ROWS = 'abcdefghi'
COLS = '123456789'
DIGITS = range(1, 10)
VARS = [row + col for row in ROWS for col in COLS]
ROWGROUPS = [[row + col for col in COLS] for row in ROWS]
COLGROUPS = [[row + col for row in ROWS] for col in COLS]
SQUAREGROUPS = [
    [ROWS[3 * rowgroup + k] + COLS[3 * colgroup + j]
     for j in range(3) for k in range(3)]
    for colgroup in range(3) for rowgroup in range(3)
]

def solve(hints):
    problem = Problem()
    for var, hint in zip(VARS, hints):
        problem.addVariables([var], [hint] if hint in DIGITS else DIGITS)
    for vargroups in [ROWGROUPS, COLGROUPS, SQUAREGROUPS]:
        for vargroup in vargroups:
            problem.addConstraint(AllDifferentConstraint(), vargroup)
    return problem.getSolution()


def pretty(var_to_value):
    board = ''
    for rownum, row in enumerate('abcdefghi'):
        for colnum, col in enumerate('123456789'):
            board += str(var_to_value[row+col])
            if colnum % 3 == 2:
                board += ' '
        board += '\n'
        if rownum % 3 == 2:
            board += '\n'
    return board

hints = (
    0, 0, 8,  0, 0, 6,  0, 0, 0,
    0, 0, 4,  3, 7, 9,  8, 0, 0,
    5, 7, 0,  0, 1, 0,  3, 2, 0,

    0, 5, 2,  0, 0, 7,  0, 0, 0,
    0, 6, 0,  5, 9, 8,  0, 4, 0,
    0, 0, 0,  4, 0, 0,  5, 7, 0,

    0, 2, 1,  0, 4, 0,  0, 9, 8,
    0, 0, 9,  6, 2, 3,  1, 0, 0,
    0, 0, 0,  9, 0, 0,  7, 0, 0,
)

print pretty(solve(hints))