Add parsing of nested containers to declarative container

src/dependency_injector/containers.pyx

@@ -144,6 +144,12 @@ class DeclarativeContainerMetaClass(type):
         cdef tuple inherited_providers
         cdef type cls
 
+        containers = tuple((name, container)
+                           for name, container in six.iteritems(attributes)
+                           if is_container(container))
+
+        attributes['containers'] = dict(containers)
+
         cls_providers = tuple((name, provider)
                               for name, provider in six.iteritems(attributes)
                               if isinstance(provider, Provider))
@@ -229,6 +235,12 @@ class DeclarativeContainer(object):
     :type: type
     """
 
+    containers = dict()
+    """Read-only dictionary of all nested containers.
+
+    :type: dict[str, :py:class:`DeclarativeContainer`]
+    """
+
     providers = dict()
     """Read-only dictionary of all providers.
 

tests/unit/containers/test_declarative.py

@@ -250,3 +250,18 @@ class DeclarativeContainerTests(unittest.TestCase):
 
         self.assertEqual(_Container1.p13(), 11)
         self.assertEqual(_Container2.p13(), 22)
+
+    def test_containers_attribute(self):
+        class Container(containers.DeclarativeContainer):
+            class Container1(containers.DeclarativeContainer):
+                pass
+
+            class Container2(containers.DeclarativeContainer):
+                pass
+
+            Container3 = containers.DynamicContainer()
+
+        self.assertEqual(Container.containers,
+                         dict(Container1=Container.Container1,
+                              Container2=Container.Container2,
+                              Container3=Container.Container3))