[Numpy-discussion] Recarray attributes writeable (original) (raw)

Francesc Altet faltet at carabos.com
Sat Jun 17 04:17:28 EDT 2006

El dv 16 de 06 del 2006 a les 14:46 -0700, en/na Andrew Straw va escriure:

Erin Sheldon wrote:

>Anyway - Recarrays have convenience attributes such that >fields may be accessed through "." in additioin to >the "field()" method. These attributes are designed for >read only; one cannot alter the data through them. >Yet they are writeable: > > > >>>>tr=numpy.recarray(10, formats='i4,f8,f8', names='id,ra,dec') >>>>tr.field('ra')[:] = 0.0 >>>>tr.ra >>>> >>>> >array([ 0., 0., 0., 0., 0., 0., 0., 0., 0., 0.]) > > > >>>>tr.ra = 3 >>>>tr.ra >>>> >>>> >3 > > >>>>tr.field('ra') >>>> >>>> >array([ 0., 0., 0., 0., 0., 0., 0., 0., 0., 0.]) > >I feel this should raise an exception, just as with trying to write >to the "size" attribute. Any thoughts? > > I have not used recarrays much, so take this with the appropriate measure of salt. I'd prefer to drop the entire pseudo-attribute thing completely before it gets entrenched. (Perhaps it's too late.)

However, I think that this has its utility, specially when accessing to nested fields (see later). In addition, I'd suggest introducing a special accessor called, say, 'fields' in order to access the fields themselves and not the attributes. For example, if you want to access the 'strides' attribute, you can do it in the usual way:

import numpy tr=numpy.recarray(10, formats='i4,f8,f8', names='id,ra,strides') tr.strides (20,)

but, if you want to access field 'strides' you could do it by issuing:

tr.fields.strides <repr of field accessor object (shape, type...)> tr.fields.strides[:] array([ 0., 0., 0., 0., 0., 0., 0., 0., 0., 0.])

We have several advantages in adopting the previous approach:

  1. You don't mix (nor pollute) the namespaces for attributes and fields.

  2. You have a clear idea when you are accessing a variable or a field.

  3. Accessing nested columns would still be very easy: tr.field('nested1').field('nested2').field('nested3') vs tr.fields.nested1.nested2.nested3

  4. You can also define a proper getitem for accessing fields: tr.fields['nested1']['nested2']['nested3']. In the same way, elements of 'nested2' field could be accessed by: tr.fields['nested1']['nested2'][2:10:2].

  5. Finally, you can even prevent setting or deleting columns by disabling the setattr and delattr.

PyTables has adopted a similar schema for accessing nested columns, except for 4, where we decided not to accept both strings and slices for the getitem() method (you know the mantra: "there should preferably be just one way of doing things", although maybe we've been a bit too much strict in this case), and I think it works reasonably well. In any case, the idea is to decouple the attributes and fields so that they doesn't get mixed.

Implementing this shouldn't be complicated at all, but I'm afraid that I can't do this right now :-(

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