[Numpy-discussion] Py-API: Deprecate `np.dtype(np.floating)` and similar dtype creation

[Sebastian Berg]

On Fri, 2020-02-14 at 14:39 -0700, Nathan wrote:
> For what it's worth, github search only finds two instances of this
> usage:
> 
> https://github.com/search?q=%22np.dtype%28np.floating%29%22&type=Code
> 

In most common thing I would expect to be `dtype=np.integer` (possibly
without the `dtype` as a positional argument).
The call your search finds is nice because it must delete `np.dtype`
call.
As is, it is doing the incorrect thing so the deprecation would flush
out a bug.

- Sebastian


> On Fri, Feb 14, 2020 at 2:28 PM Sebastian Berg <
> sebastian at sipsolutions.net> wrote:
> > Hi all,
> > 
> > In https://github.com/numpy/numpy/pull/15534 I would like to start
> > deprecating creating dtypes from "abstract" scalar classes, such
> > as:
> > 
> > np.dtype(np.floating) is np.dtype(np.float64)
> > 
> > While, at the same time, `isinstance(np.float32, np.floating)` is
> > true.
> > 
> > Right now `arr.astype(np.floating, copy=False)` and, more
> > obviously,
> > `arr.astype(np.dtype(np.floating), copy=False)` will cast a float32
> > array to float64.
> > 
> > I think we should deprecate this, to consistently enable that in
> > the
> > future `dtype=np.floating` may choose to not cast a float32 array.
> > Of
> > course for the `astype` call the DeprecationWarning would be
> > changed to
> > a FutureWarning before we change the result value.
> > 
> > A slight (but hopefully rare) annoyance is that `np.integer` might
> > be
> > used since it reads fairly well compared to `np.int_`. The large
> > upstream packages such as SciPy or astropy seem to be clean in this
> > regard, though (at least almost clean).
> > 
> > Does anyone think this is a bad idea? To me these deprecations seem
> > fairly straight forward, possibly flush out bugs/unintended
> > behaviour,
> > and necessary for consistent future behaviour. (More similar ones
> > may
> > have to follow).
> > 
> > If there is some, but not much, hesitation, I can also add this to
> > the
> > NEP 41 draft. Although I currently feel it is the right thing to do
> > even if we never had any new dtypes.
> > 
> > - Sebastian
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