TST: Add a test case that fails

currently but should pass when code is implemented.

Just simple test cases for add and sub for now. Will add more in later commits.

Also refactored some tests for arithmetic.

specutils/tests/test_arithmetic.py

@@ -1,24 +1,43 @@
+from copy import deepcopy
+
 import astropy.units as u
-from astropy.tests.helper import assert_quantity_allclose
 import numpy as np
+import pytest
+from astropy.tests.helper import assert_quantity_allclose
 
-from ..spectra.spectrum1d import Spectrum1D
+from specutils.spectra.spectrum1d import Spectrum1D
 
 
-def test_spectral_axes():
-    flux1 = (np.random.sample(49) * 100).astype(int)
-    flux2 = (np.random.sample(49) * 100).astype(int)
+class TestMathWithAllOnes:
+    def setup_class(self):
+        flux = np.ones(10) * u.nJy
+        wave = (np.arange(flux.size) + 1) * u.um
+        self.spec = Spectrum1D(spectral_axis=wave, flux=flux)
 
-    flux3 = flux1 + flux2
+    def test_add_sub_spectral_axes_same(self):
+        spec_added = self.spec + self.spec
+        assert_quantity_allclose(spec_added.flux, 2 * u.nJy)
 
-    spec1 = Spectrum1D(spectral_axis=np.arange(1, 50) * u.nm,
-                       flux=flux1 * u.Jy)
-    spec2 = Spectrum1D(spectral_axis=np.arange(1, 50) * u.nm,
-                       flux=flux2 * u.Jy)
+        spec_subbed = self.spec - self.spec
+        assert_quantity_allclose(spec_subbed.flux, 0 * u.nJy)
 
-    spec3 = spec1 + spec2
+    def test_add_sub_spectral_axes_different(self):
+        new_wave = self.spec.spectral_axis + (1 * u.um)
+        new_spec = Spectrum1D(spectral_axis=new_wave, flux=self.spec.flux)
 
-    assert np.allclose(spec3.flux.value, flux3)
+        with pytest.raises(ValueError):
+            self.spec + new_spec
+
+        with pytest.raises(ValueError):
+            self.spec - new_spec
+
+    def test_mask_nans(self):
+        new_flux = deepcopy(self.spec.flux)
+        nan_idx = [1, 3, 5]
+        new_flux[nan_idx] = np.nan
+        new_spec = Spectrum1D(spectral_axis=self.spec.spectral_axis, flux=new_flux)
+        spec_added = self.spec + new_spec
+        assert spec_added.mask[nan_idx].all()
 
 
 def test_add_basic_spectra(simulated_spectra):
@@ -105,19 +124,6 @@ def test_masks(simulated_spectra):
     assert np.all(masked_diff.mask == masked_sum.mask | masked_spec.mask)
 
 
-def test_mask_nans():
-    flux1 = np.random.random(10)
-    flux2 = np.random.random(10)
-    nan_idx = [1, 3, 5]
-    flux2[nan_idx] = np.nan
-    spec1 = Spectrum1D(spectral_axis=np.arange(10) * u.nm, flux=flux1 * u.Jy)
-    spec2 = Spectrum1D(spectral_axis=np.arange(10) * u.nm, flux=flux2 * u.Jy)
-
-    spec3 = spec1 + spec2
-
-    assert spec3.mask[nan_idx].all() == True  # noqa
-
-
 def test_with_constants(simulated_spectra):
     spec = simulated_spectra.s1_um_mJy_e1