[Numpy-discussion] Bug in np.nonzero / Should index returning functions return ndarray subclasses?

[Jaime Fernández del Río]

There is a reported bug (issue #5837
<https://github.com/numpy/numpy/issues/5837>) regarding different returns
from np.nonzero with 1-D vs higher dimensional arrays. A full summary of
the differences can be seen from the following output:

>>> class C(np.ndarray): pass
...
>>> a = np.arange(6).view(C)
>>> b = np.arange(6).reshape(2, 3).view(C)
>>> anz = a.nonzero()
>>> bnz = b.nonzero()

>>> type(anz[0])
<type 'numpy.ndarray'>
>>> anz[0].flags
  C_CONTIGUOUS : True
  F_CONTIGUOUS : True
  OWNDATA : True
  WRITEABLE : True
  ALIGNED : True
  UPDATEIFCOPY : False
>>> anz[0].base

>>> type(bnz[0])
<class '__main__.C'>
>>> bnz[0].flags
  C_CONTIGUOUS : False
  F_CONTIGUOUS : False
  OWNDATA : False
  WRITEABLE : False
  ALIGNED : True
  UPDATEIFCOPY : False
>>> bnz[0].base
array([[0, 1],
       [0, 2],
       [1, 0],
       [1, 1],
       [1, 2]])

The original bug report was only concerned with the non-writeability of
higher dimensional array returns, but there are more differences: 1-D
always returns an ndarray that owns its memory and is writeable, but higher
dimensional arrays return views, of the type of the original array, that
are non-writeable.

I have a branch that attempts to fix this by making both 1-D and n-D arrays:

   1. return a view, never the base array,
   2. return an ndarray, never a subclass, and
   3. return a writeable view.

I guess the most controversial choice is #2, and in fact making that change
breaks a few tests. I nevertheless think that all of the index returning
functions (nonzero, argsort, argmin, argmax, argpartition) should always
return a bare ndarray, not a subclass. I'd be happy to be corrected, but I
can't think of any situation in which preserving the subclass would be
needed for these functions.

Since we are changing the returns of a few other functions in 1.10
(diagonal, diag, ravel), it may be a good moment to revisit the behavior
for these other functions. Any thoughts?

Jaime

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