def unpack(positions):
    """
    Take a (n X 2) numpy array of n cell positions in 2D and return two (n X 1) numpy arrays representing the x and y coordinates
    """

    try:
        x, y = zip(*positions)  # http://hangar.runway7.net/python/packing-unpacking-arguments
        return x, y
    except ValueError:
        if not positions:  # positions is empty
            return [], []