api: prevent evaluated derivatives to be re-evaluted

Author: mloubout

devito/finite_differences/differentiable.py

@@ -952,6 +952,11 @@ def _evaluate(self, **kwargs):
 class DiffDerivative(IndexDerivative, DifferentiableOp):
     pass
 
+    def _eval_at(self, func):
+        # Like EvalDerivative, a DiffDerivative must have already been evaluated
+        # at a valid x0 and should not be re-evaluated at a different location
+        return self
+
 
 # SymPy args ordering is the same for Derivatives and IndexDerivatives
 for i in ('DiffDerivative', 'IndexDerivative'):
@@ -998,6 +1003,11 @@ def _new_rawargs(self, *args, **kwargs):
         kwargs.pop('is_commutative', None)
         return self.func(*args, **kwargs)
 
+    def _eval_at(self, func):
+        # An EvalDerivative must have already been evaluated at a valid x0
+        # and should not be re-evaluated at a different location
+        return self
+
 
 class diffify:
 

tests/test_derivatives.py

@@ -805,13 +805,11 @@ def test_param_stagg_add(self):
         eq1 = Eq(vx, (c11 * txx).dy)
         eq2 = Eq(vx, (c11 * txx + c66 * txy).dy)
 
-        # C66 is a paramater. Expects to evaluate c66 at xp then the derivative at yp
-        # and the derivative will interpolate txy at xp
+        # Expects to evaluate c66 at xp then the derivative at yp
         expect0 = (c66.subs({x: xp, y: yp}).evaluate * txy).dy.evaluate
         assert simplify(eq0.evaluate.rhs - expect0) == 0
 
-        # C11 is a paramater and txy is staggered in x.
-        # Expects to evaluate c11 and txy xp then the derivative at yp
+        # Expects to evaluate c11 and txy at xp then the derivative at yp
         expect1 = (c11._subs(x, xp).evaluate * txx._subs(x, xp).evaluate).dy.evaluate
         assert simplify(eq1.evaluate.rhs - expect1) == 0