[Python-Dev] Data descriptor doc/implementation inconsistency

Michael Foord fuzzyman at voidspace.org.uk
Mon Jan 11 23:11:44 CET 2010

On 11/01/2010 21:12, Nick Coghlan wrote:
> 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
> [snip...]
> 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.

Does that mean we need a third class of descriptors that are neither 
data descriptors nor non-data descriptors?

What should we call them: really-not-data-descriptors?

All the best,


> Cheers,
> Nick.


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