mirror of https://github.com/python/cpython
document changes to metaclasses
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@ -1484,10 +1484,11 @@ By default, classes are constructed using :func:`type`. A class definition is
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read into a separate namespace and the value of class name is bound to the
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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`. 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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When the class definition is read, if a callable ``metaclass`` keyword argument
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is passed after the bases in the class definition, the callable given will be
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called instead of :func:`type`. If other keyword arguments are passed, they
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will also be passed to the metaclass. This allows classes or functions to be
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written which monitor or alter the class creation process:
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* Modifying the class dictionary prior to the class being created.
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@ -1508,21 +1509,19 @@ 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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This variable can be any callable accepting arguments for ``name``, ``bases``,
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and ``dict``. Upon class creation, the callable is used instead of the built-in
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:func:`type`.
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If the metaclass has a :meth:`__prepare__` attribute (usually implemented as a
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class or static method), it is called before the class body is evaluated with
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the name of the class and a tuple of its bases for arguments. It should return
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an object that supports the mapping interface that will be used to store the
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namespace of the class. The default is a plain dictionary. This could be used,
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for example, to keep track of the order that class attributes are declared in by
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returning an ordered dictionary.
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The appropriate metaclass is determined by the following precedence rules:
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* If ``dict['__metaclass__']`` exists, it is used.
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* If the ``metaclass`` keyword argument is based with the bases, it is used.
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* Otherwise, if there is at least one base class, its metaclass is used (this
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looks for a *__class__* attribute first and if not found, uses its type).
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* Otherwise, if a global variable named __metaclass__ exists, it is used.
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* Otherwise, if there is at least one base class, its metaclass is used.
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* Otherwise, the default metaclass (:class:`type`) is used.
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@ -1922,8 +1921,7 @@ correctness, implicit special method lookup may also bypass the
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... print "Metaclass getattribute invoked"
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... return type.__getattribute__(*args)
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...
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>>> class C(object):
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... __metaclass__ = Meta
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>>> class C(object, metaclass=Meta):
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... def __len__(self):
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... return 10
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... def __getattribute__(*args):
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