[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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