Programming FAQ: Mention object.__setattr__ as a technique for delega…

…tion (python#124617)

This is used for example by threading.local in the stdlib.

Doc/faq/programming.rst

@@ -1613,9 +1613,16 @@ method too, and it must do so carefully.  The basic implementation of
            self.__dict__[name] = value
        ...
 
-Most :meth:`!__setattr__` implementations must modify
-:attr:`self.__dict__ <object.__dict__>` to store
-local state for self without causing an infinite recursion.
+Many :meth:`~object.__setattr__` implementations call :meth:`!object.__setattr__` to set
+an attribute on self without causing infinite recursion::
+
+   class X:
+       def __setattr__(self, name, value):
+           # Custom logic here...
+           object.__setattr__(self, name, value)
+
+Alternatively, it is possible to set attributes by inserting
+entries into :attr:`self.__dict__ <object.__dict__>` directly.
 
 
 How do I call a method defined in a base class from a derived class that extends it?