api: Fix custom coefficients inlining

Author: FabioLuporini

devito/finite_differences/finite_difference.py

@@ -162,7 +162,11 @@ def make_derivative(expr, dim, fd_order, deriv_order, side, matvec, x0, coeffici
         weights = [weights._subs(wdim, i) for i in range(len(indices))]
 
     # Enforce fixed precision FD coefficients to avoid variations in results
-    weights = [sympify(w).evalf(_PRECISION) for w in weights]
+    if scale:
+        scale = dim.spacing**(-deriv_order)
+    else:
+        scale = 1
+    weights = [sympify(scale * w).evalf(_PRECISION) for w in weights]
 
     # Transpose the FD, if necessary
     if matvec == transpose:
@@ -208,7 +212,4 @@ def make_derivative(expr, dim, fd_order, deriv_order, side, matvec, x0, coeffici
 
         deriv = EvalDerivative(*terms, base=expr)
 
-    if scale:
-        deriv = dim.spacing**(-deriv_order) * deriv
-
     return deriv

tests/test_unexpansion.py

@@ -105,6 +105,29 @@ def test_legacy_api(self, order, nweight):
             op = Operator(eqn, opt=('advanced', {'expand': False}))
             assert f'{wdef} = {wstr}' in str(op)
 
+    def test_legacy_api_v2(self):
+        grid = Grid(shape=(10, 10, 10))
+        x, y, z = grid.dimensions
+
+        u = TimeFunction(name='u', grid=grid, space_order=4)
+
+        cc = np.array([2, 2, 2, 2, 2])
+        coeffs = [Coefficient(1, u, d, cc) for d in grid.dimensions]
+        coeffs = Substitutions(*coeffs)
+
+        eq0 = Eq(u.forward, u.dx.dz + 1.0)
+        eq1 = Eq(u.forward, u.dx.dz + 1.0, coefficients=coeffs)
+
+        op0 = Operator(eq0, opt=('advanced', {'expand': False}))
+        op1 = Operator(eq1, opt=('advanced', {'expand': False}))
+
+        assert (op0._profiler._sections['section0'].sops ==
+                op1._profiler._sections['section0'].sops)
+        weights = [i for i in FindSymbols().visit(op1) if isinstance(i, Weights)]
+        w0, w1 = sorted(weights, key=lambda i: i.name)
+        assert all(i.args[1] == 1/x.spacing for i in w0.weights)
+        assert all(i.args[1] == 1/z.spacing for i in w1.weights)
+
 
 class Test1Pass: