BaseState.get_var_value helper to get a value from a Var

reflex/state.py

@@ -107,6 +107,7 @@
     StateSchemaMismatchError,
     StateSerializationError,
     StateTooLargeError,
+    UnretrievableVarValueError,
 )
 from reflex.utils.exec import is_testing_env
 from reflex.utils.serializers import serializer
@@ -143,6 +144,9 @@
     ValueError,
 )
 
+# For BaseState.get_var_value
+VAR_TYPE = TypeVar("VAR_TYPE")
+
 
 def _no_chain_background_task(
     state_cls: Type["BaseState"], name: str, fn: Callable
@@ -1596,6 +1600,42 @@ async def get_state(self, state_cls: Type[BaseState]) -> BaseState:
         # Slow case - fetch missing parent states from redis.
         return await self._get_state_from_redis(state_cls)
 
+    async def get_var_value(self, var: Var[VAR_TYPE]) -> VAR_TYPE:
+        """Get the value of an rx.Var from another state.
+
+        Args:
+            var: The var to get the value for.
+
+        Returns:
+            The value of the var.
+
+        Raises:
+            UnretrievableVarValueError: If the var does not have a literal value
+                or associated state.
+        """
+        # Oopsie case: you didn't give me a Var... so get what you give.
+        if not isinstance(var, Var):
+            return var  # type: ignore
+
+        # Fast case: this is a literal var and the value is known.
+        if hasattr(var, "_var_value"):
+            return var._var_value
+
+        var_data = var._get_all_var_data()
+        if var_data is None or not var_data.state:
+            raise UnretrievableVarValueError(
+                f"Unable to retrieve value for {var._js_expr}: not associated with any state."
+            )
+        # Fastish case: this var belongs to this state
+        if var_data.state == self.get_full_name():
+            return getattr(self, var_data.field_name)
+
+        # Slow case: this var belongs to another state
+        other_state = await self.get_state(
+            self._get_root_state().get_class_substate(var_data.state)
+        )
+        return getattr(other_state, var_data.field_name)
+
     def _get_event_handler(
         self, event: Event
     ) -> tuple[BaseState | StateProxy, EventHandler]:

reflex/utils/exceptions.py

@@ -187,3 +187,7 @@ def raise_system_package_missing_error(package: str) -> NoReturn:
 
 class InvalidLockWarningThresholdError(ReflexError):
     """Raised when an invalid lock warning threshold is provided."""
+
+
+class UnretrievableVarValueError(ReflexError):
+    """Raised when the value of a var is not retrievable."""

tests/units/test_state.py

@@ -60,6 +60,7 @@
     ReflexRuntimeError,
     SetUndefinedStateVarError,
     StateSerializationError,
+    UnretrievableVarValueError,
 )
 from reflex.utils.format import json_dumps
 from reflex.vars.base import Var, computed_var
@@ -115,7 +116,7 @@ class TestState(BaseState):
     # Set this class as not test one
     __test__ = False
 
-    num1: int
+    num1: rx.Field[int]
     num2: float = 3.14
     key: str
     map_key: str = "a"
@@ -163,7 +164,7 @@ class ChildState(TestState):
     """A child state fixture."""
 
     value: str
-    count: int = 23
+    count: rx.Field[int] = rx.field(23)
 
     def change_both(self, value: str, count: int):
         """Change both the value and count.
@@ -1663,7 +1664,7 @@ async def state_manager(request) -> AsyncGenerator[StateManager, None]:
 
 
 @pytest.fixture()
-def substate_token(state_manager, token):
+def substate_token(state_manager, token) -> str:
     """A token + substate name for looking up in state manager.
 
     Args:
@@ -3764,3 +3765,32 @@ async def test_upcast_event_handler_arg(handler, payload):
     state = UpcastState()
     async for update in state._process_event(handler, state, payload):
         assert update.delta == {UpcastState.get_full_name(): {"passed": True}}
+
+
+@pytest.mark.asyncio
+async def test_get_var_value(state_manager: StateManager, substate_token: str):
+    """Test that get_var_value works correctly.
+
+    Args:
+        state_manager: The state manager to use.
+        substate_token: Token for the substate used by state_manager.
+    """
+    state = await state_manager.get_state(substate_token)
+
+    # State Var from same state
+    assert await state.get_var_value(TestState.num1) == 0
+    state.num1 = 42
+    assert await state.get_var_value(TestState.num1) == 42
+
+    # State Var from another state
+    child_state = await state.get_state(ChildState)
+    assert await state.get_var_value(ChildState.count) == 23
+    child_state.count = 66
+    assert await state.get_var_value(ChildState.count) == 66
+
+    # LiteralVar with known value
+    assert await state.get_var_value(rx.Var.create([1, 2, 3])) == [1, 2, 3]
+
+    # Generic Var with no state
+    with pytest.raises(UnretrievableVarValueError):
+        await state.get_var_value(rx.Var("undefined"))