# Create a firewall

firewall_x = int( grid_width / 3 )

for row in grid:
    row[ firewall_x ] = 3

# Now leave some holes

porosity = 0.2
num_holes = int( grid_height * porosity )
hole_count = 0

while hole_count < num_holes:
    hole_y = random.randint( 0 , grid_height - 1 )
    if grid[ hole_y ][ firewall_x ] == 3:
        grid[ hole_y ][ firewall_x ] = 1
        hole_count += 1