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: