fix(event): only bind an EventCallback when it is read through a state (#7401)

* fix(event): only bind an EventCallback when it is read through a state

* Use starred unpack syntax in EventCallback descriptor types

---------

Co-authored-by: Masen Furer <m_github@0x26.net>

Author: benedikt-bartscher

packages/reflex-base/news/+event-callback-non-state-get.bugfix.md

@@ -0,0 +1 @@
+An event handler stored outside a state, e.g. in a component or dataclass field, now type-checks as the handler instead of a bound function, and a handler read through a state instance is typed as a function returning `Any`.

packages/reflex-base/src/reflex_base/event/__init__.py

@@ -3013,7 +3013,16 @@ def __get__(
     ) -> "EventCallback[*P]": ...
 
     @overload
-    def __get__(self, instance: Any, owner: Any) -> "Callable[[Unpack[P]]]": ...
+    def __get__(
+        self: "EventCallback[*P]", instance: "BaseState", owner: Any
+    ) -> "Callable[[Unpack[P]], Any]": ...
+
+    # Only a state binds its handlers. Anywhere else, e.g. in a component or
+    # dataclass field, a stored callback reads back as the callback itself.
+    @overload
+    def __get__(
+        self: "EventCallback[*P]", instance: object, owner: Any
+    ) -> "EventCallback[*P]": ...
 
     def __get__(self, instance: Any, owner: Any) -> Callable:
         """Get the function with the instance bound to it.

tests/type_checking/event.py

@@ -1,4 +1,4 @@
-"""Calling an event handler on the state class types as the event it creates."""
+"""Inferred types for `reflex_base.event` that are part of the public contract."""
 
 from collections.abc import Callable
 from typing import Any, assert_type
@@ -46,3 +46,22 @@ def four(self, index: int, value: str, flag: bool, count: float) -> None: ...
 # A handler is a callable of its arguments, so it can be passed where one is expected.
 _one: Callable[[int], Any] = _HandlerState.one
 _two: Callable[[int, str], Any] = _HandlerState.two
+
+
+class _State(State):
+    @event
+    def set_name(self, name: str) -> None: ...
+
+
+class _Holder:
+    on_name: EventCallback[str] | None = None
+
+
+def _reads(state: _State, holder: _Holder) -> None:
+    # Read through its state class, a handler is the callback event triggers take.
+    assert_type(_State.set_name, EventCallback[str])
+    # Read through an instance of that state, it is the function bound to it.
+    assert_type(state.set_name, Callable[[str], Any])
+    # Stored anywhere else, e.g. a component or dataclass field, it stays the
+    # callback: only a state instance binds it.
+    assert_type(holder.on_name, EventCallback[str] | None)