bpo-30449: Improve slots documentation (GH-1819) · python/cpython@6eab93c (original) (raw)

`@@ -1653,15 +1653,11 @@ 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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-

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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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`@@ -1674,9 +1670,8 @@ 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

`@@ -1695,9 +1690,11 @@ 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

`@@ -1713,6 +1710,10 @@ 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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