Add get_prior_action to Application

burr/core/application.py

@@ -2037,6 +2037,21 @@ 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 last action that ran, or None if nothing has run yet.
+
+        In a pre_run_step hook this is the action before the current one, in a post_run_step
+        hook it is the action that just finished. For streaming actions it only changes once
+        the stream is fully consumed. Raises a ValueError if the prior action is not in the
+        graph anymore, for example after restoring state saved with an older graph.
+
+        :return: The last action that ran, or None.
+        """
+        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

@@ -3184,6 +3184,49 @@ def test_app_get_next_step():
     assert app.get_next_action().name == "counter_1"
 
 
+def test_app_get_prior_action():
+    counter_action_1 = base_counter_action.with_name("counter_1")
+    counter_action_2 = base_counter_action.with_name("counter_2")
+    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],
+            transitions=[
+                Transition(counter_action_1, counter_action_2, default),
+                Transition(counter_action_2, counter_action_1, default),
+            ],
+        ),
+    )
+    assert app.get_prior_action() is None
+    app.step()
+    assert app.get_prior_action().name == "counter_1"
+    app.step()
+    assert app.get_prior_action().name == "counter_2"
+    app.reset_to_entrypoint()
+    assert app.get_prior_action() is None
+
+
+def test_app_get_prior_action_not_in_graph():
+    counter_action = base_counter_action.with_name("counter")
+    app = Application(
+        state=State({PRIOR_STEP: "removed"}),
+        entrypoint="counter",
+        partition_key="test",
+        uid="test-123",
+        sequence_id=0,
+        graph=Graph(
+            actions=[counter_action],
+            transitions=[Transition(counter_action, counter_action, default)],
+        ),
+    )
+    with pytest.raises(ValueError):
+        app.get_prior_action()
+
+
 def test_application_builder_complete():
     app = (
         ApplicationBuilder()