<!DOCTYPE html PUBLIC
"-//W3C//DTD HTML 4.0 Transitional//EN">
<link rel=
"STYLESHEET" href=
"ref.css" type='text/css'
/>
<link rel=
"SHORTCUT ICON" href=
"../icons/pyfav.png" type=
"image/png" />
<link rel='start' href='../index.html' title='Python Documentation Index'
/>
<link rel=
"first" href=
"ref.html" title='Python Reference Manual'
/>
<link rel='contents' href='contents.html'
title=
"Contents" />
<link rel='index' href='genindex.html' title='Index'
/>
<link rel='last' href='about.html' title='About this document...'
/>
<link rel='help' href='about.html' title='About this document...'
/>
<link rel=
"prev" href=
"descriptor-invocation.html" />
<link rel=
"parent" href=
"attribute-access.html" />
<link rel=
"next" href=
"metaclasses.html" />
<meta name='aesop' content='information'
/>
<title>3.3.2.4 __slots__
</title>
<div id='top-navigation-panel' xml:id='top-navigation-panel'
>
<table align=
"center" width=
"100%" cellpadding=
"0" cellspacing=
"2">
<td class='online-navigation'
><a rel=
"prev" title=
"3.3.2.3 Invoking Descriptors"
href=
"descriptor-invocation.html"><img src='../icons/previous.png'
border='
0' height='
32' alt='Previous Page' width='
32'
/></A></td><td class='online-navigation'
><a rel=
"parent" title=
"3.3.2 Customizing attribute access"
href=
"attribute-access.html"><img src='../icons/up.png'
border='
0' height='
32' alt='Up One Level' width='
32'
/></A></td><td class='online-navigation'
><a rel=
"next" title=
"3.3.3 Customizing class creation"
href=
"metaclasses.html"><img src='../icons/next.png'
border='
0' height='
32' alt='Next Page' width='
32'
/></A></td><td align=
"center" width=
"100%">Python Reference Manual
</td>
<td class='online-navigation'
><a rel=
"contents" title=
"Table of Contents"
href=
"contents.html"><img src='../icons/contents.png'
border='
0' height='
32' alt='Contents' width='
32'
/></A></td><td class='online-navigation'
><img src='../icons/blank.png'
border='
0' height='
32' alt='' width='
32'
/></td><td class='online-navigation'
><a rel=
"index" title=
"Index"
href=
"genindex.html"><img src='../icons/index.png'
border='
0' height='
32' alt='Index' width='
32'
/></A></td><div class='online-navigation'
>
<b class=
"navlabel">Previous:
</b>
<a class=
"sectref" rel=
"prev" href=
"descriptor-invocation.html">3.3.2.3 Invoking Descriptors
</A>
<b class=
"navlabel">Up:
</b>
<a class=
"sectref" rel=
"parent" href=
"attribute-access.html">3.3.2 Customizing attribute access
</A>
<b class=
"navlabel">Next:
</b>
<a class=
"sectref" rel=
"next" href=
"metaclasses.html">3.3.3 Customizing class creation
</A>
<!--End of Navigation Panel-->
<H3><A NAME=
"SECTION005324000000000000000"></A><A NAME=
"slots"></A>
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
The default can be overridden by defining
<var>__slots__
</var> in a new-style class
definition. The
<var>__slots__
</var> 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
<var>__dict__
</var> is not created for
<dl><dt><b><tt id='l2h-
217' xml:id='l2h-
217'
>__slots__
</tt></b></dt>
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,
<var>__slots__
</var> reserves space for the declared variables
and prevents the automatic creation of
<var>__dict__
</var> and
<var>__weakref__
</var><span class=
"versionnote">New in version
2.2.
</span>
Notes on using
<var>__slots__
</var><LI>Without a
<var>__dict__
</var> variable, instances cannot be assigned new
variables not listed in the
<var>__slots__
</var> definition. Attempts to assign
to an unlisted variable name raises
<tt class=
"exception">AttributeError
</tt>. If dynamic
assignment of new variables is desired, then add
<code>'__dict__'
</code> to the
sequence of strings in the
<var>__slots__
</var> declaration.
