[Python-Dev] Data descriptor doc/implementation inconsistency (original) (raw)

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 example:

x = X() print(x.attr) <__main__.Descr object at 0x7f283b51ac50> x.attr = 42 Descriptor invoked print(x.attr) 42 x.attr = 6*9 Descriptor invoked print(x.attr) 54

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.

Cheers, Nick.

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

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