mosaic.py

import numpy


def squares(matrix, square_size):
    paint = False

    for row in xrange(len(matrix)):
        # Only in the borders of the square, we change the switch to paint
        if row % square_size == 0:
            paint = not paint

        # We need to store the initial status of the row, because it
        # defines the behaviour of the pixels in the entire row. If
        # we don't store it, the status of the next row depends on the
        # status of the last pixel in this rows (and the patterns becomes
        # really weird!)
        initial_status = paint

        for col in xrange(len(matrix[row])):
            # Only in the borders of the square, we change the switch to paint
            if col % square_size == 0:
                paint = not paint

            if paint:
                matrix[row][col] = 0

        paint = initial_status

matrix = numpy.ones((10, 10))
squares(matrix, input("Square size: "))
print matrix