Get first not empty coordinates

n2.py

def create_all_positions(max_x, max_y):                                         
  all_positions = list()                                                      
  for row in xrange(1, max_y+1):                                              
    all_positions.append([(col, row) for col in xrange(1, max_x+1)])        
  return all_positions                                                        

                                                                                   
def get_next_empty(all_positions, used_positions):                              
  for row in all_positions:                                                   
    for position in row:                                                    
      if position not in used_positions:                                  
        return position 
 

test.py

from n2 import create_all_positions, get_next_empty

max_x, max_y = 400, 400
used_positions = [
    (1, 1), (2, 1), (3, 1),
    (1, 2), (2, 2),
    (1, 3), (2, 3),
]
next_position = (4, 1)
all_positions = create_all_positions(max_x, max_y)

print 'all positions: %s' % all_positions
print 'must be: '
print next_position
print 'returns: '
returned = get_next_empty(all_positions, used_positions)
print returned
assert next_position == returned