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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 | ||
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