Just to imform that my approach works:
 
if (_import_array() < 0) {
   /* Clear the error state since we are handling the error. */
   PyErr_Clear();
   /* ... set up for the sans-numpy case. */
}
else {
   /* ... set up for the with-numpy case. */
}
It is based on Roberts idea to call PyImport_ImportModule("numpy"); and check if that succeededand clean up. In fact, _import_array() is doing this.
The code of _import_array() is in the header file __multiarray_api.h in the numpy folder of the distributed files:
 
static int
_import_array(void)
{
  int st;
  PyObject *numpy = PyImport_ImportModule("numpy.core.multiarray");
  PyObject *c_api = NULL;
  if (numpy == NULL) return -1;
  c_api = PyObject_GetAttrString(numpy, "_ARRAY_API");
  if (c_api == NULL) {Py_DECREF(numpy); return -1;}
  if (PyCObject_Check(c_api)) {
      PyArray_API = (void **)PyCObject_AsVoidPtr(c_api);
  }
  Py_DECREF(c_api);
  Py_DECREF(numpy);
  if (PyArray_API == NULL) return -1;
  /* Perform runtime check of C API version */
  if (NPY_VERSION != PyArray_GetNDArrayCVersion()) {
    PyErr_Format(PyExc_RuntimeError, "module compiled against "\
        "ABI version %x but this version of numpy is %x", \
        (int) NPY_VERSION, (int) PyArray_GetNDArrayCVersion());
    return -1;
  }
  if (NPY_FEATURE_VERSION > PyArray_GetNDArrayCFeatureVersion()) {
    PyErr_Format(PyExc_RuntimeError, "module compiled against "\
        "API version %x but this version of numpy is %x", \
        (int) NPY_FEATURE_VERSION, (int) PyArray_GetNDArrayCFeatureVersion());
    return -1;
  }
 
  /*
   * Perform runtime check of endianness and check it matches the one set by
   * the headers (npy_endian.h) as a safeguard
   */
  st = PyArray_GetEndianness();
  if (st == NPY_CPU_UNKNOWN_ENDIAN) {
    PyErr_Format(PyExc_RuntimeError, "FATAL: module compiled as unknown endian");
    return -1;
  }
#if NPY_BYTE_ORDER ==NPY_BIG_ENDIAN
  if (st != NPY_CPU_BIG) {
    PyErr_Format(PyExc_RuntimeError, "FATAL: module compiled as "\
        "big endian, but detected different endianness at runtime");
    return -1;
  }
#elif NPY_BYTE_ORDER == NPY_LITTLE_ENDIAN
  if (st != NPY_CPU_LITTLE) {
    PyErr_Format(PyExc_RuntimeError, "FATAL: module compiled as "\
        "little endian, but detected different endianness at runtime");
    return -1;
  }
#endif
 
  return 0;
}
As you can see, this routine is doing the same at the beginning with additional tests and the return value indicates if ok or not.
 
So I only had to call PyErr_Clear(); when it failes and the problem is solved.
 
Thanks for your input.
 
Peter 

From: Peter Notebaert
Sent: Wednesday, February 03, 2010 16:57
To: Discussion of Numerical Python
Subject: Re: [Numpy-discussion] Determine if numpy is installed from anextension

Ah, that is maybe the idea:

if (_import_array() < 0) {
   /* Clear the error state since we are handling the error. */
   PyErr_Clear();
   /* ... set up for the sans-numpy case. */
}
else {
   /* ... set up for the with-numpy case. */
}

I did not call PyErr_Clear() when _import_array() < 0 and the error is probably still hanging and then given later.

I will try this this evening.

Thank you for the hints.

Peter

On Wed, Feb 3, 2010 at 4:22 PM, Robert Kern <robert.kern@gmail.com> wrote:
On Wed, Feb 3, 2010 at 03:41, David Cournapeau <cournape@gmail.com> wrote:
> On Wed, Feb 3, 2010 at 5:38 PM, Peter Notebaert <peno@telenet.be> wrote:
>> >From an extension? How to import numpy from there and then test if that
>> succeeded and that without any annoying message if possible...
>
> One obvious solution would be to simply call PyImport_Import, something like:
>
> #include <Python.h>
>
> PyMODINIT_FUNC initfoo(void)
> {
>        PyObject *m, *mod;
>
>        m = Py_InitModule("foo", NULL);
>        if (m == NULL) {
>                return;
>        }
>
>        mod = PyImport_ImportModule("numpy");
>        if (mod == NULL) {
>                return;
>        }
>        Py_DECREF(mod);

Or rather, to recover from the failed import as the OP wants to do:

mod = PyImport_ImportModule("numpy");
if (mod == NULL) {
   /* Clear the error state since we are handling the error. */
   PyErr_Clear();
   /* ... set up for the sans-numpy case. */
}
else {
   Py_DECREF(mod);
   import_array();
   /* ... set up for the with-numpy case. */
}

--
Robert Kern

"I have come to believe that the whole world is an enigma, a harmless
enigma that is made terrible by our own mad attempt to interpret it as
though it had an underlying truth."
 -- Umberto Eco
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