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[3.6] bpo-30449 Terse slots (GH-1819) #5815

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Feb 22, 2018
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[3.6] bpo-30449 Terse slots (GH-1819) #5815

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29 changes: 15 additions & 14 deletions Doc/reference/datamodel.rst
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Original file line number Diff line number Diff line change
Expand Up @@ -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__

Expand All @@ -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
Expand All @@ -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
Expand All @@ -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:

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