[Numpy-discussion] Docstring improvements for numpy.where?
Robert Kern
robert.kern@gmail....
Wed Sep 12 21:16:09 CDT 2007
Fernando Perez wrote:
> Hi all,
>
> A couple of times I've been confused by numpy.where(), and I think
> part of it comes from the docstring. Searching my gmail archive seems
> to indicate I'm not the only one bitten by this.
>
> Compare:
>
> In [14]: pdoc numpy.where
> Class Docstring:
> where(condition, | x, y)
>
> The result is shaped like condition and has elements of x and y where
> condition is respectively true or false. If x or y are not given,
> then it is equivalent to condition.nonzero().
>
> To group the indices by element, rather than dimension, use
>
> transpose(where(condition, | x, y))
>
> instead. This always results in a 2d array, with a row of indices for
> each element that satisfies the condition.
>
> with (b is just any array):
>
> In [17]: pdoc b.nonzero
> Class Docstring:
> a.nonzero() returns a tuple of arrays
>
> Returns a tuple of arrays, one for each dimension of a,
> containing the indices of the non-zero elements in that
> dimension. The corresponding non-zero values can be obtained
> with
> a[a.nonzero()].
>
> To group the indices by element, rather than dimension, use
> transpose(a.nonzero())
> instead. The result of this is always a 2d array, with a row for
> each non-zero element.;
>
>
> The sentence "The result is shaped like condition" in the where()
> docstring is misleading, since the behavior is really that of
> nonzero(). Where() *always* returns a tuple, not an array shaped like
> condition. If this were more clearly explained, along with a simple
> example for the usual case that seems to trip everyone:
>
> In [21]: a=arange(10)
>
> In [22]: N.where(a>5)
> Out[22]: (array([6, 7, 8, 9]),)
>
> In [23]: N.where(a>5)[0]
> Out[23]: array([6, 7, 8, 9])
>
> I think we'd get a lot less confusion.
>
> Or am I missing something, or just being dense (quite likely)?
That sentence applies to the 3-argument form, which has nothing to do with
nonzero() and does not yield a tuple. But in general, yes, the docstring leaves
much to be desired.
--
Robert Kern
"I have come to believe that the whole world is an enigma, a harmless enigma
that is made terrible by our own mad attempt to interpret it as though it had
an underlying truth."
-- Umberto Eco
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