Implement lazy initialization and improve copying for Selector provider

src/dependency_injector/providers.pyi

@@ -389,10 +389,16 @@ class Container(Provider[T]):
 
 
 class Selector(Provider[Any]):
-    def __init__(self, selector: _Callable[..., Any], **providers: Provider): ...
+    def __init__(self, selector: Optional[_Callable[..., Any]] = None, **providers: Provider): ...
     def __getattr__(self, name: str) -> Provider: ...
+
+    @property
+    def selector(self) -> Optional[_Callable[..., Any]]: ...
+    def set_selector(self, selector: Optional[_Callable[..., Any]]) -> Selector: ...
+
     @property
     def providers(self) -> _Dict[str, Provider]: ...
+    def set_providers(self, **providers: Provider) -> Selector: ...
 
 
 class ProvidedInstanceFluentInterface:

src/dependency_injector/providers.pyx

@@ -3718,10 +3718,14 @@ cdef class Selector(Provider):
         assert isinstance(instance_2, SomeOtherClass)
     """
 
-    def __init__(self, selector, **providers):
+    def __init__(self, selector=None, **providers):
         """Initialize provider."""
-        self.__selector = selector
-        self.__providers = providers
+        self.__selector = None
+        self.set_selector(selector)
+
+        self.__providers = {}
+        self.set_providers(**providers)
+
         super(Selector, self).__init__()
 
     def __deepcopy__(self, memo):
@@ -3730,10 +3734,10 @@ cdef class Selector(Provider):
         if copied is not None:
             return copied
 
-        copied = self.__class__(
-            deepcopy(self.__selector, memo),
-            **deepcopy(self.__providers, memo),
-        )
+        copied = _memorized_duplicate(self, memo)
+        copied.set_selector(deepcopy(self.__selector, memo))
+        copied.set_providers(**deepcopy(self.__providers, memo))
+
         self._copy_overridings(copied, memo)
 
         return copied
@@ -3766,11 +3770,26 @@ cdef class Selector(Provider):
             address=hex(id(self)),
         )
 
+    @property
+    def selector(self):
+        """Return selector."""
+        return self.__selector
+
+    def set_selector(self, selector):
+        """Set selector."""
+        self.__selector = selector
+        return self
+
     @property
     def providers(self):
         """Return providers."""
         return dict(self.__providers)
 
+    def set_providers(self, **providers: Provider):
+        """Set providers."""
+        self.__providers = providers
+        return self
+
     @property
     def related(self):
         """Return related providers generator."""

tests/unit/providers/test_selector_py2_py3.py

@@ -16,6 +16,28 @@ class SelectorTests(unittest.TestCase):
     def test_is_provider(self):
         self.assertTrue(providers.is_provider(providers.Selector(self.selector)))
 
+    def test_init_optional(self):
+        one = providers.Object(1)
+        two = providers.Object(2)
+
+        provider = providers.Selector()
+        provider.set_selector(self.selector)
+        provider.set_providers(one=one, two=two)
+
+        self.assertEqual(provider.providers, {'one': one, 'two': two})
+        with self.selector.override('one'):
+            self.assertEqual(provider(), one())
+        with self.selector.override('two'):
+            self.assertEqual(provider(), two())
+
+    def test_set_selector_returns_self(self):
+        provider = providers.Selector()
+        self.assertIs(provider.set_selector(self.selector), provider)
+
+    def test_set_providers_returns_self(self):
+        provider = providers.Selector()
+        self.assertIs(provider.set_providers(one=providers.Provider()), provider)
+
     def test_provided_instance_provider(self):
         provider = providers.Selector(self.selector)
         self.assertIsInstance(provider.provided, providers.ProvidedInstance)