[Numpy-discussion] Follow-up Numarray header PEP

gerard.vermeulen at grenoble.cnrs.fr gerard.vermeulen at grenoble.cnrs.fr
Sun Jul 18 14:25:10 CDT 2004

Hi Todd,

This is a follow-up on the 'header pep' discussion.

The attachment numnum-0.1.tar.gz contains the sources for the
extension modules pep and numnum.  At least on my systems, both
modules behave as described in the 'numarray header PEP' when the
extension modules implementing the C-API are not present (a situation
not foreseen by the macros import_array() of Numeric and especially
numarray).  IMO, my solution is 'bona fide', but requires further

The pep module shows how to handle the colliding C-APIs of the Numeric
and numarray extension modules and how to implement automagical
conversion between Numeric and numarray arrays.

For a technical reason explained in the README, the hard work of doing
the conversion between Numeric and numarray arrays has been delegated
to the numnum module.  The numnum module is useful when one needs to
convert from one array type to the other to use an extension module
which only exists for the other type (eg. combining numarray's image
processing extensions with pygame's Numeric interface):

Python 2.3+ (#1, Jan  7 2004, 09:17:35)
[GCC 3.3.1 (SuSE Linux)] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import numnum; import Numeric as np; import numarray as na
>>> np1 = np.array([[1, 2], [3, 4]]); na1 = numnum.toNA(np1)
>>> na2 = na.array([[1, 2, 3], [4, 5, 6]]); np2 = numnum.toNP(na2)
>>> print type(np1); np1; type(np2); np2
<type 'array'>
array([[1, 2],
       [3, 4]])
<type 'array'>
array([[1, 2, 3],
       [4, 5, 6]],'i')
>>> print type(na1); na1; type(na2); na2
<class 'numarray.numarraycore.NumArray'>
array([[1, 2],
       [3, 4]])
<class 'numarray.numarraycore.NumArray'>
array([[1, 2, 3],
       [4, 5, 6]])

The pep module shows how to implement array processing functions which
use the Numeric, numarray or Sequence C-API:

static PyObject *
wysiwyg(PyObject *dummy, PyObject *args)
    PyObject *seq1, *seq2;
    PyObject *result;

    if (!PyArg_ParseTuple(args, "OO", &seq1, &seq2))
        return NULL;

    switch(API) {
    case NumericAPI:
        PyObject *np1 = NN_API->toNP(seq1);
        PyObject *np2 = NN_API->toNP(seq2);
        result = np_wysiwyg(np1, np2);
    case NumarrayAPI:
        PyObject *na1 = NN_API->toNA(seq1);
        PyObject *na2 = NN_API->toNA(seq2);
        result = na_wysiwyg(na1, na2);
    case SequenceAPI:
        result = seq_wysiwyg(seq1, seq2);
        PyErr_SetString(PyExc_RuntimeError, "Should never happen");
        return 0;

    return result;

See the README for an example session using the pep module showing that
it is possible pass a mix of Numeric and numarray arrays to pep.wysiwyg().


- it is straightforward to adapt pep and numnum so that the conversion
  functions are linked into pep instead of imported.

- numnum is still 'proof of concept'.  I am thinking about methods to
  make those techniques safer if the numarray (and Numeric?) header
  files make it never into the Python headers (or make it safer to
  use those techniques with Python < 2.4).  In particular it would
  be helpful if the numerical C-APIs export an API version number,
  similar to the versioning scheme of shared libraries -- see the
  libtool->versioning info pages. 

I am considering three possibilities to release a more polished
version of numnum (3rd party extension writers may prefer to link
rather than import numnum's functionality):

1. release it from PyQwt's project page
2. register an independent numnum project at SourceForge
3. hand numnum over to the Numerical Python project (frees me from
   worrying about API changes).

Regards -- Gerard Vermeulen

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