Throw error when trying to sympify `pystencils.Field` (e.g. using it in an...

Throw error when trying to sympify `pystencils.Field` (e.g. using it in an Assignment without indexing)

pystencils/field.py

@@ -15,7 +15,7 @@ import pystencils
 from pystencils.alignedarray import aligned_empty
 from pystencils.data_types import StructType, TypedSymbol, create_type
 from pystencils.kernelparameters import FieldShapeSymbol, FieldStrideSymbol
-from pystencils.stencil import direction_string_to_offset, offset_to_direction_string, inverse_direction
+from pystencils.stencil import direction_string_to_offset, inverse_direction, offset_to_direction_string
 from pystencils.sympyextensions import is_integer_sequence
 
 __all__ = ['Field', 'fields', 'FieldType', 'AbstractField']
@@ -413,6 +413,12 @@ class Field(AbstractField):
     def __str__(self):
         return self.name
 
+    def __getattribute__(self, name):
+        if name == '__sympy__':
+            raise RuntimeError("pystencils.Field cannot by sympified!\n"
+                               "Please use a proper indexing to access elements of the Field!")
+        return super().__getattribute__(name)
+
     def neighbor(self, coord_id, offset):
         offset_list = [0] * self.spatial_dimensions
         offset_list[coord_id] = offset

pystencils_tests/test_assignment_collection.py

+import pytest
 import sympy as sp
+import pystencils
 
 from pystencils import Assignment, AssignmentCollection
 from pystencils.astnodes import Conditional
@@ -40,3 +42,15 @@ def test_free_and_defined_symbols():
 
     print(ac)
     print(ac.__repr__)
+
+
+def test_sympifying_fields():
+    x, y = pystencils.fields('x, y: double[1d]')
+    # Apparently, we trigger our helpful error message only for Sympy>=1.5
+    with pytest.raises((RuntimeError, sp.SympifyError)):
+        AssignmentCollection({
+            x: y
+            })
+
+    with pytest.raises((RuntimeError, sp.SympifyError)):
+        sp.S(x)