fix dumping of empty YAMLObject on Python 3.11+

* translate __getstate__ returning `None` to empty mapping (arguably wrong, but backward-compatibie)
* adds constructor/representer tests to exercise this case

lib/yaml/constructor.py

@@ -416,6 +416,7 @@ def construct_yaml_map(self, node):
     def construct_yaml_object(self, node, cls):
         data = cls.__new__(cls)
         yield data
+        # FIXME: __getstate__/__setstate__ support non-mapping state
         if hasattr(data, '__setstate__'):
             state = self.construct_mapping(node, deep=True)
             data.__setstate__(state)

lib/yaml/representer.py

@@ -224,7 +224,19 @@ def represent_yaml_object(self, tag, data, cls, flow_style=None):
         if hasattr(data, '__getstate__'):
             state = data.__getstate__()
         else:
+            # FIXME: this isn't always possible (eg, __slots__)
             state = data.__dict__.copy()
+
+        # fix for https://github.com/yaml/pyyaml/issues/692
+        # __getstate__() has always supported non-mapping state blobs, and as of Python 3.11 returns None by default for
+        # objects with no instance attrs. Ideally, we'd attempt to serialize and round-trip a null scalar for full
+        # fidelity with the types supported by __getstate__/__setstate__, but we can't be guaranteed that it'll be
+        # interpreted as such. For compatibility with existing behavior on older Pythons, we'll continue to represent
+        # this case as an empty mapping.
+        # (see https://github.com/python/cpython/issues/70766 for a specific case)
+        if state is None:
+            state = {}
+
         return self.represent_mapping(tag, state, flow_style=flow_style)
 
     def represent_undefined(self, data):

tests/data/construct-python-object.code

@@ -1,4 +1,5 @@
 [
+EmptyObject(),
 AnObject(1, 'two', [3,3,3]),
 AnInstance(1, 'two', [3,3,3]),
 

tests/data/construct-python-object.data

@@ -1,3 +1,4 @@
+- !!python/object:test_constructor.EmptyObject {}
 - !!python/object:test_constructor.AnObject { foo: 1, bar: two, baz: [3,3,3] }
 - !!python/object:test_constructor.AnInstance { foo: 1, bar: two, baz: [3,3,3] }
 

tests/lib/test_constructor.py

@@ -15,7 +15,7 @@ def execute(code):
 
 def _make_objects():
     global MyLoader, MyDumper, MyTestClass1, MyTestClass2, MyTestClass3, YAMLObject1, YAMLObject2,  \
-            AnObject, AnInstance, AState, ACustomState, InitArgs, InitArgsWithState,    \
+            EmptyObject, AnObject, AnInstance, AState, ACustomState, InitArgs, InitArgsWithState,    \
             NewArgs, NewArgsWithState, Reduce, ReduceWithState, Slots, MyInt, MyList, MyDict,  \
             FixedOffset, today, execute, MyFullLoader
 
@@ -128,6 +128,18 @@ def __eq__(self, other):
             return type(self) is type(other) and    \
                     (self.foo, self.bar, self.baz) == (other.foo, other.bar, other.baz)
 
+    # Python 3.11+ implements __getstate__() returning `None` on objects with no attrs https://github.com/python/cpython/issues/70766
+    class EmptyObject(yaml.YAMLObject):
+        yaml_tag = '!emptyobj'
+
+        def __eq__(self, other):
+            return type(other) is EmptyObject
+
+        # simulate Python 3.11 behavior with a generated __getstate__ returning None on objects with no attrs
+        def __getstate__(self):
+            return None
+
+
     class AnInstance:
         def __init__(self, foo=None, bar=None, baz=None):
             self.foo = foo