From 37614227499dcf270a02d2c87e4e8e955ac5cc3b Mon Sep 17 00:00:00 2001 From: Georg Brandl Date: Thu, 23 Aug 2007 21:42:54 +0000 Subject: [PATCH] Bug #1758696: more info about descriptors. --- Doc/reference/datamodel.rst | 13 ++++++++++--- 1 file changed, 10 insertions(+), 3 deletions(-) diff --git a/Doc/reference/datamodel.rst b/Doc/reference/datamodel.rst index 030d1d9272a..18ebe76df93 100644 --- a/Doc/reference/datamodel.rst +++ b/Doc/reference/datamodel.rst @@ -1546,11 +1546,11 @@ Super Binding ``A.__dict__['m'].__get__(obj, A)``. For instance bindings, the precedence of descriptor invocation depends on the -which descriptor methods are defined. Data descriptors define both -:meth:`__get__` and :meth:`__set__`. Non-data descriptors have just the +which descriptor methods are defined. Normally, data descriptors define both +:meth:`__get__` and :meth:`__set__`, while non-data descriptors have just the :meth:`__get__` method. Data descriptors always override a redefinition in an instance dictionary. In contrast, non-data descriptors can be overridden by -instances. +instances. [#]_ Python methods (including :func:`staticmethod` and :func:`classmethod`) are implemented as non-data descriptors. Accordingly, instances can redefine and @@ -2242,6 +2242,13 @@ For more information on context managers, see :ref:`typecontextmanager`. .. [#] This, and other statements, are only roughly true for instances of new-style classes. +.. [#] A descriptor can define any combination of :meth:`__get__`, + :meth:`__set__` and :meth:`__delete__`. If it does not define :meth:`__get__`, + then accessing the attribute even on an instance will return the descriptor + object itself. If the descriptor defines :meth:`__set__` and/or + :meth:`__delete__`, it is a data descriptor; if it defines neither, it is a + non-data descriptor. + .. [#] For operands of the same type, it is assumed that if the non-reflected method (such as :meth:`__add__`) fails the operation is not supported, which is why the reflected method is not called.