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54 | <H1><A NAME="SECTION009600000000000000000"></A><A NAME="class"></A> | |
55 | <BR> | |
56 | 7.6 Class definitions | |
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58 | <a id='l2h-614' xml:id='l2h-614'></a><a id='l2h-615' xml:id='l2h-615'></a> | |
59 | <P> | |
60 | A class definition defines a class object (see section <A href="types.html#types">3.2</A>): | |
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62 | <P> | |
63 | <dl><dd class="grammar"> | |
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65 | <table> | |
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67 | <td><a id='tok-classdef' xml:id='tok-classdef'>classdef</a></td> | |
68 | <td>::=</td> | |
69 | <td>"class" <a class='grammartoken' href="class.html#tok-classname">classname</a> [<a class='grammartoken' href="class.html#tok-inheritance">inheritance</a>] ":" | |
70 | <a class='grammartoken' href="compound.html#tok-suite">suite</a></td></tr> | |
71 | <tr> | |
72 | <td><a id='tok-inheritance' xml:id='tok-inheritance'>inheritance</a></td> | |
73 | <td>::=</td> | |
74 | <td>"(" <a class='grammartoken' href="exprlists.html#tok-expression_list">expression_list</a> ")"</td></tr> | |
75 | <tr> | |
76 | <td><a id='tok-classname' xml:id='tok-classname'>classname</a></td> | |
77 | <td>::=</td> | |
78 | <td><a class='grammartoken' href="identifiers.html#tok-identifier">identifier</a></td></tr> | |
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82 | href="grammar.txt" type="text/plain" | |
83 | >Download entire grammar as text.</a> | |
84 | </dd></dl> | |
85 | ||
86 | <P> | |
87 | A class definition is an executable statement. It first evaluates the | |
88 | inheritance list, if present. Each item in the inheritance list | |
89 | should evaluate to a class object or class type which allows | |
90 | subclassing. The class's suite is then executed | |
91 | in a new execution frame (see section <A href="naming.html#naming">4.1</A>), using a newly | |
92 | created local namespace and the original global namespace. | |
93 | (Usually, the suite contains only function definitions.) When the | |
94 | class's suite finishes execution, its execution frame is discarded but | |
95 | its local namespace is saved. A class object is then created using | |
96 | the inheritance list for the base classes and the saved local | |
97 | namespace for the attribute dictionary. The class name is bound to this | |
98 | class object in the original local namespace. | |
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100 | <a id='l2h-617' xml:id='l2h-617'></a><a id='l2h-618' xml:id='l2h-618'></a><a id='l2h-619' xml:id='l2h-619'></a> | |
101 | <P> | |
102 | <strong>Programmer's note:</strong> Variables defined in the class definition | |
103 | are class variables; they are shared by all instances. To define | |
104 | instance variables, they must be given a value in the | |
105 | <tt class="method">__init__()</tt> method or in another method. Both class and | |
106 | instance variables are accessible through the notation | |
107 | ``<code>self.name</code>'', and an instance variable hides a class variable | |
108 | with the same name when accessed in this way. Class variables with | |
109 | immutable values can be used as defaults for instance variables. | |
110 | For new-style classes, descriptors can be used to create instance | |
111 | variables with different implementation details. | |
112 | ||
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147 | <span class="release-info">Release 2.4.2, documentation updated on 28 September 2005.</span> | |
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