feat(core): add get_prior_action() to Application

burr/core/application.py

@@ -2034,6 +2034,20 @@ def _set_state(self, new_state: State[ApplicationStateType]):
     def get_next_action(self) -> Optional[Action]:
         return self._graph.get_next_node(self._state.get(PRIOR_STEP), self._state, self.entrypoint)
 
+    def get_prior_action(self) -> Optional[Action]:
+        """Returns the action that was executed in the prior step.
+
+        This is the symmetric counterpart to :py:meth:`get_next_action`.
+        Returns ``None`` if no step has been executed yet (i.e. the application
+        is still at its entrypoint and has not run any action).
+
+        :return: The last-executed :py:class:`Action`, or ``None`` if no step has run.
+        """
+        prior_step = self._state.get(PRIOR_STEP)
+        if prior_step is None:
+            return None
+        return self._graph.get_action(prior_step)
+
     def update_state(self, new_state: State[ApplicationStateType]):
         """Updates state -- this is meant to be called if you need to do
         anything with the state. For example:

tests/core/test_application.py

@@ -3128,6 +3128,38 @@ def test_app_get_next_step():
     assert app.get_next_action().name == "counter_1"
 
 
+def test_app_get_prior_step():
+    counter_action_1 = base_counter_action.with_name("counter_1")
+    counter_action_2 = base_counter_action.with_name("counter_2")
+    counter_action_3 = base_counter_action.with_name("counter_3")
+    app = Application(
+        state=State(),
+        entrypoint="counter_1",
+        partition_key="test",
+        uid="test-123",
+        sequence_id=0,
+        graph=Graph(
+            actions=[counter_action_1, counter_action_2, counter_action_3],
+            transitions=[
+                Transition(counter_action_1, counter_action_2, default),
+                Transition(counter_action_2, counter_action_3, default),
+                Transition(counter_action_3, counter_action_1, default),
+            ],
+        ),
+    )
+    # uninitialized -- no prior step yet
+    assert app.get_prior_action() is None
+    app.step()
+    # ran counter_1
+    assert app.get_prior_action().name == "counter_1"
+    app.step()
+    # ran counter_2
+    assert app.get_prior_action().name == "counter_2"
+    app.step()
+    # ran counter_3
+    assert app.get_prior_action().name == "counter_3"
+
+
 def test_application_builder_complete():
     app = (
         ApplicationBuilder()