Add check to check if slots is defined.

Slots prevents the operation of Event and Callback descriptors, this change adds a check for presents of __slots__ and raises an error

Author: timsavage

pyapp/events.py

@@ -2,13 +2,19 @@
 Events
 ~~~~~~
 
-A simple framework publish events or callbacks from a class to subscribed listener(s).
+A simple framework to publish events or callbacks to subscribed listener(s).
 
 Async events/callbacks are also supported via the `AsyncEvent` and `AsyncCallback`
 descriptors.
 
-Event can have multiple listening functions where callbacks can only have a single
-function bound (assigning a second will remove the previous).
+An event can have multiple listening functions where as callbacks can only have
+a single function bound (assigning a second will remove the previous binding).
+
+.. note::
+
+    Event and Callback descriptors do not work when ``__slots__`` are defined.
+    If slots are defined a ``InstanceHasNoDictError`` will be raised on access
+
 
 Example::
 
@@ -49,8 +55,9 @@ async def on_new_message(message: str):
 from typing import TypeVar
 from typing import Union
 
-__all__ = ("Event", "AsyncEvent", "listen_to", "Callback", "AsyncCallback", "bind_to")
+from pyapp.exceptions import UnsupportedObject
 
+__all__ = ("Event", "AsyncEvent", "listen_to", "Callback", "AsyncCallback", "bind_to")
 
 _CT = TypeVar("_CT")
 
@@ -148,6 +155,10 @@ def __get__(self, instance, owner) -> ListenerSet[_CT]:
             return listeners
 
     def __set_name__(self, owner, name):
+        if hasattr(owner, "__slots__"):
+            raise UnsupportedObject(
+                "An Event cannot be used on an object with __slots__ defined."
+            )
         self.name = name  # pylint: disable=attribute-defined-outside-init
 
 
@@ -260,6 +271,10 @@ def __get__(self, instance, owner) -> CallbackBinding[_CT]:
             return wrapper
 
     def __set_name__(self, owner, name):
+        if hasattr(owner, "__slots__"):
+            raise UnsupportedObject(
+                "A Callback cannot be used on an object with __slots__ defined."
+            )
         self.name = name  # pylint: disable=attribute-defined-outside-init
 
 

pyapp/exceptions.py

@@ -71,3 +71,15 @@ class UnsupportedContentType(Exception):
     """
     Content type of the file is not supported
     """
+
+
+class EventException(Exception):
+    """
+    Exception caused by event/callback definition
+    """
+
+
+class UnsupportedObject(EventException, TypeError):
+    """
+    Instance does not have a dict that descriptor can use to store callbacks
+    """

tests/test_events.py

@@ -4,6 +4,7 @@
 import pytest
 
 from pyapp import events
+from pyapp.exceptions import UnsupportedObject
 
 
 class TestListenerSet:
@@ -102,6 +103,14 @@ def on_target():
 
         assert len(instance.target) == 1
 
+    def test__when_object_has_slots(self):
+        with pytest.raises(RuntimeError):
+
+            class MyObject:
+                __slots__ = ("foo",)
+
+                target = events.Event[Callable[[], None]]()
+
 
 class TestAsyncListenerSet:
     def test_call(self, event_loop):
@@ -197,6 +206,14 @@ def on_target():
         target = instance.target
         assert target._callback is on_target
 
+    def test__when_object_has_slots(self):
+        with pytest.raises(RuntimeError):
+
+            class MyObject:
+                __slots__ = ("foo",)
+
+                target = events.Callback[Callable[[], None]]()
+
 
 class TestAsyncCallbackBinding:
     def test_call__bound(self, event_loop):