Implement lazy initialization and improve copying for FactoryAggregate provider

src/dependency_injector/providers.pyi

@@ -270,6 +270,7 @@ class FactoryAggregate(Provider):
 
     @property
     def factories(self) -> _Dict[str, Factory]: ...
+    def set_factories(self, **factories: Factory) -> FactoryAggregate: ...
 
 
 class BaseSingleton(Provider[T]):

src/dependency_injector/providers.pyx

@@ -2377,29 +2377,19 @@ cdef class FactoryAggregate(Provider):
     __IS_DELEGATED__ = True
 
     def __init__(self, **factories):
-        """Initializer.
-
-        :param factories: Dictionary of aggregate factories.
-        :type factories: dict[str, :py:class:`Factory`]
-        """
-        for factory in factories.values():
-            if isinstance(factory, Factory) is False:
-                raise Error(
-                    '{0} can aggregate only instances of {1}, given - {2}'
-                    .format(self.__class__, Factory, factory))
-        self.__factories = factories
+        """Initialize provider."""
+        self.__factories = {}
+        self.set_factories(**factories)
         super(FactoryAggregate, self).__init__()
 
     def __deepcopy__(self, memo):
         """Create and return full copy of provider."""
-        cdef FactoryAggregate copied
-
         copied = memo.get(id(self))
         if copied is not None:
             return copied
 
-        copied = self.__class__()
-        copied.__factories = deepcopy(self.__factories, memo)
+        copied = _memorized_duplicate(self, memo)
+        copied.set_factories(**deepcopy(self.factories, memo))
 
         self._copy_overridings(copied, memo)
 
@@ -2421,6 +2411,16 @@ cdef class FactoryAggregate(Provider):
         """Return dictionary of factories, read-only."""
         return self.__factories
 
+    def set_factories(self, **factories):
+        """Set factories."""
+        for factory in factories.values():
+            if isinstance(factory, Factory) is False:
+                raise Error(
+                    '{0} can aggregate only instances of {1}, given - {2}'
+                    .format(self.__class__, Factory, factory))
+        self.__factories = factories
+        return self
+
     def override(self, _):
         """Override provider with another provider.
 

tests/unit/providers/test_factories_py2_py3.py

@@ -512,6 +512,26 @@ class FactoryAggregateTests(unittest.TestCase):
                 example_a=providers.Factory(self.ExampleA),
                 example_b=object())
 
+    def test_init_optional_factories(self):
+        provider = providers.FactoryAggregate()
+        provider.set_factories(
+            example_a=self.example_a_factory,
+            example_b=self.example_b_factory,
+        )
+        self.assertEqual(
+            provider.factories,
+            {
+                'example_a': self.example_a_factory,
+                'example_b': self.example_b_factory,
+            },
+        )
+        self.assertIsInstance(provider('example_a'), self.ExampleA)
+        self.assertIsInstance(provider('example_b'), self.ExampleB)
+
+    def test_set_provides_returns_self(self):
+        provider = providers.FactoryAggregate()
+        self.assertIs(provider.set_factories(example_a=self.example_a_factory), provider)
+
     def test_call(self):
         object_a = self.factory_aggregate('example_a',
                                           1, 2, init_arg3=3, init_arg4=4)