Allow bound method as event handler (reflex-dev#4348)

* subtract 1 arg if the method is a bound method

* fix it early in user_args

* only bound methods pls

* add test

Author: benedikt-bartscher

reflex/event.py

@@ -1346,6 +1346,10 @@ def check_fn_match_arg_spec(
         EventFnArgMismatch: Raised if the number of mandatory arguments do not match
     """
     user_args = inspect.getfullargspec(user_func).args
+    # Drop the first argument if it's a bound method
+    if inspect.ismethod(user_func) and user_func.__self__ is not None:
+        user_args = user_args[1:]
+
     user_default_args = inspect.getfullargspec(user_func).defaults
     number_of_user_args = len(user_args) - number_of_bound_args
     number_of_user_default_args = len(user_default_args) if user_default_args else 0

tests/units/test_event.py

@@ -2,6 +2,7 @@
 
 import pytest
 
+import reflex as rx
 from reflex.event import (
     Event,
     EventChain,
@@ -439,3 +440,17 @@ def _args_spec(value: Var[int]) -> tuple[Var[int]]:
     # Ensure chain carries _var_data
     chain_var = Var.create(EventChain(events=[S.s(S.x)], args_spec=_args_spec))
     assert chain_var._get_all_var_data() == S.x._get_all_var_data()
+
+
+def test_event_bound_method() -> None:
+    class S(BaseState):
+        @event
+        def e(self, arg: str):
+            print(arg)
+
+    class Wrapper:
+        def get_handler(self, arg: str):
+            return S.e(arg)
+
+    w = Wrapper()
+    _ = rx.input(on_change=w.get_handler)