Update docs and examples

Author: rmk135

docs/providers/dependency.rst

@@ -15,7 +15,12 @@ Dependency provider will control that returned object is an instance of ``instan
 
 To provide a dependency you need to override the ``Dependency`` provider. You can call
 provider ``.override()`` method or provide an overriding provider when creating a container.
-See :ref:`provider-overriding`.
+See :ref:`provider-overriding`. If you don't provide a dependency, ``Dependency`` provider
+will raise an error:
+
+.. literalinclude:: ../../examples/providers/dependency_undefined_error.py
+   :language: python
+   :lines: 18-
 
 You also can provide a default for the dependency. To provide a default use ``default`` argument:
 ``Dependency(..., default=...)``. Default can be a value or a provider. If default is not a provider,

examples/providers/dependency.py

@@ -3,7 +3,7 @@
 import abc
 import dataclasses
 
-from dependency_injector import containers, providers, errors
+from dependency_injector import containers, providers
 
 
 class DbAdapter(metaclass=abc.ABCMeta):
@@ -41,10 +41,5 @@ class Container(containers.DeclarativeContainer):
     assert isinstance(container2.user_service().database, PostgresDbAdapter)
 
     container3 = Container(database=providers.Singleton(object))
-    try:
-        container3.user_service()
-    except errors.Error as exception:
-        print(exception)
-        # The output is:
-        # <object object at 0x107ce5c40> is not an
-        # instance of <class '__main__.DbAdapter'>
+    container3.user_service()  # <-- raises error:
+    # <object ...> is not an instance of DbAdapter

examples/providers/dependency_undefined_error.py

@@ -0,0 +1,31 @@
+"""`Dependency` provider undefined error example."""
+
+import abc
+import dataclasses
+
+from dependency_injector import containers, providers
+
+
+class DbAdapter(metaclass=abc.ABCMeta):
+    ...
+
+
+@dataclasses.dataclass
+class UserService:
+    database: DbAdapter
+
+
+class Container(containers.DeclarativeContainer):
+
+    database = providers.Dependency(instance_of=DbAdapter)
+
+    user_service = providers.Factory(
+        UserService,
+        database=database,
+    )
+
+
+if __name__ == '__main__':
+    container = Container()
+    container.user_service()  # <-- raises error:
+    # Dependency "Container.database" is not defined