From 881afcbde44a7d72cdba4a39ec673b462ece8313 Mon Sep 17 00:00:00 2001
From: mloubout <mathias.louboutin@gmail.com>
Date: Tue, 12 Mar 2024 20:25:40 -0400
Subject: [PATCH] api: add priority to fd coefficients for mixed derivatives

---
 devito/finite_differences/differentiable.py | 9 +++------
 devito/finite_differences/tools.py          | 1 +
 tests/test_symbolic_coefficients.py         | 7 +++++++
 3 files changed, 11 insertions(+), 6 deletions(-)

diff --git a/devito/finite_differences/differentiable.py b/devito/finite_differences/differentiable.py
index ee3e2ba38b..d2df797df6 100644
--- a/devito/finite_differences/differentiable.py
+++ b/devito/finite_differences/differentiable.py
@@ -11,7 +11,7 @@
 from sympy.core.decorators import call_highest_priority
 from sympy.core.evalf import evalf_table
 
-from devito.finite_differences.tools import make_shift_x0
+from devito.finite_differences.tools import make_shift_x0, coeff_priority
 from devito.logger import warning
 from devito.tools import (as_tuple, filter_ordered, flatten, frozendict,
                           infer_dtype, is_integer, split)
@@ -130,11 +130,8 @@ def coefficients(self):
         coefficients = {f.coefficients for f in self._functions}
         # If there is multiple ones, we have to revert to the highest priority
         # i.e we have to remove symbolic
-        if len(coefficients) == 2:
-            return (coefficients - {'symbolic'}).pop()
-        else:
-            assert len(coefficients) == 1
-            return coefficients.pop()
+        key = lambda x: coeff_priority[x]
+        return sorted(coefficients, key=key, reverse=True)[0]
 
     @cached_property
     def _coeff_symbol(self, *args, **kwargs):
diff --git a/devito/finite_differences/tools.py b/devito/finite_differences/tools.py
index 0aa56f316b..a52b9625be 100644
--- a/devito/finite_differences/tools.py
+++ b/devito/finite_differences/tools.py
@@ -248,6 +248,7 @@ def numeric_weights(function, deriv_order, indices, x0):
 
 fd_weights_registry = {'taylor': numeric_weights, 'standard': numeric_weights,
                        'symbolic': symbolic_weights}
+coeff_priority = {'taylor': 1, 'standard': 1, 'symbolic': 0}
 
 
 def generate_indices(expr, dim, order, side=None, matvec=None, x0=None):
diff --git a/tests/test_symbolic_coefficients.py b/tests/test_symbolic_coefficients.py
index 1b66aac3e5..abbdbdb3b1 100644
--- a/tests/test_symbolic_coefficients.py
+++ b/tests/test_symbolic_coefficients.py
@@ -510,3 +510,10 @@ def test_compound_nested_subs(self):
 
         # `str` simply because some objects are of type EvalDerivative
         assert str(eq.evaluate.rhs) == str(term0 + term1 + term2)
+
+    def test_priority(self):
+        grid = Grid(shape=(11,))
+        m = Function(name='m', grid=grid, space_order=2, coefficients='symbolic')
+        p = Function(name='p', grid=grid, space_order=2)
+
+        assert (p*m).coefficients == 'taylor'