[Numpy-discussion] PR to add an initializer kwarg to ufunc.reduce (and similar functions)

Mon Mar 26 11:16:34 EDT 2018

OK, the new documentation is actually clear:

    initializer : scalar, optional
        The value with which to start the reduction.
        Defaults to the `~numpy.ufunc.identity` of the ufunc.
        If ``None`` is given, the first element of the reduction is used,
        and an error is thrown if the reduction is empty. If ``a.dtype`` is
        ``object``, then the initializer is _only_ used if reduction is empty.

I would actually like to say that I do not like the object special case
much (and it is probably the reason why I was confused), nor am I quite
sure this is what helps a lot? Logically, I would argue there are two
things:

 1. initializer/start (always used)
 2. default (oly used for empty reductions)

For example, I might like to give `np.nan` as the default for some
empty reductions, this will not work. I understand that this is a
minimal invasive PR and I am not sure I find the solution bad enough to
really dislike it, but what do other think? My first expectation was
the default behaviour (in all cases, not just object case) for some
reason.

To be honest, for now I just wonder a bit: How hard would it be to do
both, or is that too annoying? It would at least get rid of that
annoying thing with object ufuncs (which currently have a default, but
not really an identity/initializer).

Best,

Sebastian

On Mon, 2018-03-26 at 08:20 -0400, Hameer Abbasi wrote:
> Actually, the behavior right now isn’t that of `default` but that of
> `initializer` or `start`.
> 
> This was discussed further down in the PR but to reiterate:
> `np.sum([10], initializer=5)` becomes `15`.
> 
> Also, `np.min([5], initializer=0)` becomes `0`, so it isn’t really
> the default value, it’s the initial value among which the reduction
> is performed.
> 
> This was the reason to call it initializer in the first place. I like
> `initial` and `initial_value` as well, and `start` also makes sense
> but isn’t descriptive enough.
> 
> Hameer
> Sent from Astro for Mac
> 
> > On Mar 26, 2018 at 12:06, Sebastian Berg <sebastian at sipsolutions.ne
> > t> wrote:
> > 
> > Initializer or this sounds fine to me. As an other data point which
> > I
> > think has been mentioned before, `sum` uses start and min/max use
> > default. `start` does not work, unless we also change the code to
> > always use the identity if given (currently that is not the case),
> > in
> > which case it might be nice. However, "start" seems a bit like
> > solving
> > a different issue in any case.
> > 
> > Anyway, mostly noise. I really like adding this, the only thing
> > worth
> > discussing a bit is the name :).
> > 
> > - Sebastian
> > 
> > 
> > On Mon, 2018-03-26 at 05:57 -0400, Hameer Abbasi wrote:
> > > It calls it `initializer` - See https://docs.python.org/3.5/libra
> > > ry/f
> > > unctools.html#functools.reduce
> > > 
> > > Sent from Astro for Mac
> > > 
> > > > On Mar 26, 2018 at 09:54, Eric Wieser <wieser.eric+numpy at gmail.
> > > > com>
> > > > wrote:
> > > > 
> > > > It turns out I mispoke - functools.reduce calls the argument
> > > > `initial`
> > > > 
> > > > On Mon, 26 Mar 2018 at 00:17 Stephan Hoyer <shoyer at gmail.com>
> > > > wrote:
> > > > > This looks like a very logical addition to the reduce
> > > > > interface.
> > > > > It has my support!
> > > > > 
> > > > > I would have preferred the more descriptive name
> > > > > "initial_value",
> > > > > but consistency with functools.reduce makes a compelling case
> > > > > for
> > > > > "initializer".
> > > > > 
> > > > > On Sun, Mar 25, 2018 at 1:15 PM Eric Wieser <wieser.eric+nump
> > > > > y at gm
> > > > > ail.com> wrote:
> > > > > > To reiterate my comments in the issue - I'm in favor of
> > > > > > this.
> > > > > > 
> > > > > > It seems seem especially valuable for identity-less
> > > > > > functions
> > > > > > (`min`, `max`, `lcm`), and the argument name is consistent
> > > > > > with
> > > > > > `functools.reduce`. too.
> > > > > > 
> > > > > > The only argument I can see against merging this would be
> > > > > > `kwarg`-creep of `reduce`, and I think this has enough use
> > > > > > cases to justify that.
> > > > > > 
> > > > > > I'd like to merge in a few days, if no one else has any
> > > > > > opinions.
> > > > > > 
> > > > > > Eric
> > > > > > 
> > > > > > On Fri, 16 Mar 2018 at 10:13 Hameer Abbasi <einstein.edison
> > > > > > @gma
> > > > > > il.com> wrote:
> > > > > > > Hello, everyone. I’ve submitted a PR to add a initializer
> > > > > > > kwarg to ufunc.reduce. This is useful in a few cases,
> > > > > > > e.g.,
> > > > > > > it allows one to supply a “default” value for identity-
> > > > > > > less
> > > > > > > ufunc reductions, and specify an initial value for
> > > > > > > reductions
> > > > > > > such as sum (other than zero.)
> > > > > > > 
> > > > > > > Please feel free to review or leave feedback, (although I
> > > > > > > think Eric and Marten have picked it apart pretty well).
> > > > > > > 
> > > > > > > https://github.com/numpy/numpy/pull/10635
> > > > > > > 
> > > > > > > Thanks,
> > > > > > > 
> > > > > > > Hameer
> > > > > > > Sent from Astro for Mac
> > > > > > > 
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