bpo-30449: Improve __slots__ documentation (GH-1819)

(cherry picked from commit 2b44e302)
Co-authored-by: Aaron Hall, MBA <aaronchall@yahoo.com>

Doc/reference/datamodel.rst

@@ -1653,15 +1653,11 @@ instances cannot override the behavior of a property.
 __slots__
 ^^^^^^^^^
 
-By default, instances of classes have a dictionary for attribute storage.  This
-wastes space for objects having very few instance variables.  The space
-consumption can become acute when creating large numbers of instances.
-
-The default can be overridden by defining *__slots__* in a class definition.
-The *__slots__* declaration takes a sequence of instance variables and reserves
-just enough space in each instance to hold a value for each variable.  Space is
-saved because *__dict__* is not created for each instance.
+*__slots__* allow us to explicitly declare data members (like
+properties) and deny the creation of *__dict__* and *__weakref__*
+(unless explicitly declared in *__slots__* or available in a parent.)
 
+The space saved over using *__dict__* can be significant.
 
 .. data:: object.__slots__
 
@@ -1674,9 +1670,8 @@ saved because *__dict__* is not created for each instance.
 Notes on using *__slots__*
 """"""""""""""""""""""""""
 
-* When inheriting from a class without *__slots__*, the *__dict__* attribute of
-  that class will always be accessible, so a *__slots__* definition in the
-  subclass is meaningless.
+* When inheriting from a class without *__slots__*, the *__dict__* and
+  *__weakref__* attribute of the instances will always be accessible.
 
 * Without a *__dict__* variable, instances cannot be assigned new variables not
   listed in the *__slots__* definition.  Attempts to assign to an unlisted
@@ -1695,9 +1690,11 @@ Notes on using *__slots__*
   *__slots__*; otherwise, the class attribute would overwrite the descriptor
   assignment.
 
-* The action of a *__slots__* declaration is limited to the class where it is
-  defined.  As a result, subclasses will have a *__dict__* unless they also define
-  *__slots__* (which must only contain names of any *additional* slots).
+* The action of a *__slots__* declaration is not limited to the class
+  where it is defined.  *__slots__* declared in parents are available in
+  child classes. However, child subclasses will get a *__dict__*  and
+  *__weakref__* unless they also define *__slots__* (which should only
+  contain names of any *additional* slots).
 
 * If a class defines a slot also defined in a base class, the instance variable
   defined by the base class slot is inaccessible (except by retrieving its
@@ -1713,6 +1710,10 @@ Notes on using *__slots__*
 
 * *__class__* assignment works only if both classes have the same *__slots__*.
 
+* Multiple inheritance with multiple slotted parent classes can be used,
+  but only one parent is allowed to have attributes created by slots
+  (the other bases must have empty slot layouts) - violations raise
+  :exc:`TypeError`.
 
 .. _class-customization: