bpo-30449 Terse slots (#1819)
* correct __slots__ documentation with minimal changes * add multiple inheritance info * remove mapping from description
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@ -1614,15 +1614,11 @@ instances cannot override the behavior of a property.
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__slots__
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^^^^^^^^^
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By default, instances of classes have a dictionary for attribute storage. This
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wastes space for objects having very few instance variables. The space
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consumption can become acute when creating large numbers of instances.
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The default can be overridden by defining *__slots__* in a class definition.
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The *__slots__* declaration takes a sequence of instance variables and reserves
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just enough space in each instance to hold a value for each variable. Space is
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saved because *__dict__* is not created for each instance.
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*__slots__* allow us to explicitly declare data members (like
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properties) and deny the creation of *__dict__* and *__weakref__*
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(unless explicitly declared in *__slots__* or available in a parent.)
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The space saved over using *__dict__* can be significant.
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.. data:: object.__slots__
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@ -1635,9 +1631,8 @@ saved because *__dict__* is not created for each instance.
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Notes on using *__slots__*
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""""""""""""""""""""""""""
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* When inheriting from a class without *__slots__*, the *__dict__* attribute of
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that class will always be accessible, so a *__slots__* definition in the
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subclass is meaningless.
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* When inheriting from a class without *__slots__*, the *__dict__* and
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*__weakref__* attribute of the instances will always be accessible.
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* Without a *__dict__* variable, instances cannot be assigned new variables not
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listed in the *__slots__* definition. Attempts to assign to an unlisted
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@ -1656,9 +1651,11 @@ Notes on using *__slots__*
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*__slots__*; otherwise, the class attribute would overwrite the descriptor
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assignment.
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* The action of a *__slots__* declaration is limited to the class where it is
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defined. As a result, subclasses will have a *__dict__* unless they also define
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*__slots__* (which must only contain names of any *additional* slots).
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* The action of a *__slots__* declaration is not limited to the class
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where it is defined. *__slots__* declared in parents are available in
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child classes. However, child subclasses will get a *__dict__* and
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*__weakref__* unless they also define *__slots__* (which should only
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contain names of any *additional* slots).
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* If a class defines a slot also defined in a base class, the instance variable
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defined by the base class slot is inaccessible (except by retrieving its
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@ -1674,6 +1671,10 @@ Notes on using *__slots__*
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* *__class__* assignment works only if both classes have the same *__slots__*.
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* Multiple inheritance with multiple slotted parent classes can be used,
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but only one parent is allowed to have attributes created by slots
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(the other bases must have empty slot layouts) - violations raise
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:exc:`TypeError`.
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.. _class-customization:
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