Fix conditional for non-Kramers WCC

Author: greschd

tests/test_z2_invariant.py

@@ -38,13 +38,25 @@ def test_trivial(num_lines, num_wcc, patch_surface_data):
     assert z2pack.invariant.z2(data, check_kramers_pairs=False) == 0
 
 
-def test_no_kramers_pairs(N, M, patch_surface_data):  # pylint: disable=invalid-name
+def test_not_even_number_wcc(N, M, patch_surface_data):  # pylint: disable=invalid-name
+    """
+    Test that the Kramers pairs check raises when an odd number of WCC is present.
+    """
     wcc = [np.linspace(0, 1, 2 * M + 1) for j in range(N + 1)]
     data = SurfaceData(wcc)
     with pytest.raises(ValueError):
         z2pack.invariant.z2(data)
 
 
+def test_not_kramers_pairs(patch_surface_data):
+    """
+    Test that the check for Kramers pairs raises when WCC are not pairs.
+    """
+    data = SurfaceData([[0., 0.], [0.5, 0.6]])
+    with pytest.raises(ValueError):
+        print(z2pack.invariant.z2(data))
+
+
 def test_linear(num_lines_nonzero, x, patch_surface_data):
     """
     Test non-zero Z2 invariant with linear WCC evolution.

z2pack/invariant.py

@@ -46,9 +46,10 @@ def z2(surface_result, check_kramers_pairs=True):  # pylint: disable=invalid-nam
         print(z2pack.invariant.z2(result)) # Prints the Z2 invariant
     """
     wcc = surface_result.wcc
-    if check_kramers_pairs and len(wcc) > 0:  # pylint: disable=len-as-condition
-        if not _check_kramers_pairs(list(wcc[0])) and _check_kramers_pairs(
-            list(wcc[-1])
+    if check_kramers_pairs and wcc:
+        if not (
+            _check_kramers_pairs(list(wcc[0]))
+            and _check_kramers_pairs(list(wcc[-1]))
         ):
             raise ValueError(
                 'The given WCC are not degenerate Kramers pairs at the edges of the surface.'