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Diffstat (limited to 'Doc/reference/datamodel.rst')
| -rw-r--r-- | Doc/reference/datamodel.rst | 21 |
1 files changed, 18 insertions, 3 deletions
diff --git a/Doc/reference/datamodel.rst b/Doc/reference/datamodel.rst index 92fece1..e6cba75 100644 --- a/Doc/reference/datamodel.rst +++ b/Doc/reference/datamodel.rst @@ -1086,7 +1086,8 @@ Basic customization :meth:`__init__` method will not be invoked. :meth:`__new__` is intended mainly to allow subclasses of immutable types (like - int, str, or tuple) to customize instance creation. + int, str, or tuple) to customize instance creation. It is also commonly + overridden in custom metaclasses in order to customize class creation. .. method:: object.__init__(self[, ...]) @@ -1527,7 +1528,7 @@ read into a separate namespace and the value of class name is bound to the result of ``type(name, bases, dict)``. When the class definition is read, if *__metaclass__* is defined then the -callable assigned to it will be called instead of :func:`type`. The allows +callable assigned to it will be called instead of :func:`type`. This allows classes or functions to be written which monitor or alter the class creation process: @@ -1536,7 +1537,21 @@ process: * Returning an instance of another class -- essentially performing the role of a factory function. -.. XXX needs to be updated for the "new metaclasses" PEP +These steps will have to be performed in the metaclass's :meth:`__new__` method +-- :meth:`type.__new__` can then be called from this method to create a class +with different properties. This example adds a new element to the class +dictionary before creating the class:: + + class metacls(type): + def __new__(mcs, name, bases, dict): + dict['foo'] = 'metacls was here' + return type.__new__(mcs, name, bases, dict) + +You can of course also override other class methods (or add new methods); for +example defining a custom :meth:`__call__` method in the metaclass allows custom +behavior when the class is called, e.g. not always creating a new instance. + + .. data:: __metaclass__ This variable can be any callable accepting arguments for ``name``, ``bases``, |