events: move arity check to runtime

We cannot add attributes to a method, and I do not want to wrap the
callback function or to modify the callback function set, so I
moved the check to the runtime. It should not change the performance.

lib/level/events.py

@@ -30,17 +30,11 @@ class EventList:
 
         :param event: The name of the event to listen.
         :param function: The function to call on update.
-                         If the function arity is 2, then on update,
-                         it will be called with the variable name and its
-                         new value. If the function arity is 1, it will be
-                         called with the updated value.
-
+        If the function arity is 2, then on update,
+        it will be called with the variable name and its
+        new value. If the function arity is 1, it will be
+        called with the updated value.
         """
-        # Checking the function arity
-        arity = len(signature(function).parameters)
-        if arity not in [1, 2]:
-            raise Exception(f"Wrong arity for event callback funtion {function.__name__}")
-        function._arity = arity
         self.events[event].add(function)
 
     def disconnect(
@@ -67,7 +61,11 @@ class EventList:
         """
         for variable in variable_list:
             for function in self.events[f"value_change:{variable}"]:
-                if function._arity == 1:
+                # Checking the function arity
+                arity = len(signature(function).parameters)
+                if arity not in [1, 2]:
+                    raise RuntimeError(f"Wrong arity for event callback funtion {function.__name__}")
+                if arity == 1:
                     function(self.variables[variable])
                 else:
                     function(variable, self.variables[variable])