bug: d*k methods for SOC/NC returned None

They should return NotImplemented to be more specific.

Fixes #183.

Signed-off-by: Nick Papior <[email protected]>

Author: zerothi

sisl/physics/sparse.py

@@ -622,8 +622,10 @@ def _reset(self):
                 raise ValueError('Currently not implemented')
             self.Pk = self._Pk_non_colinear
             self.Sk = self._Sk_non_colinear
-            self.dPk = None
-            self.dSk = None
+            self.dPk = NotImplemented
+            self.dSk = NotImplemented
+            self.ddPk = NotImplemented
+            self.ddSk = NotImplemented
 
         elif self.spin.is_spinorbit:
             if self.spin.dkind == 'f':
@@ -647,8 +649,10 @@ def _reset(self):
             # The overlap is the same as non-collinear
             self.Pk = self._Pk_spin_orbit
             self.Sk = self._Sk_non_colinear
-            self.dPk = None
-            self.dSk = None
+            self.dPk = NotImplemented
+            self.dSk = NotImplemented
+            self.ddPk = NotImplemented
+            self.ddSk = NotImplemented
 
         if self.orthogonal:
             self.Sk = self._Sk_diagonal