[Numpy-discussion] Contiguity of result of astype changed - intentional?

12 Sep 2012

      Hi,

We hit a subtle behavior change for the ``astype`` array method
between 1.6.1 and 1.7.0 beta.

In 1.6.1:

In [18]: a = np.arange(24).reshape((2, 3, 4)).transpose((1, 2, 0))

In [19]: a.flags
Out[19]:
  C_CONTIGUOUS : False
  F_CONTIGUOUS : False
  OWNDATA : False
  WRITEABLE : True
  ALIGNED : True
  UPDATEIFCOPY : False

In [20]: b = a.astype(float)

In [21]: b.flags
Out[21]:
  C_CONTIGUOUS : True
  F_CONTIGUOUS : False
  OWNDATA : True
  WRITEABLE : True
  ALIGNED : True
  UPDATEIFCOPY : False

In [22]: b.strides
Out[22]: (64, 16, 8)

So - ``a.astype(float)`` here has made a new C-contiguous array,
somewhat as implied by the 'copy' explanation in the docstring.  In
1.7.0 beta, ``a`` is the same but:

In [22]: b.flags
Out[22]:
  C_CONTIGUOUS : False
  F_CONTIGUOUS : False
  OWNDATA : True
  WRITEABLE : True
  ALIGNED : True
  UPDATEIFCOPY : False

In [23]: b.strides
Out[23]: (32, 8, 96)

Is this intended?  Is there a performance reason to keep the same
strides in 1.7.0?

Thanks for any pointers,

Matthew

[Numpy-discussion] Contiguity of result of astype changed - intentional?

Matthew Brett