[Tutor] Why is an OrderedDict not sliceable?

[Albert-Jan Roskam]

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> Date: Wed, 27 Jan 2016 02:24:35 +1100
> From: steve at pearwood.info
> To: tutor at python.org
> Subject: Re: [Tutor] Why is an OrderedDict not sliceable?
>
> On Sun, Jan 24, 2016 at 07:47:47PM +0000, Albert-Jan Roskam wrote:
>
>>> You appear to be confusing ordered and sorted.
>>
>> You are correct. Is there a difference in the way those terms are
>> used colloquially vs. in the field of Computer Science (Note: English
>> is not my mother tongue)?
>
> In ordinary English, "ordered" and "sorted" often are used to mean the
> same thing. People do often use sorted and ordered as interchangeable,
> but the definitions are slightly different:
>
>
>
> ordered \ordered\ adj.
> 1. having or evincing a systematic arrangement; especially,
> having elements succeeding in order according to rule; as,
> an ordered sequence; an ordered pair. Opposite of
> disordered or unordered. [Narrower terms:
> abecedarian, alphabetical; {consecutive, sequent,
> sequential, serial, successive ]
> [WordNet 1.5 +PJC]
>
> 2. arranged in order.
>
> Sort \Sort\, v. t. [imp. & p. p. Sorted; p. pr. & vb. n.
> Sorting.]
> 1. To separate, and place in distinct classes or divisions,
> as things having different qualities; as, to sort cloths
> according to their colors; to sort wool or thread
> according to its fineness.
> [1913 Webster]
>
>
> The way I would put it is that "sorted" means the items are ordered
> according to some specific rule or property of the items themselves,
> e.g. to sort your clothes by colour. "Ordered" is more general: it just
> means to have some order, which may be according to a rule or property,
> or it may be in whatever sequence the items happen to have.
>
> Books on a shelf have some order, the order that they appear when you
> read them from left to right, regardless of whether they are sorted by
> author, title, height, colour or at random. Books jumbled up in a bag
> have no order.
>
> Ordinary dicts are like books in a bag. You reach in and grab whatever
> book happens to come to hand first. OrderedDicts are like books on a
> shelf: you can systematically touch each book in order starting from the
> left, and new books are always added to the right.

Thank you! That distinction is indeed quite subtle. With "ordered" I tend to think of "ordinal measurement level" or something like that. E.g., sort a list of heights, calculate the median, ntiles etc. But your description makes it a whole lot clearer. Sometimes analogies work better!