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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