fix: Add descriptive exception when fail to add instance to weakref d…

…ictionary (#189)

* add weakref information and test

* more information

* Update src/superqt/utils/_throttler.py

---------

Co-authored-by: Talley Lambert <[email protected]>

src/superqt/utils/_throttler.py

@@ -259,7 +259,15 @@ def _get_throttler(self, instance, owner, parent, obj):
                 throttler,
             )
         except AttributeError:
-            self._obj_dkt[obj] = throttler
+            try:
+                self._obj_dkt[obj] = throttler
+            except TypeError as e:
+                raise TypeError(
+                    "To use qthrottled or qdebounced as a method decorator, "
+                    "objects must have  `__dict__` or be weak referenceable. "
+                    "Please either add `__weakref__` to `__slots__` or use"
+                    "qthrottled/qdebounced as a function (not a decorator)."
+                ) from e
         return throttler
 
     def __get__(self, instance, owner):

tests/test_throttler.py

@@ -87,6 +87,41 @@ def call2():
     mock2.assert_called_once()
 
 
+def test_class_with_slots(qtbot):
+    class A:
+        __slots__ = ("count", "__weakref__")
+
+        def __init__(self):
+            self.count = 0
+
+        @qdebounced(timeout=4)
+        def callback(self):
+            self.count += 1
+
+    a = A()
+    for _ in range(10):
+        a.callback()
+
+    qtbot.wait(5)
+    assert a.count == 1
+
+
+@pytest.mark.usefixtures("qapp")
+def test_class_with_slots_except():
+    class A:
+        __slots__ = ("count",)
+
+        def __init__(self):
+            self.count = 0
+
+        @qdebounced(timeout=4)
+        def callback(self):
+            self.count += 1
+
+    with pytest.raises(TypeError, match="To use qthrottled or qdebounced"):
+        A().callback()
+
+
 def test_throttled(qtbot):
     mock1 = Mock()
     mock2 = Mock()