api: Patch imperfect cross derivs w symbolic coeffs

devito/finite_differences/differentiable.py

@@ -115,6 +115,13 @@ def _symbolic_functions(self):
     def _uses_symbolic_coefficients(self):
         return bool(self._symbolic_functions)
 
+    @cached_property
+    def _coeff_symbol(self, *args, **kwargs):
+        if self._uses_symbolic_coefficients:
+            return W
+        else:
+            raise ValueError("Couldn't find any symbolic coefficients")
+
     def _eval_at(self, func):
         if not func.is_Staggered:
             # Cartesian grid, do no waste time
@@ -327,6 +334,10 @@ def highest_priority(DiffOp):
     return sorted(DiffOp._args_diff, key=prio, reverse=True)[0]
 
 
+# Abstract symbol representing a symbolic coefficient
+W = sympy.Function('W')
+
+
 class DifferentiableOp(Differentiable):
 
     __sympy_class__ = None
@@ -766,9 +777,6 @@ def _new_rawargs(self, *args, **kwargs):
         kwargs.pop('is_commutative', None)
         return self.func(*args, **kwargs)
 
-    def _coeff_symbol(self, *args, **kwargs):
-        return self.base._coeff_symbol(*args, **kwargs)
-
 
 class diffify(object):
 

devito/types/dense.py

@@ -198,14 +198,6 @@ def coefficients(self):
         """Form of the coefficients of the function."""
         return self._coefficients
 
-    @cached_property
-    def _coeff_symbol(self):
-        if self.coefficients == 'symbolic':
-            return sympy.Function('W')
-        else:
-            raise ValueError("Function was not declared with symbolic "
-                             "coefficients.")
-
     @cached_property
     def shape(self):
         """

tests/test_symbolic_coefficients.py

@@ -380,6 +380,7 @@ def test_aggregate_w_custom_coeffs(self):
 
     def test_cross_derivs(self):
         grid = Grid(shape=(11, 11, 11))
+
         q = TimeFunction(name='q', grid=grid, space_order=8, time_order=2,
                          coefficients='symbolic')
         q0 = TimeFunction(name='q', grid=grid, space_order=8, time_order=2)
@@ -389,3 +390,20 @@ def test_cross_derivs(self):
 
         assert(eq0.evaluate.evalf(_PRECISION).__repr__() ==
                eq1.evaluate.evalf(_PRECISION).__repr__())
+
+    def test_cross_derivs_imperfect(self):
+        grid = Grid(shape=(11, 11, 11))
+
+        p = TimeFunction(name='p', grid=grid, space_order=4, time_order=2,
+                         coefficients='symbolic')
+        q = TimeFunction(name='q', grid=grid, space_order=4, time_order=2,
+                         coefficients='symbolic')
+
+        p0 = TimeFunction(name='p', grid=grid, space_order=4, time_order=2)
+        q0 = TimeFunction(name='q', grid=grid, space_order=4, time_order=2)
+
+        eq0 = Eq(q0.forward, (q0.dx + p0.dx).dy)
+        eq1 = Eq(q.forward, (q.dx + p.dx).dy)
+
+        assert(eq0.evaluate.evalf(_PRECISION).__repr__() ==
+               eq1.evaluate.evalf(_PRECISION).__repr__())

tests/test_unexpansion.py

@@ -22,7 +22,7 @@ def test_backward_dt2(self):
         assert_structure(op, ['t,x,y'], 't,x,y')
 
 
-class TestSymbolicCoefficients(object):
+class TestSymbolicCoeffs(object):
 
     def test_fallback_to_default(self):
         grid = Grid(shape=(8, 8, 8))
@@ -40,12 +40,22 @@ def test_fallback_to_default(self):
 
     def test_numeric_coeffs(self):
         grid = Grid(shape=(11,), extent=(10.,))
+
         u = Function(name='u', grid=grid, coefficients='symbolic', space_order=2)
+        v = Function(name='v', grid=grid, coefficients='symbolic', space_order=2)
 
         coeffs = Substitutions(Coefficient(2, u, grid.dimensions[0], np.zeros(3)))
 
-        op = Operator(Eq(u, u.dx2, coefficients=coeffs), opt=({'expand': False},))
-        op.cfunction
+        opt = ('advanced', {'expand': False})
+
+        # Pure derivative
+        Operator(Eq(u, u.dx2, coefficients=coeffs), opt=opt).cfunction
+
+        # Mixed derivative
+        Operator(Eq(u, u.dx.dx, coefficients=coeffs), opt=opt).cfunction
+
+        # Non-perfect mixed derivative
+        Operator(Eq(u, (u.dx + v.dx).dx, coefficients=coeffs), opt=opt).cfunction
 
 
 class Test1Pass(object):