tests: Update tests to use DevitoWarning

Author: JDBetteridge

devito/finite_differences/derivative.py

@@ -2,7 +2,6 @@
 from collections.abc import Iterable
 from functools import cached_property
 from itertools import chain
-from warnings import warn
 
 import sympy
 
@@ -13,6 +12,7 @@
 from devito.tools import (as_mapper, as_tuple, frozendict, is_integer,
                           Pickable)
 from devito.types.utils import DimensionTuple
+from devito.warnings import warn
 
 __all__ = ['Derivative']
 
@@ -96,8 +96,8 @@ class Derivative(sympy.Derivative, Differentiable, Pickable):
     def __new__(cls, expr, *dims, **kwargs):
         # TODO: Delete this
         if kwargs.get('preprocessed', False):
-            from warnings import warn
-            warn('I removed the `preprocessed` kwarg')
+            from warnings import warn as pywarn
+            pywarn('I removed the `preprocessed` kwarg')
 
         # Validate the input arguments `expr`, `dims` and `deriv_order`
         expr = cls._validate_expr(expr)

tests/test_derivatives.py

@@ -10,6 +10,7 @@
                                                       DiffDerivative)
 from devito.symbolics import indexify, retrieve_indexed
 from devito.types.dimension import StencilDimension
+from devito.warnings import DevitoWarning
 
 _PRECISION = 9
 
@@ -1160,17 +1161,26 @@ def test_expand_add(self):
     def test_expand_nest(self):
         """
         Check valid nested derivative expands (combining x derivatives)
+        Note that it is important to specify `fd_order` to avoid raising a warning
         """
+        a = self.u.dx(fd_order=2)
+        b = a.subs({self.u: -5*self.u.dx(fd_order=2) + 4*self.u + 3}, postprocess=False)
+        expanded = 4*Derivative(self.u, self.x, fd_order=2) \
+            - 5*Derivative(self.u, (self.x, 2), fd_order=4)
+        assert b.expand(add=True, nest=True) == expanded
+
+        # Check that doing the naïve thing raises a warning, but asserts equal
         expanded = 4*Derivative(self.u, self.x) - 5*Derivative(self.u, (self.x, 2))
-        assert self.b.expand(add=True, nest=True) == expanded
+        with pytest.warns(DevitoWarning):
+            assert self.b.expand(add=True, nest=True) == expanded
 
     def test_nested_orders(self):
         """
         Check nested expansion results in correct derivative and fd order
         """
         # Default fd_order
         du22 = Derivative(Derivative(self.u, (self.x, 2)), (self.x, 2))
-        with pytest.warns(UserWarning):
+        with pytest.warns(DevitoWarning):
             du22_expanded = du22.expand(nest=True)
         du4 = Derivative(self.u, (self.x, 4))
         assert du22_expanded == du4