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Python Language Reference Manual
by Guido van Rossum and Fred L. Drake, Jr.
Paperback (6"x9"), 120 pages
ISBN 0954161785
RRP £12.95 ($19.95)

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3.4.2.4 __slots__

By default, instances of both old and new-style 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 new-style 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__
This class variable can be assigned a string, iterable, or sequence of strings with variable names used by instances. If defined in a new-style class, __slots__ reserves space for the declared variables and prevents the automatic creation of __dict__ and __weakref__ for each instance. (Added in Python version 2.2)

Notes on using __slots__

ISBN 0954161785Python Language Reference ManualSee the print edition