feat(EvaluateAt): add support for children evaluated at edges

src/pybamm/expression_tree/unary_operators.py

@@ -1288,6 +1288,9 @@ def _evaluate_for_shape(self):
         """See :meth:`pybamm.Symbol.evaluate_for_shape_using_domain()`"""
         return pybamm.evaluate_for_shape_using_domain(self.domains)
 
+    def _evaluates_on_edges(self, dimension: str) -> bool:
+        return False
+
 
 class UpwindDownwind(SpatialOperator):
     """

src/pybamm/spatial_methods/finite_volume.py

@@ -1357,7 +1357,10 @@ def evaluate_at(self, symbol, discretised_child, position):
         # Get mesh nodes
         domain = discretised_child.domain
         mesh = self.mesh[domain]
-        nodes = mesh.nodes
+        if symbol.children[0].evaluates_on_edges("primary"):
+            nodes = mesh.edges
+        else:
+            nodes = mesh.nodes
         if hasattr(mesh, "length"):
             domain = discretised_child.domain
             raise NotImplementedError(
@@ -1369,7 +1372,7 @@ def evaluate_at(self, symbol, discretised_child, position):
         index = np.argmin(np.abs(nodes - position.value))
 
         # Create a sparse matrix with a 1 at the index
-        sub_matrix = csr_matrix(([1], ([0], [index])), shape=(1, mesh.npts))
+        sub_matrix = csr_matrix(([1], ([0], [index])), shape=(1, len(nodes)))
         # repeat across auxiliary domains
         matrix = csr_matrix(kron(eye(repeats), sub_matrix))
 

tests/unit/test_spatial_methods/test_finite_volume/test_finite_volume.py

@@ -673,6 +673,22 @@ def test_evaluate_at(self):
         y = np.arange(n)[:, np.newaxis]
         assert evaluate_at_disc.evaluate(y=y) == y[idx]
 
+        disc.bcs = {
+            var: {
+                "left": (pybamm.Scalar(0), "Dirichlet"),
+                "right": (pybamm.Scalar(n - 1), "Dirichlet"),
+            }
+        }
+        position = pybamm.Scalar(mesh["negative electrode"].edges[-1])
+        downwind_var = pybamm.downwind(var)
+        evaluate_at = pybamm.EvaluateAt(downwind_var, position)
+
+        assert evaluate_at.evaluates_on_edges("primary") is False
+
+        evaluate_at_disc = disc.process_symbol(evaluate_at)
+
+        assert evaluate_at_disc.evaluate(y=y) == n - 1
+
         mesh = get_mesh_for_testing_symbolic()
         spatial_methods = {"domain": pybamm.FiniteVolume()}
         var = pybamm.Variable("var", domain="domain")