projects / OpenSPARC-T2-SAM / blame
| Commit | Line | Data |
|---|---|---|
| 920dae64 AT |
1 | <!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN"> |
| 2 | <html> | |
| 3 | <head> | |
| 4 | <link rel="STYLESHEET" href="ref.css" type='text/css' /> | |
| 5 | <link rel="SHORTCUT ICON" href="../icons/pyfav.png" type="image/png" /> | |
| 6 | <link rel='start' href='../index.html' title='Python Documentation Index' /> | |
| 7 | <link rel="first" href="ref.html" title='Python Reference Manual' /> | |
| 8 | <link rel='contents' href='contents.html' title="Contents" /> | |
| 9 | <link rel='index' href='genindex.html' title='Index' /> | |
| 10 | <link rel='last' href='about.html' title='About this document...' /> | |
| 11 | <link rel='help' href='about.html' title='About this document...' /> | |
| 12 | <link rel="prev" href="descriptor-invocation.html" /> | |
| 13 | <link rel="parent" href="attribute-access.html" /> | |
| 14 | <link rel="next" href="metaclasses.html" /> | |
| 15 | <meta name='aesop' content='information' /> | |
| 16 | <title>3.3.2.4 __slots__</title> | |
| 17 | </head> | |
| 18 | <body> | |
| 19 | <DIV CLASS="navigation"> | |
| 20 | <div id='top-navigation-panel' xml:id='top-navigation-panel'> | |
| 21 | <table align="center" width="100%" cellpadding="0" cellspacing="2"> | |
| 22 | <tr> | |
| 23 | <td class='online-navigation'><a rel="prev" title="3.3.2.3 Invoking Descriptors" | |
| 24 | href="descriptor-invocation.html"><img src='../icons/previous.png' | |
| 25 | border='0' height='32' alt='Previous Page' width='32' /></A></td> | |
| 26 | <td class='online-navigation'><a rel="parent" title="3.3.2 Customizing attribute access" | |
| 27 | href="attribute-access.html"><img src='../icons/up.png' | |
| 28 | border='0' height='32' alt='Up One Level' width='32' /></A></td> | |
| 29 | <td class='online-navigation'><a rel="next" title="3.3.3 Customizing class creation" | |
| 30 | href="metaclasses.html"><img src='../icons/next.png' | |
| 31 | border='0' height='32' alt='Next Page' width='32' /></A></td> | |
| 32 | <td align="center" width="100%">Python Reference Manual</td> | |
| 33 | <td class='online-navigation'><a rel="contents" title="Table of Contents" | |
| 34 | href="contents.html"><img src='../icons/contents.png' | |
| 35 | border='0' height='32' alt='Contents' width='32' /></A></td> | |
| 36 | <td class='online-navigation'><img src='../icons/blank.png' | |
| 37 | border='0' height='32' alt='' width='32' /></td> | |
| 38 | <td class='online-navigation'><a rel="index" title="Index" | |
| 39 | href="genindex.html"><img src='../icons/index.png' | |
| 40 | border='0' height='32' alt='Index' width='32' /></A></td> | |
| 41 | </tr></table> | |
| 42 | <div class='online-navigation'> | |
| 43 | <b class="navlabel">Previous:</b> | |
| 44 | <a class="sectref" rel="prev" href="descriptor-invocation.html">3.3.2.3 Invoking Descriptors</A> | |
| 45 | <b class="navlabel">Up:</b> | |
| 46 | <a class="sectref" rel="parent" href="attribute-access.html">3.3.2 Customizing attribute access</A> | |
| 47 | <b class="navlabel">Next:</b> | |
| 48 | <a class="sectref" rel="next" href="metaclasses.html">3.3.3 Customizing class creation</A> | |
| 49 | </div> | |
| 50 | <hr /></div> | |
| 51 | </DIV> | |
| 52 | <!--End of Navigation Panel--> | |
| 53 | ||
| 54 | <H3><A NAME="SECTION005324000000000000000"></A><A NAME="slots"></A> | |
| 55 | <BR> | |
| 56 | 3.3.2.4 __slots__ | |
| 57 | </H3> | |
| 58 | ||
| 59 | <P> | |
| 60 | By default, instances of both old and new-style classes have a dictionary | |
