[Numpy-discussion] numpy all unexpected result (generator)

Travis Oliphant travis@continuum...
Tue Jan 31 16:17:28 CST 2012


I also agree that an exception should be raised at the very least.

It might also be possible to make the NumPy any, all, and sum functions behave like the builtins when given a generator.  It seems worth exploring at least.

Travis 

--
Travis Oliphant
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512-826-7480


On Jan 31, 2012, at 10:46 AM, Benjamin Root <ben.root@ou.edu> wrote:

> 
> On Tue, Jan 31, 2012 at 10:05 AM, Olivier Delalleau <shish@keba.be> wrote:
> Le 31 janvier 2012 10:50, Robert Kern <robert.kern@gmail.com> a écrit :
> On Tue, Jan 31, 2012 at 15:35, Benjamin Root <ben.root@ou.edu> wrote:
> >
> >
> > On Tue, Jan 31, 2012 at 9:18 AM, Robert Kern <robert.kern@gmail.com> wrote:
> >>
> >> On Tue, Jan 31, 2012 at 15:13, Benjamin Root <ben.root@ou.edu> wrote:
> >>
> >> > Is np.all() using np.array() or np.asanyarray()?  If the latter, I would
> >> > expect it to return a numpy array from a generator.
> >>
> >> Why would you expect that?
> >>
> >> [~/scratch]
> >> |37> np.asanyarray(i>5 for i in range(10))
> >> array(<generator object <genexpr> at 0xdc24a08>, dtype=object)
> >>
> >> --
> >> Robert Kern
> >
> >
> > What possible use-case could there be for a numpy array of generators?
> 
> Not many. This isn't an intentional feature, just a logical
> consequence of all of the other intentional features being applied
> consistently.
> 
> > Furthermore, from the documentation:
> >
> > numpy.asanyarray = asanyarray(a, dtype=None, order=None, maskna=None,
> > ownmaskna=False)
> >      Convert the input to an ndarray, but pass ndarray subclasses through.
> >
> >      Parameters
> >      ----------
> >      a : array_like
> >          Input data, in any form that can be converted to an array.  This
> >          includes scalars, lists, lists of tuples, tuples, tuples of tuples,
> >          tuples of lists, and ndarrays.
> >
> > Emphasis mine.  A generator is an input that could be converted into an
> > array.  (Setting aside the issue of non-terminating generators such as those
> > from cycle()).
> 
> I'm sorry, but this is not true. In general, it's too hard to do all
> of the magic autodetermination that asarray() and array() do when
> faced with an indeterminate-length iterable. We tried. That's why we
> have fromiter(). By restricting the domain to an iterable yielding
> scalars and requiring that the user specify the desired dtype,
> fromiter() can figure out the rest.
> 
> Like it or not, "array_like" is practically defined by the behavior of
> np.asarray(), not vice-versa.
> 
> In that case I agree with whoever said ealier it would be best to detect this case and throw an exception, as it'll probably save some headaches.
> 
> -=- Olivier
> 
> 
> I'll agree with this statement.  This bug has popped up a few times in the mpl bug tracker due to the pylab mode.  While I would prefer if it were possible to evaluate the generator into an array, silently returning True incorrectly for all() and any() is probably far worse.
> 
> That said, is it still impossible to make np.all() and np.any() special to have similar behavior to the built-in all() and any()?  Maybe it could catch the above exception and then return the result from python's built-ins?
> 
> Cheers!
> Ben Root
> 
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