Clarify metaclass docs and add example.
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@ -1177,7 +1177,8 @@ Basic customization
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:meth:`__init__` method will not be invoked.
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:meth:`__new__` is intended mainly to allow subclasses of immutable types (like
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int, str, or tuple) to customize instance creation.
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int, str, or tuple) to customize instance creation. It is also commonly
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overridden in custom metaclasses in order to customize class creation.
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.. method:: object.__init__(self[, ...])
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@ -1651,7 +1652,7 @@ definition is read into a separate namespace and the value of class name is
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bound to the result of ``type(name, bases, dict)``.
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When the class definition is read, if *__metaclass__* is defined then the
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callable assigned to it will be called instead of :func:`type`. The allows
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callable assigned to it will be called instead of :func:`type`. This allows
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classes or functions to be written which monitor or alter the class creation
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process:
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@ -1660,6 +1661,20 @@ process:
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* Returning an instance of another class -- essentially performing the role of a
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factory function.
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These steps will have to be performed in the metaclass's :meth:`__new__` method
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-- :meth:`type.__new__` can then be called from this method to create a class
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with different properties. This example adds a new element to the class
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dictionary before creating the class::
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class metacls(type):
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def __new__(mcs, name, bases, dict):
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dict['foo'] = 'metacls was here'
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return type.__new__(mcs, name, bases, dict)
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You can of course also override other class methods (or add new methods); for
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example defining a custom :meth:`__call__` method in the metaclass allows custom
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behavior when the class is called, e.g. not always creating a new instance.
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.. data:: __metaclass__
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