<span class=
"versionnote">Changed in version
2.3:
Previously, adding
<code>'__dict__'
</code> to the
<var>__slots__
</var>declaration would not enable the assignment of new attributes not
specifically listed in the sequence of instance variable names.
</span><LI>Without a
<var>__weakref__
</var> variable for each instance, classes
defining
<var>__slots__
</var> do not support weak references to its instances.
If weak reference support is needed, then add
<code>'__weakref__'
</code> to the
sequence of strings in the
<var>__slots__
</var> declaration.
<span class=
"versionnote">Changed in version
2.3:
Previously, adding
<code>'__weakref__'
</code> to the
<var>__slots__
</var>declaration would not enable support for weak references.
</span><LI><var>__slots__
</var> are implemented at the class level by creating
descriptors (
<A href=
"descriptors.html#descriptors">3.3.2</A>) for each variable name. As a result,
class attributes cannot be used to set default values for instance
variables defined by
<var>__slots__
</var>; otherwise, the class attribute would
overwrite the descriptor assignment.
<LI>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 descriptor directly from the base class). This renders the meaning of the
program undefined. In the future, a check may be added to prevent this.
<LI>The action of a
<var>__slots__
</var> declaration is limited to the class
where it is defined. As a result, subclasses will have a
<var>__dict__
</var>unless they also define
<var>__slots__
</var>.
<LI><var>__slots__
</var> do not work for classes derived from ``variable-length''
built-in types such as
<tt class=
"class">long
</tt>,
<tt class=
"class">str
</tt> and
<tt class=
"class">tuple
</tt>.
<LI>Any non-string iterable may be assigned to
<var>__slots__
</var>.
Mappings may also be used; however, in the future, special meaning may
be assigned to the values corresponding to each key.
<div class='online-navigation'
>
<table align=
"center" width=
"100%" cellpadding=
"0" cellspacing=
"2">
<td class='online-navigation'
><a rel=
"prev" title=
"3.3.2.3 Invoking Descriptors"
href=
"descriptor-invocation.html"><img src='../icons/previous.png'
border='
0' height='
32' alt='Previous Page' width='
32'
/></A></td><td class='online-navigation'
><a rel=
"parent" title=
"3.3.2 Customizing attribute access"
href=
"attribute-access.html"><img src='../icons/up.png'
border='
0' height='
32' alt='Up One Level' width='
32'
/></A></td><td class='online-navigation'
><a rel=
"next" title=
"3.3.3 Customizing class creation"
href=
"metaclasses.html"><img src='../icons/next.png'
border='
0' height='
32' alt='Next Page' width='
32'
/></A></td><td align=
"center" width=
"100%">Python Reference Manual
</td>
<td class='online-navigation'
><a rel=
"contents" title=
"Table of Contents"
href=
"contents.html"><img src='../icons/contents.png'
border='
0' height='
32' alt='Contents' width='
32'
/></A></td><td class='online-navigation'
><img src='../icons/blank.png'
border='
0' height='
32' alt='' width='
32'
/></td><td class='online-navigation'
><a rel=
"index" title=
"Index"
href=
"genindex.html"><img src='../icons/index.png'
border='
0' height='
32' alt='Index' width='
32'
/></A></td><div class='online-navigation'
>
<b class=
"navlabel">Previous:
</b>
<a class=
"sectref" rel=
"prev" href=
"descriptor-invocation.html">3.3.2.3 Invoking Descriptors
</A>
<b class=
"navlabel">Up:
</b>
<a class=
"sectref" rel=
"parent" href=
"attribute-access.html">3.3.2 Customizing attribute access
</A>
<b class=
"navlabel">Next:
</b>
<a class=
"sectref" rel=
"next" href=
"metaclasses.html">3.3.3 Customizing class creation
</A>
<span class=
"release-info">Release
2.4.2, documentation updated on
28 September
2005.
</span>
<!--End of Navigation Panel-->
See
<i><a href=
"about.html">About this document...
</a></i> for information on suggesting changes.
projects / OpenSPARC-T2-SAM / blob