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data descriptors do not override the class dictionary if __get__ is not defined

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Adjust documentation and add a test to verify this behavior.

See http://mail.python.org/pipermail/python-dev/2010-January/095637.html for
discussion.
This commit is contained in:
Benjamin Peterson 2010-01-18 23:07:56 +00:00
parent c07336c673
commit 9179dab3f4
2 changed files with 30 additions and 11 deletions
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21
Doc/reference/datamodel.rst
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@ -1603,11 +1603,17 @@ Super Binding
``A.__dict__['m'].__get__(obj, A)``.
For instance bindings, the precedence of descriptor invocation depends on 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
which descriptor methods are defined. 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 will return the descriptor
object itself unless there is a value in the object's instance dictionary. 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. Normally, data
descriptors define both :meth:`__get__` and :meth:`__set__`, while non-data
descriptors have just the :meth:`__get__` method. Data descriptors with
:meth:`__set__` and :meth:`__get__` defined 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
@ -2434,13 +2440,6 @@ object itself in order to be consistently invoked by the interpreter).
controlled conditions. It generally isn't a good idea though, since it can
lead to some very strange behaviour if it is handled incorrectly.
.. [#] 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.