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[Python-Dev] what is a dict_keys and where can I import it from?

Steven D'Aprano steve at pearwood.info
Tue Feb 12 12:27:21 CET 2013 
On 12/02/13 18:05, Chris Withers wrote:
> Hi all,
>
> So, dicts in Python 3 return "something different" from their keys and values methods:
>
>> >> dict(x=1, y=2).keys()
> dict_keys(['y', 'x'])
>> >> type(dict(x=1, y=2).keys())
> <class 'dict_keys'>
>
> I have vague memories of these things being referred to as views or some such? Where can I learn more?

The Fine Manual is usually a good place to refresh your vague memories :-)

http://docs.python.org/3/library/stdtypes.html#dict.keys

By the way, they're also in Python 2.7, only they're called  "viewkeys" instead.



> More importantly, how can I tell if I have one of them?

Depends why you care. You may not care, but for those times where you do, they are in collections.


py> from collections import KeysView
py> keys = {}.keys()
py> isinstance(keys, KeysView)
True


An anomaly, which I cannot explain:

py> issubclass(type(keys), KeysView)
True
py> type(keys) is KeysView
False
py> type(keys).__mro__
(<class 'dict_keys'>, <class 'object'>)


This disturbs my calm, because I expect that if issubclass returns True, the two classes will either be identical, or the second will be in the MRO of the first. What have I missed?



> I guess I can keep a reference to type({}.keys()) somewhere, but that feels a bit yucky.

I remember Python 1.4 days when the only way to type-test something was:

if type(something) is type([]):
     ...

so dynamically grabbing the type from a literal when needed does not seem the least bit yucky to me.


-- 
Steven
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