Implement lazy initialization and improve copying for Delegate provider

src/dependency_injector/providers.pyi

@@ -94,7 +94,7 @@ class Delegate(Provider[Provider]):
     def __init__(self, provides: Optional[Provider] = None) -> None: ...
     @property
     def provides(self) -> Optional[Provider]: ...
-    def set_provides(self, provides: Optional[Provider]): ...
+    def set_provides(self, provides: Optional[Provider]) -> Delegate: ...
 
 
 class Dependency(Provider[T]):

src/dependency_injector/providers.pyx

@@ -520,11 +520,7 @@ cdef class Delegate(Provider):
     """
 
     def __init__(self, provides=None):
-        """Initializer.
-
-        :param provides: Value that have to be provided.
-        :type provides: :py:class:`Provider`
-        """
+        """Initialize provider."""
         self.__provides = None
         self.set_provides(provides)
         super(Delegate, self).__init__()
@@ -535,11 +531,13 @@ cdef class Delegate(Provider):
         if copied is not None:
             return copied
 
-        copied = self.__class__()
-        memo[id(self)] = copied
+        provides = self.provides
+        if provides:
+            provides = deepcopy(self.provides, memo)
+
+        copied = _memorized_duplicate(self, memo)
+        copied.set_provides(provides)
 
-        if self.provides:
-            copied.set_provides(deepcopy(self.provides, memo))
         self._copy_overridings(copied, memo)
 
         return copied
@@ -560,12 +558,15 @@ cdef class Delegate(Provider):
 
     @property
     def provides(self):
+        """Return provider's provides."""
         return self.__provides
 
     def set_provides(self, provides):
+        """Set provider's provides."""
         if provides:
             provides = ensure_is_provider(provides)
         self.__provides = provides
+        return self
 
     @property
     def related(self):

tests/unit/providers/test_base_py2_py3.py

@@ -291,6 +291,16 @@ class DelegateTests(unittest.TestCase):
     def test_is_provider(self):
         self.assertTrue(providers.is_provider(self.delegate))
 
+    def test_init_optional_provides(self):
+        provider = providers.Delegate()
+        provider.set_provides(self.delegated)
+        self.assertIs(provider.provides, self.delegated)
+        self.assertIs(provider(), self.delegated)
+
+    def test_set_provides_returns_self(self):
+        provider = providers.Delegate()
+        self.assertIs(provider.set_provides(self.delegated), provider)
+
     def test_init_with_not_provider(self):
         self.assertRaises(errors.Error, providers.Delegate, object())