[Python-ideas] Putting `blist` into collections module

[Guido van Rossum]

On Mon, Sep 22, 2014 at 11:48 AM, <random832 at fastmail.us> wrote:

> On Sun, Sep 21, 2014, at 01:18, David Wilson wrote:
> > Coming from this perspective, I'd prefer that further additions were
> > limited to clean and far better understood structures. In this light,
> > could we perhaps instead discuss the merits of a collections.Tree,
> > collections.SortedDict or similar?
>
> As I understand it, the problem with a tree, SortedDict/SortedSet, or in
> general any collection that relies on comparison relationships (heap,
> etc), is: unlike hashing (where Hashable implies an immutable hash
> relationship), there is no way to detect whether an object implements an
> immutable well-defined ordering. And that, unlike a mutable hashed
> object (which can AIUI only lose itself), a mutable sorted object (or a
> badly-behaved one like NaN) can cause other objects in the set to be
> inserted in the wrong place or not found.
>

A good point, too often overlooked.

We could introduce a convention requiring __hash__() even though it is not
used by the implementation. After all, if the object is immutable when it
comes to a well-defined ordering, it is immutable when it comes to
equality. I don't really want to think about classes that are immutable
when it comes to == but not when it comes to <; that seems terrible.

-- 
--Guido van Rossum (python.org/~guido)
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