| 61 | for attribute storage. This wastes space for objects having very few instance | |
| 62 | variables. The space consumption can become acute when creating large numbers | |
| 63 | of instances. | |
| 64 | ||
| 65 | <P> | |
| 66 | The default can be overridden by defining <var>__slots__</var> in a new-style class | |
| 67 | definition. The <var>__slots__</var> declaration takes a sequence of instance | |
| 68 | variables and reserves just enough space in each instance to hold a value | |
| 69 | for each variable. Space is saved because <var>__dict__</var> is not created for | |
| 70 | each instance. | |
| 71 | ||
| 72 | <P> | |
| 73 | <dl><dt><b><tt id='l2h-217' xml:id='l2h-217'>__slots__</tt></b></dt> | |
| 74 | <dd> | |
| 75 | This class variable can be assigned a string, iterable, or sequence of strings | |
| 76 | with variable names used by instances. If defined in a new-style class, | |
| 77 | <var>__slots__</var> reserves space for the declared variables | |
| 78 | and prevents the automatic creation of <var>__dict__</var> and <var>__weakref__</var> | |
| 79 | for each instance. | |
| 80 | ||
| 81 | <span class="versionnote">New in version 2.2.</span> | |
| 82 | ||
| 83 | </dd></dl> | |
| 84 | ||
| 85 | <P> | |
| 86 | Notes on using <var>__slots__</var> | |
| 87 | ||
| 88 | <P> | |
| 89 | ||
| 90 | <UL> | |
| 91 | <LI>Without a <var>__dict__</var> variable, instances cannot be assigned new | |
| 92 | variables not listed in the <var>__slots__</var> definition. Attempts to assign | |
| 93 | to an unlisted variable name raises <tt class="exception">AttributeError</tt>. If dynamic | |
| 94 | assignment of new variables is desired, then add <code>'__dict__'</code> to the | |
| 95 | sequence of strings in the <var>__slots__</var> declaration. | |
| 96 | ||
| 97 | <span class="versionnote">Changed in version 2.3: | |
| 98 | Previously, adding <code>'__dict__'</code> to the <var>__slots__</var> | |
| 99 | declaration would not enable the assignment of new attributes not | |
| 100 | specifically listed in the sequence of instance variable names.</span> | |
| 101 | ||
| 102 | <P> | |
| 103 | </LI> | |
| 104 | <LI>Without a <var>__weakref__</var> variable for each instance, classes | |
| 105 | defining <var>__slots__</var> do not support weak references to its instances. | |
| 106 | If weak reference support is needed, then add <code>'__weakref__'</code> to the | |
| 107 | sequence of strings in the <var>__slots__</var> declaration. | |
| 108 | ||
| 109 | <span class="versionnote">Changed in version 2.3: | |
| 110 | Previously, adding <code>'__weakref__'</code> to the <var>__slots__</var> | |
| 111 | declaration would not enable support for weak references.</span> | |
| 112 | ||
| 113 | <P> | |
| 114 | </LI> | |
| 115 | <LI><var>__slots__</var> are implemented at the class level by creating | |
| 116 | descriptors (<A href="descriptors.html#descriptors">3.3.2</A>) for each variable name. As a result, | |
| 117 | class attributes cannot be used to set default values for instance | |
| 118 | variables defined by <var>__slots__</var>; otherwise, the class attribute would | |
| 119 | overwrite the descriptor assignment. | |
| 120 | ||
| 121 | <P> | |
| 122 | </LI> | |
| 123 | <LI>If a class defines a slot also defined in a base class, the instance | |
| 124 | variable defined by the base class slot is inaccessible (except by retrieving | |
| 125 | its descriptor directly from the base class). This renders the meaning of the | |
