implement Axis.astype

[gdementen]

I relatively often need to change an axis labels dtype. For example to convert labels to int64, I currently have to do:

>>> arr = ndtest("a=1..3")
>>> arr.a.dtype
dtype('int32')
>>> arr.set_labels('a', arr.a.labels.astype(np.int64))
>>> # this would be a tad shorter and makes sense IMO
... arr.set_labels('a', arr.a.astype(np.int64))

In an ideal world, arr.set_labels('a', np.int64) would work too (but I am unsure it is possible). It currently returns an object array for the axis labels:

>>> arr.set_labels('a', np.int64).a.dtype
dtype('O')

[gdementen]

Here is a more realistic example: convert "string" labels to integers

>>> arr = zeros('a=2000,2001,total').drop('total')
>>> arr.a
Axis(['2000', '2001'], 'a')
>>> arr.set_labels('a', int).a
Axis([2000, 2001], 'a')
>>> arr.set_labels('a', int).a.dtype
dtype('O')