Honor falsey callable callbacks in WeakFunctionProxy

docs/changes.rst

@@ -100,6 +100,13 @@ Version 2.5.0
 
 **Bugs Fixed**
 
+* A callable callback which evaluated as false, such as an initially empty
+  callable collection, was passed a weak reference instead of the
+  ``wrapt.WeakFunctionProxy`` when its wrapped function expired. Such a
+  callback could also be called twice when both a bound method and its
+  instance expired. These callbacks now receive the proxy and are called
+  only once, as with other callable callbacks.
+
 * Deleting the ``__module__`` or ``__doc__`` attribute of a proxy object,
   using ``del proxy.__doc__`` for example, crashed the Python interpreter
   when the C extension was in use. The attribute setters, which CPython

src/wrapt/weakrefs.py

@@ -65,9 +65,11 @@ def __init__(self, wrapped, callback=None):
         # then call it. Note that we avoid using a nested function for
         # the callback here so as not to cause any odd reference cycles.
 
-        _callback = callback and functools.partial(
-            _weak_function_proxy_callback, proxy=self, callback=callback
-        )
+        _callback = None
+        if callback is not None:
+            _callback = functools.partial(
+                _weak_function_proxy_callback, proxy=self, callback=callback
+            )
 
         self._self_expired = False
         self._self_owner = None

tests/core/test_weak_function_proxy.py

@@ -144,6 +144,43 @@ def callback(proxy):
         self.assertEqual(len(result), 1)
         self.assertEqual(id(proxy), result[0])
 
+    def test_falsey_callback_receives_proxy(self):
+        class Callback(list):
+            def __call__(self, proxy):
+                self.append(id(proxy))
+
+        def function():
+            return 42
+
+        callback = Callback()
+        proxy = wrapt.WeakFunctionProxy(function, callback)
+        self.assertEqual(proxy(), 42)
+        del function
+        gc.collect()
+        self.assertEqual(callback, [id(proxy)])
+
+    @unittest.skipIf(
+        sys.version_info[:2] == (3, 13) and sysconfig.get_config_var("Py_GIL_DISABLED"),
+        "Free-threaded CPython 3.13 immortalizes classes after a thread starts",
+    )
+    def test_falsey_callback_runs_once_when_method_and_instance_expire(self):
+        class Callback(list):
+            def __call__(self, proxy):
+                self.append(id(proxy))
+
+        class Class:
+            def method(self):
+                return 42
+
+        callback = Callback()
+        instance = Class()
+        proxy = wrapt.WeakFunctionProxy(instance.method, callback)
+        self.assertEqual(proxy(), 42)
+        del instance
+        del Class
+        gc.collect()
+        self.assertEqual(callback, [id(proxy)])
+
     def test_instancemethod_delete_instance(self):
         class Class:
             def function(self, a, b):