output rel_shift if assertion false

field.py

@@ -371,7 +371,7 @@ class Field3d:
         assert isinstance(rel_shift,(tuple,list,numpy.ndarray)) and len(rel_shift)==3,\
             "'shift' must be tuple/list with length 3."
         assert all([rel_shift[ii]>=-1.0 and rel_shift[ii]<=1.0 for ii in range(3)]),\
-            "'shift' must be in (-1.0,1.0)."
+            "'shift' must be in (-1.0,1.0). {}".format(rel_shift)
         data = self.data.copy()
         origin = list(self.origin)
         sl = 3*[slice(None)]