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:
@@ -482,9 +483,13 @@ class Field(AbstractField):
         for i, o in enumerate(offset):
             if (o + sp.Rational(1, 2)).is_Integer:
                 offset[i] += sp.Rational(1, 2)
-                neighbor[i] = 1
+                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
@@ -510,20 +515,25 @@ class Field(AbstractField):
         assert FieldType.is_staggered(self)
         stencils = {
             2: {
-                2: ["E", "N"],  # D2Q5
-                4: ["E", "N", "NE", "SE"]  # D2Q9
+                2: ["W", "S"],  # D2Q5
+                4: ["W", "S", "SW", "NW"]  # D2Q9
             },
             3: {
-                3: ["E", "N", "T"],  # D3Q7
-                7: ["E", "N", "T", "TNE", "BNE", "TSE", "BSE "],  # D3Q15
-                9: ["E", "N", "T", "NE", "SE", "TE", "BE", "TN", "BN"],  # D3Q19
-                13: ["E", "N", "T", "NE", "SE", "TE", "BE", "TN", "BN", "TNE", "BNE", "TSE", "BSE"]  # D3Q27
+                3: ["W", "S", "B"],  # D3Q7
+                7: ["W", "S", "B", "BSW", "TSW", "BNW", "TNW"],  # D3Q15
+                9: ["W", "S", "B", "SW", "NW", "BW", "TW", "BS", "TS"],  # D3Q19
+                13: ["W", "S", "B", "SW", "NW", "BW", "TW", "BS", "TS", "BSW", "TSW", "BNW", "TNW"]  # D3Q27
             }
         }
         if not self.index_shape[0] in stencils[self.spatial_dimensions]:
             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)