[Python-ideas] OrderedDict.peekitem()

[Andrew Barnert]

On Jul 6, 2015, at 14:30, Neil Girdhar <mistersheik at gmail.com> wrote:
> 
> SortedDict (http://www.grantjenks.com/docs/sortedcontainers/sorteddict.html) manages to support indexing. 

Only by having a special view, accessed as .index. If it just took indices as subscripts, that would be ambiguous with integer keys.

> Can OrderedDict do the same thing?

It's worth noting that most of the various different sorted containers on PyPI support something equivalent, but all with different interfaces.

More importantly, they all use logarithmic data structures (binary trees, b-trees, skip lists, the hybrid thing blist uses, ...), which give you O(log N) indexing, and some of them can do even better by giving you O(log N) to find a slice and O(1) within that slice; OrderedDict uses a linked list, so it would be O(N).

> 
>> On Monday, July 6, 2015 at 10:49:44 AM UTC-4, Kale Kundert wrote:
>> Today I was trying to use collections.OrderedDict to manage a LIFO queue, and I 
>> was surprised to realize that OrderedDict doesn't provide a way to look at its 
>> first or last item.  There is an OrderedDict.popitem() method, which removes and 
>> returns either the first or last item, but it's not hard to imagine cases where 
>> you would want to see what's on the queue without popping it right away. 
>> 
>> My proposal is to add a peekitem() method to OrderedDict.  This method would 
>> have the same signature and would return the same thing as popitem(), it just 
>> wouldn't modify the data structure. 
>> 
>> -Kale 
>> 
>> P.S. There is already a way to peek at the last item an OrderedDict, but it 
>> hides the intent of the code and you wouldn't think of it if you weren't 
>> familiar with python: next(reversed(ordered_dict)) 
>> 
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