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<H2><A NAME="SECTION005330000000000000000"></A><A NAME="metaclasses"></A>
<BR>
3.3.3 Customizing class creation
</H2>

<P>
By default, new-style classes are constructed using <tt class="function">type()</tt>.
A class definition is read into a separate namespace and the value
of class name is bound to the result of <code>type(name, bases, dict)</code>.

<P>
When the class definition is read, if <var>__metaclass__</var> is defined
then the callable assigned to it will be called instead of <tt class="function">type()</tt>.
The allows classes or functions to be written which monitor or alter the class
creation process:

<P>

<UL>
<LI>Modifying the class dictionary prior to the class being created.
</LI>
<LI>Returning an instance of another class - essentially performing
the role of a factory function.
</LI>
</UL>

<P>
<dl><dt><b><tt id='l2h-218' xml:id='l2h-218'>__metaclass__</tt></b></dt>
<dd>
This variable can be any callable accepting arguments for <code>name</code>,
<code>bases</code>, and <code>dict</code>.  Upon class creation, the callable is
used instead of the built-in <tt class="function">type()</tt>.

<span class="versionnote">New in version 2.2.</span>
                     
</dd></dl>

<P>
The appropriate metaclass is determined by the following precedence rules:

<P>

<UL>
<LI>If <code>dict['__metaclass__']</code> exists, it is used.

<P>
</LI>
<LI>Otherwise, if there is at least one base class, its metaclass is used
(this looks for a <var>__class__</var> attribute first and if not found, uses its
type).

<P>
</LI>
<LI>Otherwise, if a global variable named __metaclass__ exists, it is used.

<P>
</LI>
<LI>Otherwise, the old-style, classic metaclass (types.ClassType) is used.

<P>
</LI>
</UL>      

<P>
The potential uses for metaclasses are boundless. Some ideas that have
been explored including logging, interface checking, automatic delegation,
automatic property creation, proxies, frameworks, and automatic resource
locking/synchronization.

<P>

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	54
	55	<H2><A NAME="SECTION005330000000000000000"></A><A NAME="metaclasses"></A>
	56	<BR>
	57	3.3.3 Customizing class creation
	58	</H2>
	59
	60	<P>
	61	By default, new-style classes are constructed using <tt class="function">type()</tt>.
	62	A class definition is read into a separate namespace and the value
	63	of class name is bound to the result of <code>type(name, bases, dict)</code>.
	64
65	<P>
66	When the class definition is read, if <var>__metaclass__</var> is defined
67	then the callable assigned to it will be called instead of <tt class="function">type()</tt>.
68	The allows classes or functions to be written which monitor or alter the class
69	creation process:
70
71	<P>
72
73	<UL>
74	<LI>Modifying the class dictionary prior to the class being created.
75	</LI>
76	<LI>Returning an instance of another class - essentially performing
77	the role of a factory function.
78	</LI>
79	</UL>
80
81	<P>
82	<dl><dt><b><tt id='l2h-218' xml:id='l2h-218'>__metaclass__</tt></b></dt>
83	<dd>
84	This variable can be any callable accepting arguments for <code>name</code>,
85	<code>bases</code>, and <code>dict</code>. Upon class creation, the callable is
86	used instead of the built-in <tt class="function">type()</tt>.
87
88	<span class="versionnote">New in version 2.2.</span>
89
90	</dd></dl>
91
92	<P>
93	The appropriate metaclass is determined by the following precedence rules:
94
95	<P>
96
97	<UL>
98	<LI>If <code>dict['__metaclass__']</code> exists, it is used.
99
100	<P>
101	</LI>
102	<LI>Otherwise, if there is at least one base class, its metaclass is used
103	(this looks for a <var>__class__</var> attribute first and if not found, uses its
104	type).
105
106	<P>
107	</LI>
108	<LI>Otherwise, if a global variable named __metaclass__ exists, it is used.
109
110	<P>
111	</LI>
112	<LI>Otherwise, the old-style, classic metaclass (types.ClassType) is used.
113
114	<P>
115	</LI>
116	</UL>
117
118	<P>
119	The potential uses for metaclasses are boundless. Some ideas that have
120	been explored including logging, interface checking, automatic delegation,
121	automatic property creation, proxies, frameworks, and automatic resource
122	locking/synchronization.
123
124	<P>
125
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