api: add fallback from wrong sized custom coeffs

Author: mloubout

devito/finite_differences/finite_difference.py

@@ -1,5 +1,6 @@
 from sympy import sympify
 
+from devito.logger import warning
 from .differentiable import EvalDerivative, DiffDerivative, Weights
 from .tools import (left, right, generate_indices, centered, direct, transpose,
                     check_input, fd_weights_registry, process_weights)
@@ -158,6 +159,12 @@ def make_derivative(expr, dim, fd_order, deriv_order, side, matvec, x0, coeffici
     # `coefficients` method (`taylor` or `symbolic`)
     if weights is None:
         weights = fd_weights_registry[coefficients](expr, deriv_order, indices, x0)
+    if len(weights) != len(indices):
+        warning(f"Number of weights ({len(weights)}) does not match "
+                f"number of indices ({len(indices)}), reverting to Taylor")
+        scale = False
+        weights = fd_weights_registry['taylor'](expr, deriv_order, indices, x0)
+
     # Did fd_weights_registry return a new Function/Expression instead of a values?
     _, wdim, _ = process_weights(weights, expr, dim)
     if wdim is not None:

requirements-optional.txt

@@ -3,4 +3,5 @@ pillow>11,<11.3.1
 pyrevolve==2.2.6
 scipy<1.15.4
 distributed<2025.7.1
-click<9.0
+click<9.0
+cloudpickle<3.1.2

requirements-testing.txt

@@ -6,3 +6,4 @@ nbval<0.11.1
 scipy<1.15.4
 pooch<1.8.3
 click<9.0
+cloudpickle<3.1.2

requirements.txt

@@ -7,5 +7,4 @@ cgen>=2020.1,<2026
 codepy>=2019.1,<2025
 multidict<6.3
 anytree>=2.4.3,<=2.13.0
-cloudpickle<3.1.2
 packaging<25.1

tests/test_derivatives.py

@@ -1229,3 +1229,24 @@ def test_expand_product_rule(self):
             + 10*self.f*Derivative(self.v, self.x)*Derivative(self.u, self.x) \
             + 5*self.f*self.u*Derivative(self.v, (self.x, 2))
         assert diffify(expr.expand(product_rule=True)) == expanded
+
+    def test_fallback_wrong_custom_size(self):
+        """
+        Check an exception is raised when a custom size is not compatible
+        with the derivative order
+        """
+        grid = Grid((10,))
+        x, = grid.dimensions
+        u = Function(name="u", grid=grid, space_order=2, staggered=x)
+        v = Function(name="v", grid=grid, space_order=2, staggered=NODE)
+
+        w = [-2, 2]  # Should 4 coeff since this is staggered
+
+        eq0 = Eq(u, v.dx(w=w)).evaluate
+        exp0 = -2 * v / x.spacing + 2 * v._subs(x, x + x.spacing)/x.spacing
+        # This one should fallback to taylor coeffs sinc w is too short
+        # for a centered derivative
+        eq1 = Eq(v, v.dx(w=w)).evaluate
+        exp1 = - .5 * (v._subs(x, x - x.spacing) - v._subs(x, x + x.spacing))/x.spacing
+        assert simplify(eq0.rhs - exp0) == 0
+        assert simplify(eq1.rhs - exp1) == 0