WIP nested class support

* could be done with existing "all dots" tag syntax instead of the `@` separator, but discarding the explicit distinction between the module/package and type name makes the import/construction logic a lot less precise when dealing with nested types.

lib/yaml/constructor.py

@@ -541,7 +541,9 @@ def find_python_name(self, name, mark, unsafe=False):
         if not name:
             raise ConstructorError("while constructing a Python object", mark,
                     "expected non-empty name appended to the tag", mark)
-        if '.' in name:
+        if '@' in name:  # handles nested objects via __qualname__
+            module_name, object_name = name.rsplit('@', 1)
+        elif '.' in name:  # handle old-style references
             module_name, object_name = name.rsplit('.', 1)
         else:
             module_name = 'builtins'
@@ -556,11 +558,16 @@ def find_python_name(self, name, mark, unsafe=False):
             raise ConstructorError("while constructing a Python object", mark,
                     "module %r is not imported" % module_name, mark)
         module = sys.modules[module_name]
-        if not hasattr(module, object_name):
+
+        # descend multi-part object_name to support nested classes
+        cur_obj = module
+        for attr in object_name.split('.'):
+            cur_obj = getattr(cur_obj, attr, None)
+        if not cur_obj:
             raise ConstructorError("while constructing a Python object", mark,
                     "cannot find %r in the module %r"
-                    % (object_name, module.__name__), mark)
-        return getattr(module, object_name)
+                    % (object_name, module_name), mark)
+        return cur_obj
 
     def construct_python_name(self, suffix, node):
         value = self.construct_scalar(node)

lib/yaml/representer.py

@@ -336,24 +336,32 @@ def represent_object(self, data):
         else:
             tag = 'tag:yaml.org,2002:python/object/apply:'
             newobj = False
-        function_name = '%s.%s' % (function.__module__, function.__name__)
-        if not args and not listitems and not dictitems \
-                and isinstance(state, dict) and newobj:
-            return self.represent_mapping(
-                    'tag:yaml.org,2002:python/object:'+function_name, state)
-        if not listitems and not dictitems  \
-                and isinstance(state, dict) and not state:
-            return self.represent_sequence(tag+function_name, args)
+
         value = {}
-        if args:
-            value['args'] = args
-        if state or not isinstance(state, dict):
-            value['state'] = state
-        if listitems:
-            value['listitems'] = listitems
-        if dictitems:
-            value['dictitems'] = dictitems
-        return self.represent_mapping(tag+function_name, value)
+
+        represent_impl = self.represent_mapping
+
+        if not args and not listitems and not dictitems and isinstance(state, dict) and newobj:
+            # object supports simple object state w/ __newobj__
+            tag = 'tag:yaml.org,2002:python/object:'
+            value = state
+        elif not listitems and not dictitems and isinstance(state, dict) and not state:
+            value = args
+            represent_impl = self.represent_sequence
+        else:
+            if args:
+                value['args'] = args
+            if state or not isinstance(state, dict):
+                value['state'] = state
+            if listitems:
+                value['listitems'] = listitems
+            if dictitems:
+                value['dictitems'] = dictitems
+
+        type_qualname = getattr(function, '__qualname__', getattr(function, '__name__', None))
+        type_separator = '@' if '.' in type_qualname else '.'  # if nested class, use @ in tag to disambiguate module name and object qualname
+        tag = f'{tag}{function.__module__}{type_separator}{type_qualname}'
+        return represent_impl(tag, value)
 
     def represent_ordered_dict(self, data):
         # Provide uniform representation across different Python versions.

tests/legacy_tests/data/construct-python-object.code

@@ -2,6 +2,8 @@
 AnObject(1, 'two', [3,3,3]),
 AnInstance(1, 'two', [3,3,3]),
 
+NestedOuterObject.NestedInnerObject1.NestedInnerObject2.NestedInnerObject3('hi mom'),
+
 AnObject(1, 'two', [3,3,3]),
 AnInstance(1, 'two', [3,3,3]),
 

tests/legacy_tests/data/construct-python-object.data

@@ -1,5 +1,6 @@
 - !!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] }
+- !!python/object:test_constructor@NestedOuterObject.NestedInnerObject1.NestedInnerObject2.NestedInnerObject3 { data: hi mom }
 
 - !!python/object/new:test_constructor.AnObject { args: [1, two], kwds: {baz: [3,3,3]} }
 - !!python/object/apply:test_constructor.AnInstance { args: [1, two], kwds: {baz: [3,3,3]} }

tests/legacy_tests/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,    \
+            AnObject, AnInstance, NestedOuterObject, AState, ACustomState, InitArgs, InitArgsWithState,    \
             NewArgs, NewArgsWithState, Reduce, ReduceWithState, Slots, MyInt, MyList, MyDict,  \
             FixedOffset, today, execute, MyFullLoader
 
@@ -128,6 +128,16 @@ def __eq__(self, other):
             return type(self) is type(other) and    \
                     (self.foo, self.bar, self.baz) == (other.foo, other.bar, other.baz)
 
+    class NestedOuterObject:
+        class NestedInnerObject1:
+            class NestedInnerObject2:
+                class NestedInnerObject3:
+                    def __init__(self, data):
+                        self.data = data
+                    def __eq__(self, other):
+                        return type(self) is type(other) and self.data == other.data
+
+
     class AnInstance:
         def __init__(self, foo=None, bar=None, baz=None):
             self.foo = foo