From e6fa6af0ab6d0c141e17de4b56847ae016f5b70e Mon Sep 17 00:00:00 2001 From: "Aaron Hall, MBA" Date: Thu, 25 May 2017 22:33:26 -0700 Subject: [PATCH] bpo-30449 Terse slots (GH-1819) * correct __slots__ documentation with minimal changes * add multiple inheritance info * remove mapping from description (cherry picked from commit 2b44e302ec3079363c4d5c875677945953705c58) Co-authored-by: Aaron Hall, MBA --- Doc/reference/datamodel.rst | 29 +++++++++++++++-------------- 1 file changed, 15 insertions(+), 14 deletions(-) diff --git a/Doc/reference/datamodel.rst b/Doc/reference/datamodel.rst index 773eeb23354016..c95f4a97093037 100644 --- a/Doc/reference/datamodel.rst +++ b/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: pFad - Phonifier reborn

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