Rather than looping through and testing like this, you should probably grab some of numpy's testing tools and rewrite using `assert_array_almost_equal()`  This would be done with:

```python
from numpy.testing import assert_array_almost_equal

..

def test_add_mesh(self):
        self.arithmetic_mesh_setup()
        self.mesh_1.add(self.mesh_2)
        exp_res = [2.1, 4.2, 6.3, 8.4]
        obs_res = [self.mesh_1.mesh.getTagHandle("flux")[vol] for vol in self.mesh_1.structured_iterate_hex("xyz")]
        assert_array_almost_equal(exp_res, obs_res)

```