Update docs on creating custom providers with a requirement to specify .related property

Author: rmk135

docs/main/changelog.rst

@@ -13,6 +13,7 @@ Development version
 - Fix ``container.reset_singleton()`` to reset all provider types, not only ``Singleton``.
 - Improve ``container.traverse(types=[...])`` and ``provider.traverse(types=[...])`` typing stubs
   to return ``types`` -typed iterator.
+- Update docs on creating custom providers with a requirement to specify ``.related`` property.
 
 4.18.0
 ------

docs/providers/custom.rst

@@ -20,8 +20,11 @@ To create a custom provider you need to follow these rules:
    from the ``providers`` module. After the a new provider object is created use
    ``Provider._copy_overriding()`` method to copy all overriding providers. See the example
    below.
-4. If the new provider has a ``__init__()`` method, it should call the parent
+4. If new provider has a ``__init__()`` method, it should call the parent
    ``Provider.__init__()``.
+5. If new provider stores any other providers, these providers should be listed in
+   ``.related`` property. Property ``.related`` also should yield providers from parent
+   ``.related`` property.
 
 .. literalinclude:: ../../examples/providers/custom_factory.py
    :language: python

examples/providers/custom_factory.py

@@ -25,6 +25,12 @@ def __deepcopy__(self, memo):
 
         return copied
 
+    @property
+    def related(self):
+        """Return related providers generator."""
+        yield from [self._factory]
+        yield from super().related
+
     def _provide(self, args, kwargs):
         return self._factory(*args, **kwargs)