Why are there no ordered dictionaries?

Bengt Richter bokr at oz.net
Wed Nov 23 02:53:46 CET 2005


On Tue, 22 Nov 2005 20:37:40 +0100, Christoph Zwerschke <cito at online.de> wrote:

>One implementation detail that I think needs further consideration is in 
>which way to expose the keys and to mix in list methods for ordered 
>dictionaries.
>
>In http://aspn.activestate.com/ASPN/Cookbook/Python/Recipe/107747
>the keys are exposed via the keys() method which is bad. It should be a 
>copy only, like for ordinary dicts (one comment also mentions that).
>
>In Foord/Larosa's odict, the keys are exposed as a public member which 
>also seems to be a bad idea ("If you alter the sequence list so that it 
>no longer reflects the contents of the dictionary, you have broken your 
>OrderedDict").
>
>I think it would be probably the best to hide the keys list from the 
>public, but to provide list methods for reordering them (sorting, 
>slicing etc.).
>
>For instance:
>
>d1 = OrderedDict( (1, 11), (2, 12), 3, 13) )
>
>d1[1:] ==> OrderedDict( (2, 12), 3, 13) )
>
>d1[0] + d1[2] ==> OrderedDict( (1, 11), (3, 13) )
>
>d1.reverse() ==> OrderedDict( (3, 13), (2, 12), 1, 11) )
>
>d1.insert(1, (4, 14))
>     ==> OrderedDict( (1, 11), (4, 14), (2, 12), 3, 13) )
>
>etc.
>
>But no other way to directly manipulate the keys should be provided.
>
 >>> from odictb import OrderedDict
 >>> d1 = OrderedDict([(1, 11), (2, 12), (3, 13)])
 >>> d1
 {1: 11, 2: 12, 3: 13}
 >>> d1[1:]
 {2: 12, 3: 13}
 >>> d1[0:1] + d1[2:3]
 {1: 11, 3: 13}
 >>> d1.reverse()
 >>> d1
 {3: 13, 2: 12, 1: 11}
 >>> d1.insert(1, (4,14))
 >>> d1
 {3: 13, 4: 14, 2: 12, 1: 11}
 >>> d1.items()
 [(3, 13), (4, 14), (2, 12), (1, 11)]
 >>> d1.keys()
 [3, 4, 2, 1]
 >>> d1.values()
 [13, 14, 12, 11]
 >>> d1[1:2]
 {4: 14}
 >>> d1[-1:]
 {1: 11}

Que mas?

Regards,
Bengt Richter



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