output rel_shift if assertion false

field.py

@@ -6,7 +6,7 @@ class Field3d:
         self.data      = numpy.array(data)
         self.origin    = tuple([float(x) for x in origin])
         self.spacing   = tuple([float(x) for x in spacing])
-        self.eps_collapse = 1e-12
+        self.eps_collapse = 1e-7
         self._dim = None
         return
 
@@ -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)]