fix: do not allow instantiation of State mixins

Closes #4343

reflex/state.py

@@ -386,6 +386,10 @@ class BaseState(Base, ABC, extra=pydantic.Extra.allow):
                 "State classes should not be instantiated directly in a Reflex app. "
                 "See https://reflex.dev/docs/state/ for further information."
             )
+        if type(self)._mixin:
+            raise ReflexRuntimeError(
+                f"{type(self).__name__} is a state mixin and cannot be instantiated directly."
+            )
         kwargs["parent_state"] = parent_state
         super().__init__()
         for name, value in kwargs.items():

tests/units/components/test_component_state.py

@@ -1,7 +1,10 @@
 """Ensure that Components returned by ComponentState.create have independent State classes."""
 
+import pytest
+
 import reflex as rx
 from reflex.components.base.bare import Bare
+from reflex.utils.exceptions import ReflexRuntimeError
 
 
 def test_component_state():
@@ -40,3 +43,21 @@ def test_component_state():
     assert len(cs2.children) == 1
     assert cs2.children[0].render() == Bare.create("b").render()
     assert cs2.id == "b"
+
+
+def test_init_component_state() -> None:
+    """Ensure that ComponentState subclasses cannot be instantiated directly."""
+
+    class CS(rx.ComponentState):
+        @classmethod
+        def get_component(cls, *children, **props):
+            return rx.el.div()
+
+    with pytest.raises(ReflexRuntimeError):
+        CS()
+
+    class SubCS(CS):
+        pass
+
+    with pytest.raises(ReflexRuntimeError):
+        SubCS()

tests/units/test_state.py

@@ -43,7 +43,7 @@ from reflex.state import (
 )
 from reflex.testing import chdir
 from reflex.utils import format, prerequisites, types
-from reflex.utils.exceptions import SetUndefinedStateVarError
+from reflex.utils.exceptions import ReflexRuntimeError, SetUndefinedStateVarError
 from reflex.utils.format import json_dumps
 from reflex.vars.base import Var, computed_var
 from tests.units.states.mutation import MutableSQLAModel, MutableTestState
@@ -3411,3 +3411,19 @@ def test_typed_state() -> None:
         field: rx.Field[str] = rx.field("")
 
     _ = TypedState(field="str")
+
+
+def test_init_mixin() -> None:
+    """Ensure that State mixins can not be instantiated directly."""
+
+    class Mixin(BaseState, mixin=True):
+        pass
+
+    with pytest.raises(ReflexRuntimeError):
+        Mixin()
+
+    class SubMixin(Mixin, mixin=True):
+        pass
+
+    with pytest.raises(ReflexRuntimeError):
+        SubMixin()