[Python-ideas] Sorted lists

Brice Parent contact at brice.xyz
Wed Apr 10 04:01:32 EDT 2019

It surely would be helpful. I still find it a bit too single-case oriented.

Maybe having an equivalent __sorting_algo__ property with a value of the 
current sorting algorithm would be more general?
There could be a SortingAlgo base class, which could be extended into 
classes like:
  - SortingAlgoNone(SortingAlgo) or SortingAlgoUnsorted(SortingAlgo) 
which would be the default for non-sorted lists (or just the value None)
  - SortingAlgoAscending(SortingAlgo)
  - SortingAlgoAscendingNumbers(SortingAlgoAscending)
  - MyCustomSortingAlgo(SortingAlgo)
  - ...
It would allow to mark a list as sorted with any algorithm, and of 
course, any code that would use these lists would be able to read/write 
this __sorting_algo__.

And complementary idea, we could have an extra arg to sort() (and other 
functions like this one) like `trust_declared_algo=True`, that if set 
would only sort the list if its list.__sorting_algo__ is compatible (a 
subclass of the sorting algo it uses, or the class itself).

The rest of the behaviours (when the __sorting_algo__ would be set or 
reset) would be as described by Steven in the original proposal.


Le 8/4/19 à 4:32, Steven D'Aprano a écrit :
> There are quite a few important algorithms which require lists to be
> sorted. For example, the bisect module, and for statistics median and
> other quantiles.
> Sorting a list is potentially expensive: while Timsort is very
> efficient, it is still ultimately an O(N log N) algorithm which limits
> how efficient it can be. Checking whether a list is sorted is O(N).
> What if we could check that lists were sorted in constant time?
> Proposal: let's give lists a dunder flag, __issorted__, that tracks
> whether the list is definitely sorted or not:
> - Empty lists, or lists with a single item, are created with
>    __issorted__ = True; lists with two or more items are created
>    with the flag set to False.
> - Appending or inserting items sets the flag to False.
> - Deleting or popping items doesn't change the flag.
> - Reversing the list doesn't change the flag.
> - Sorting it sets the flag to True. (The sort method should NOT
>    assume the list is sorted just because the flag is set.)
> Functions that require the list to be sorted can use the flag as a
> quick check:
>      if not alist.__issorted__:
>          alist.sort()
>      ...
> The flag will be writable, so that functions such as those in
> bisect can mark that they have kept the sorted invariant:
>      bisect.insort(alist, x)
>      assert alist.__issorted__
> Being writable, the flag is advisory, not a guarantee, and "consenting
> adults" applies. People can misuse the flag:
>      alist = [1, 4, 2, 0, 5]
>      alist.__issorted__ = True
> but then they have nobody to blame but themselves if they shoot
> themselves in the foot. That's no worse than the situation we have now,
> were you might pass an unsorted list to bisect.
> The flag doesn't guarantee that the list is sorted the way you want
> (e.g. biggest to smallest, by some key, etc) only that it has been
> sorted. Its up to the user to ensure they sort it the right way:
>      # Don't do this and expect it to work!
>      alist.sort(key=random.random)
>      bisect.insort(alist, 1)
> If you really want to be sure about the state of the list, you have to
> make a copy and sort it. But that's no different from the situation
> right now. But for those willing to assume "consenting adults", you
> might trust the flag and avoid sorting.
> Downsides:
> - Every list grows an extra attribute; however, given that lists are
>    already quite big data structures and are often over-allocated, I
>    don't think this will matter much.
> - insert(), append(), extend(), __setitem__() will be a tiny bit
>    slower due to the need to set the flag.
> Thoughts?

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