diff options
Diffstat (limited to 'Doc/howto/descriptor.rst')
| -rw-r--r-- | Doc/howto/descriptor.rst | 27 |
1 files changed, 15 insertions, 12 deletions
diff --git a/Doc/howto/descriptor.rst b/Doc/howto/descriptor.rst index f8763d888e..a0c698899b 100644 --- a/Doc/howto/descriptor.rst +++ b/Doc/howto/descriptor.rst @@ -36,9 +36,7 @@ continuing through the base classes of ``type(a)`` excluding metaclasses. If the looked-up value is an object defining one of the descriptor methods, then Python may override the default behavior and invoke the descriptor method instead. Where this occurs in the precedence chain depends on which descriptor methods -were defined. Note that descriptors are only invoked for new style objects or -classes (a class is new style if it inherits from :class:`object` or -:class:`type`). +were defined. Descriptors are a powerful, general purpose protocol. They are the mechanism behind properties, methods, static methods, class methods, and :func:`super()`. @@ -89,8 +87,6 @@ of ``obj``. If ``d`` defines the method :meth:`__get__`, then ``d.__get__(obj)` is invoked according to the precedence rules listed below. The details of invocation depend on whether ``obj`` is an object or a class. -Either way, descriptors only work for new style objects and classes. A class is -new style if it is a subclass of :class:`object`. For objects, the machinery is in :meth:`object.__getattribute__` which transforms ``b.x`` into ``type(b).__dict__['x'].__get__(b, type(b))``. The @@ -115,7 +111,6 @@ The important points to remember are: * descriptors are invoked by the :meth:`__getattribute__` method * overriding :meth:`__getattribute__` prevents automatic descriptor calls -* :meth:`__getattribute__` is only available with new style classes and objects * :meth:`object.__getattribute__` and :meth:`type.__getattribute__` make different calls to :meth:`__get__`. * data descriptors always override instance dictionaries. @@ -124,14 +119,11 @@ The important points to remember are: The object returned by ``super()`` also has a custom :meth:`__getattribute__` method for invoking descriptors. The call ``super(B, obj).m()`` searches ``obj.__class__.__mro__`` for the base class ``A`` immediately following ``B`` -and then returns ``A.__dict__['m'].__get__(obj, A)``. If not a descriptor, +and then returns ``A.__dict__['m'].__get__(obj, B)``. If not a descriptor, ``m`` is returned unchanged. If not in the dictionary, ``m`` reverts to a search using :meth:`object.__getattribute__`. -Note, in Python 2.2, ``super(B, obj).m()`` would only invoke :meth:`__get__` if -``m`` was a data descriptor. In Python 2.3, non-data descriptors also get -invoked unless an old-style class is involved. The implementation details are -in :c:func:`super_getattro()` in +The implementation details are in :c:func:`super_getattro()` in `Objects/typeobject.c <http://svn.python.org/view/python/trunk/Objects/typeobject.c?view=markup>`_ and a pure Python equivalent can be found in `Guido's Tutorial`_. @@ -218,6 +210,8 @@ here is a pure Python equivalent:: self.fget = fget self.fset = fset self.fdel = fdel + if doc is None and fget is not None: + doc = fget.__doc__ self.__doc__ = doc def __get__(self, obj, objtype=None): @@ -237,6 +231,15 @@ here is a pure Python equivalent:: raise AttributeError("can't delete attribute") self.fdel(obj) + def getter(self, fget): + return type(self)(fget, self.fset, self.fdel, self.__doc__) + + def setter(self, fset): + return type(self)(self.fget, fset, self.fdel, self.__doc__) + + def deleter(self, fdel): + return type(self)(self.fget, self.fset, fdel, self.__doc__) + The :func:`property` builtin helps whenever a user interface has granted attribute access and then subsequent changes require the intervention of a method. @@ -398,7 +401,7 @@ is to create alternate class constructors. In Python 2.3, the classmethod :func:`dict.fromkeys` creates a new dictionary from a list of keys. The pure Python equivalent is:: - class Dict: + class Dict(object): . . . def fromkeys(klass, iterable, value=None): "Emulate dict_fromkeys() in Objects/dictobject.c" |