bpo-33816: Remove outdated metaclass example (GH-7566) · python/cpython@c2ccac7 (original) (raw)
`` @@ -1998,46 +1998,14 @@ becomes the :attr:~object.__dict__ attribute of the class object.
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``` Describes the implicit __class__ closure reference
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Metaclass example
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^^^^^^^^^^^^^^^^^
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Uses for metaclasses
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^^^^^^^^^^^^^^^^^^^^
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`The potential uses for metaclasses are boundless. Some ideas that have been
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`explored include enum, logging, interface checking, automatic delegation,
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`automatic property creation, proxies, frameworks, and automatic resource
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`locking/synchronization.
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Here is an example of a metaclass that uses an :class:`collections.OrderedDict`
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to remember the order that class variables are defined::
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class OrderedClass(type):
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@classmethod
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def __prepare__(metacls, name, bases, **kwds):
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return collections.OrderedDict()
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def __new__(cls, name, bases, namespace, **kwds):
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result = type.__new__(cls, name, bases, dict(namespace))
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result.members = tuple(namespace)
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return result
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class A(metaclass=OrderedClass):
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def one(self): pass
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def two(self): pass
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def three(self): pass
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def four(self): pass
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>>> A.members
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('__module__', 'one', 'two', 'three', 'four')
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When the class definition for *A* gets executed, the process begins with
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calling the metaclass's :meth:`__prepare__` method which returns an empty
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:class:`collections.OrderedDict`. That mapping records the methods and
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attributes of *A* as they are defined within the body of the class statement.
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Once those definitions are executed, the ordered dictionary is fully populated
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and the metaclass's :meth:`__new__` method gets invoked. That method builds
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the new type and it saves the ordered dictionary keys in an attribute
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called ``members``.
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`Customizing instance and subclass checks
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`----------------------------------------
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