add post_order

CHANGELOG.md

@@ -1,8 +1,11 @@
 # [Unreleased](https://github.com/pybamm-team/PyBaMM/)
 
 ## Features
+
+- Added `Symbol.post_order()` method to return an iterable that steps through the tree in post-order fashion. ([#4684](https://github.com/pybamm-team/PyBaMM/pull/4684))
 - Added two more submodels (options) for the SEI: Lars von Kolzenberg (2020) model and Tunneling Limit model ([#4394](https://github.com/pybamm-team/PyBaMM/pull/4394))
 
+
 ## Breaking changes
 
 - The conda distribution (`pybamm`) now installs all optional dependencies available on conda-forge. Use the new `pybamm-base` conda

src/pybamm/expression_tree/symbol.py

@@ -570,6 +570,13 @@ def pre_order(self):
         anytree = import_optional_dependency("anytree")
         return anytree.PreOrderIter(self)
 
+    def post_order(self):
+        """
+        returns an iterable that steps through the tree in post-order fashion.
+        """
+        anytree = import_optional_dependency("anytree")
+        return anytree.PostOrderIter(self)
+
     def __str__(self):
         """return a string representation of the node and its children."""
         return self._name

tests/unit/test_expression_tree/test_symbol.py

@@ -227,8 +227,27 @@ def test_multiple_symbols(self):
             "c",
             "a",
         ]
+        expected_postorder = [
+            "a",
+            "c",
+            "*",
+            "a",
+            "b",
+            "*",
+            "c",
+            "*",
+            "a",
+            "+",
+            "c",
+            "a",
+            "*",
+            "-",
+            "*",
+        ]
         for node, expect in zip(exp.pre_order(), expected_preorder):
             assert node.name == expect
+        for node, expect in zip(exp.post_order(), expected_postorder):
+            assert node.name == expect
 
     def test_symbol_diff(self):
         a = pybamm.Symbol("a")