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Bug #1758696: more info about descriptors.

This commit is contained in:
Georg Brandl 2007-08-23 21:42:54 +00:00
parent 604c121eee
commit 3761422749
1 changed files with 10 additions and 3 deletions
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13
Doc/reference/datamodel.rst
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@ -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.