[Numpy-discussion] Syntax Improvement for Array Transpose

Tue Jun 25 17:49:33 EDT 2019

On Tue, 2019-06-25 at 17:00 -0400, Marten van Kerkwijk wrote:
> Hi Kirill, others,
> 
> Indeed, it is becoming long! That said, while initially I was quite
> charmed by Eric's suggestion of deprecating and then changing `.T`, I
> think the well-argued opposition to it has changed my opinion.
> Perhaps most persuasive to me was Matthew's point just now that code
> (or a code snippet) that worked on an old numpy should not silently
> do something different on a new numpy (unless the old behaviour was a
> true bug, of course; but here `.T` has always had a very well-defined 
> meaning - even though you are right that the documentation does not
> exactly lead the novice user away from using it for matrix transpose!
> If someone has the time to open a PR that clarifies it.........).
> 
> Note that I do agree with the sentiment that the deprecation/change
> would likely expose some hidden bugs - and, as noted, it is hard to
> know where those bugs are if they are hidden! (FWIW, I did find some
> in astropy's coordinate implementation, which was initially written
> for scalar coordinates where `.T` worked was just fine; as a result,
> astropy gained a `matrix_transpose` utility function.) Still, it does
> not quite outweigh to me the disadvantages enumerated.
> 

True, eventually switching is much more problematic than only
deprecation, and yes, I guess the last step is likely forbidding.

I do not care too much, but the at least the deprecation/warning does
not seem too bad to me unless it is really widely used for high
dimensions. Sure, it requires to touch code and may make it uglier, but
a change requiring to touch a fair amount of scripts is not all that
uncommon, especially if it can find some bugs (e.g. for me
scipy.misc.factorial moving for example meant I had to change a lot of
scripts, annoying but I could live with it).

Although, I might prefer to spend our "force users to do annoying code
changes" chips on better things. And I guess there may not be much of a
point in a mere deprecation.

> One thing seems clear: if `.T` is out, that means `.H` is out as well
> (at least as a matrix transpose, the only sensible meaning I think it
> has). Having `.H` as a conjugate matrix transpose would just cause
> more confusion about the meaning of `.T`.
> 

I tend to agree, the only way that could work seems if T was deprecated
for high dimensions.

> For the names, my suggestion of lower-casing the M in the initial
> one, i.e., `.mT` and `.mH`, so far seemed most supported (and I think
> we should discuss *assuming* those would eventually involve not
> copying data; let's not worry about implementation details). 

It would be a nice assumption, but as I said, I do see an issue with
object array support. Which makes it likely that `.H` could only be
supported on some dtypes (similar to `.real/.imag`).
(Strictly speaking it would be possible to make a ConugateObject dtype
and define casting for it, I have some doubt that the added complexity
is worth it though). The no-copy conjugate is a cool idea but
ultimately may be a bit too cool?

> So, specific items to confirm:
> 
> 1) Is this a worthy addition? (certainly, their existence would
> reduce confusion about `.T`... so far, my sense is tentative yes)
> 
> 2) Are `.mT` and `.mH` indeed the consensus? [1]
> 

It is likely the only reasonable option, unless you make `H` object
which does `arr_like**H` but I doubt that is a good idea.

> 3) What, if anything, should these new properties do for 0-d and 1-d
> arrays: pass through, change shape, or error? (logically, I think
> *new* properties should never emit warnings: either do something or
> error).
<snip>
> Marten
> 
> [1] Some sadness about mᵀ and mᴴ - but, then, there is 
> http://www.modernemacs.com/post/prettify-mode/
> 

Hehe, you are using a block for Phonetic Extensions, and that block has
a second H which looks the same on my font but is Cyrillic. Lucky us,
we could make one of them for row vectors and the other for column
vectors ;).

- Sebastian

> On Tue, Jun 25, 2019 at 4:17 PM Kirill Balunov <
> kirillbalunov at gmail.com> wrote:
> > вт, 25 июн. 2019 г. в 21:20, Cameron Blocker <
> > cameronjblocker at gmail.com>:
> > > It seems to me that the general consensus is that we shouldn't be
> > > changing .T to do what we've termed matrix transpose or conjugate
> > > transpose.
> > > 
> >  
> > Reading through this thread, I can not say that I have the same
> > opinion - at first, many looked positively at the possibility of
> > change - `arr.T` to mean a transpose of the last two dimensions by
> > default, and then people start discussing several different (albeit
> > related) topics at once. So, I want to point out that it is rather
> > difficult to follow what is currently discussed in this thread,
> > probably because several different (albeit related) topics are
> > being discussed at once. I would suggest at first discuss `arr.T`
> > change, because other topics somewhat depend on that
> > (`arr.MT`/`arr.CT`/`arr.H` and others).
> > 
> > p.s:  Documentation about  `.T` shows only two examples, for 1d -
> > to show that it works and for 2d case. Maybe it means something?
> > (especially for new `numpy` users. )
> > 
> > with kind regards,
> > -gdg
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