[Python-Dev] Data descriptor doc/implementation inconsistency

Nick Coghlan ncoghlan at gmail.com
Mon Jan 11 22:12:39 CET 2010

Benjamin Peterson wrote:
 > My question is: Is this a doc bug or a implementation bug? If the
> former, it will be the description of a data descriptor much less
> consistent, since it will require that a __get__ method be present,
> too. If the latter, the fix may break some programs relying on the
> ability to "cache" a value in the instance dictionary.
> [1] http://docs.python.org/reference/datamodel#invoking-descriptors

I would call it a documentation bug: by leaving out the __get__ you're
telling Python to override *just* the setting of the attribute, while
still using the normal lookup process for retrieving the attribute (i.e.
allowing the attribute to be shadowed in the instance dictionary).

Adding a "print('Descriptor Invoked')" to the __set__ method in your

>>> x = X()
>>> print(x.attr)
<__main__.Descr object at 0x7f283b51ac50>
>>> x.attr = 42
Descriptor invoked
>>> print(x.attr)
>>> x.attr = 6*9
Descriptor invoked
>>> print(x.attr)

Note that the behaviour here is still different from that of a data
descriptor: with a data descriptor, once it gets shadowed in the
instance dictionary, the descriptor is ignored *completely*. The only
way to get the descriptor involved again is to eliminate the shadowing.
The non-data descriptor with only __set__ is just choosing not to
override the attribute lookup process.


Nick Coghlan   |   ncoghlan at gmail.com   |   Brisbane, Australia

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