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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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<span class="release-info">Release 2.4.2, documentation updated on 28 September 2005.</span>
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