Grid walk problem

#!/usr/bin/env python


def sum_of_digits(number):
    """
    Calculate the sum of the digits of the specified parameter.
    """
    return sum(map(int, str(number)))


def is_accessible_point(x, y):
    """
    Verify if the given point is accessible to the monkey or not.
    """
    return (sum_of_digits(abs(x)) + sum_of_digits(abs(y))) <= 19


def append_neighbors(points, x, y):
    """
    Append accessible neighbors to the accessible points `points`
    if does not exists.
    """
    for (i, j) in ((x+1, y), (x, y+1), (x-1, y), (x, y-1)):
        # print "%d, %d" % (i, j)
        if is_accessible_point(i, j) and ((i, j) not in points):
            points.append((i, j))


if __name__ == '__main__':
    points = [(0, 0)]
    i = 0
    while i < len(points):
        append_neighbors(points, points[i][0], points[i][1])
        i += 1
    print "Number of accessible points is: %d" % len(points)