fix minor regressions introduced with !86

• DataHandling.array_like did not preserve staggeredness.
• Printing a staggered FieldAccess sometimes threw errors because of int vs. numpy.int64.
• Accessing an invalid staggered neighbor did not produce a good error message.
• I had reversed the index convention so that it was inconsistent with the documentation of create_staggered_kernel.

pystencils/field.py

@@ -466,6 +466,7 @@ class Field(AbstractField):
         """
         assert FieldType.is_staggered(self)
 
+        offset_orig = offset
         if type(offset) is np.ndarray:
             offset = tuple(offset)
         if type(offset) is str:
@@ -484,7 +485,11 @@ class Field(AbstractField):
                 offset[i] += sp.Rational(1, 2)
                 neighbor[i] = 1
         neighbor = offset_to_direction_string(neighbor)
-        idx = self.staggered_stencil.index(neighbor)
+        try:
+            idx = self.staggered_stencil.index(neighbor)
+        except ValueError:
+            raise ValueError("{} is not a valid neighbor for the {} stencil".format(offset_orig,
+                             self.staggered_stencil_name))
         offset = tuple(offset)
 
         if self.index_dimensions == 1:  # this field stores a scalar value at each staggered position
@@ -524,6 +529,11 @@ class Field(AbstractField):
             raise ValueError("No known stencil has {} staggered points".format(self.index_shape[0]))
         return stencils[self.spatial_dimensions][self.index_shape[0]]
 
+    @property
+    def staggered_stencil_name(self):
+        assert FieldType.is_staggered(self)
+        return "D%dQ%d" % (self.spatial_dimensions, self.index_shape[0] * 2 + 1)
+
     def __call__(self, *args, **kwargs):
         center = tuple([0] * self.spatial_dimensions)
         return Field.Access(self, center)(*args, **kwargs)