| 126 | program undefined. In the future, a check may be added to prevent this. | |
| 127 | ||
| 128 | <P> | |
| 129 | </LI> | |
| 130 | <LI>The action of a <var>__slots__</var> declaration is limited to the class | |
| 131 | where it is defined. As a result, subclasses will have a <var>__dict__</var> | |
| 132 | unless they also define <var>__slots__</var>. | |
| 133 | ||
| 134 | <P> | |
| 135 | </LI> | |
| 136 | <LI><var>__slots__</var> do not work for classes derived from ``variable-length'' | |
| 137 | built-in types such as <tt class="class">long</tt>, <tt class="class">str</tt> and <tt class="class">tuple</tt>. | |
| 138 | ||
| 139 | <P> | |
| 140 | </LI> | |
| 141 | <LI>Any non-string iterable may be assigned to <var>__slots__</var>. | |
| 142 | Mappings may also be used; however, in the future, special meaning may | |
| 143 | be assigned to the values corresponding to each key. | |
| 144 | ||
| 145 | <P> | |
| 146 | </LI> | |
| 147 | </UL> | |
| 148 | ||
| 149 | <P> | |
| 150 | ||
| 151 | <DIV CLASS="navigation"> | |
| 152 | <div class='online-navigation'> | |
| 153 | <p></p><hr /> | |
| 154 | <table align="center" width="100%" cellpadding="0" cellspacing="2"> | |
| 155 | <tr> | |
| 156 | <td class='online-navigation'><a rel="prev" title="3.3.2.3 Invoking Descriptors" | |
| 157 | href="descriptor-invocation.html"><img src='../icons/previous.png' | |
| 158 | border='0' height='32' alt='Previous Page' width='32' /></A></td> | |
| 159 | <td class='online-navigation'><a rel="parent" title="3.3.2 Customizing attribute access" | |
| 160 | href="attribute-access.html"><img src='../icons/up.png' | |
| 161 | border='0' height='32' alt='Up One Level' width='32' /></A></td> | |
| 162 | <td class='online-navigation'><a rel="next" title="3.3.3 Customizing class creation" | |
| 163 | href="metaclasses.html"><img src='../icons/next.png' | |
| 164 | border='0' height='32' alt='Next Page' width='32' /></A></td> | |
| 165 | <td align="center" width="100%">Python Reference Manual</td> | |
| 166 | <td class='online-navigation'><a rel="contents" title="Table of Contents" | |
| 167 | href="contents.html"><img src='../icons/contents.png' | |
| 168 | border='0' height='32' alt='Contents' width='32' /></A></td> | |
| 169 | <td class='online-navigation'><img src='../icons/blank.png' | |
| 170 | border='0' height='32' alt='' width='32' /></td> | |
| 171 | <td class='online-navigation'><a rel="index" title="Index" | |
| 172 | href="genindex.html"><img src='../icons/index.png' | |
| 173 | border='0' height='32' alt='Index' width='32' /></A></td> | |
| 174 | </tr></table> | |
| 175 | <div class='online-navigation'> | |
| 176 | <b class="navlabel">Previous:</b> | |
| 177 | <a class="sectref" rel="prev" href="descriptor-invocation.html">3.3.2.3 Invoking Descriptors</A> | |
| 178 | <b class="navlabel">Up:</b> | |
| 179 | <a class="sectref" rel="parent" href="attribute-access.html">3.3.2 Customizing attribute access</A> | |
| 180 | <b class="navlabel">Next:</b> | |
| 181 | <a class="sectref" rel="next" href="metaclasses.html">3.3.3 Customizing class creation</A> | |
| 182 | </div> | |
| 183 | </div> | |
| 184 | <hr /> | |
| 185 | <span class="release-info">Release 2.4.2, documentation updated on 28 September 2005.</span> | |
| 186 | </DIV> | |
| 187 | <!--End of Navigation Panel--> | |
| 188 | <ADDRESS> | |
| 189 | See <i><a href="about.html">About this document...</a></i> for information on suggesting changes. | |
| 190 | </ADDRESS> | |
| 191 | </BODY> | |
| 192 | </HTML> |