def test_intersect():
    g = ProximityGrid()
    g['a'] = ((1, 2), (3, 4))
    g['b'] = ((5, 6), (7, 8))
    assert(g.intersect(((1, 5), (2, 6))) == set())
    assert(g.intersect(((2, 3), (2, 3))) == set(['a']))
    assert(g.intersect(((6, 7), (6, 7))) == set(['b']))
    assert(g.intersect(((2, 3), (6, 7))) == set(['a', 'b']))
def test_cell_size():
    g = ProximityGrid(3)
    assert g.cell_size() == 3
def test_default_cell_size():
    g = ProximityGrid()
    assert g.cell_size() == 1