Hi guys, I have a problem with looping over numpy arrays in C. I modify the array in-place (and return None), but after modification, the array doesn't seem to play nice any more. Below, I have the C code for a function do_something (a stripped version of my original function), which has as arguments a 1D numpy array (float or complex) and either an int or a sequence of ints. In python, I do the following using only the integer:

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a = array([0., 1., 2., 3., 2., 1., 0.]) do_something(a,3) savetxt('bla',a)

Which works fine. However, when I do the same, but with a list of any length larger than 0:

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a = array([0., 1., 2., 3., 2., 1., 0.]) do_something(a,[3]) savetxt('bla',a) Traceback (most recent call last): File "<stdin>", line 1, in <module> File "/usr/lib/python2.5/site-packages/numpy/core/numeric.py", line 767, in savetxt X = asarray(X) File "/usr/lib/python2.5/site-packages/numpy/core/numeric.py", line 132, in asarray return array(a, dtype, copy=False, order=order) TypeError: an integer is required savetxt('bla',a)

For some reason, the first time it doesn't work; the asarray fails but then succeeds on a second try. The resulting file is identical to when I would have looped over the list in python, and called the do_something function with each integer separately. For instance: for i in [3,1,0]: do_something(a, i) works fine as well. So apparently looping in C temporarily leaves the array in a weird state but manages to automatically be restored after one exception. I've checked that type(a), a.dtype, a.shape and a.ndim remain the same before and after calling do_something with a sequence or with an integer as second argument. That doesn't seem to be the problem. The reason that I want do the loop in C is that I need some precalculations, before being able to do the actual loop. But they might not be time-consuming enough to warrant writing it in C, so I could just do the loop in python and not have this problem any more. However, if the loop in C manages to somehow (temporarily) corrupt the array, how can I be sure that the single-shot call doesn't succeed by accident? If anyone can help, suggest something to try or spot my mistake, I'd appreciate it. Thanks, Sumant static PyObject* do_something(PyObject *self, PyObject *args, PyObject *kwargs) { PyObject *input, *s, *item; PyArrayObject *array = NULL; npy_intp i,n; static char *kwlist[] = {"a", "index", NULL}; if (!PyArg_ParseTupleAndKeywords(args, kwargs, "Oi", kwlist, &input, &i)) goto sequence; if ((array = (PyArrayObject *) PyArray_FromAny(input, NULL, 1, 1, NPY_CARRAY | NPY_UPDATEIFCOPY, NULL)) == NULL) goto error; n = PyArray_DIM(array,0); if (PyArray_ISCOMPLEX(array)) do_something_complex((complex*)(PyArray_DATA(array)), n, i); else if (PyArray_ISFLOAT(array)) do_something_double((double*)(PyArray_DATA(array)), n, i); else goto error; Py_DECREF(array); Py_RETURN_NONE; sequence: if (!PyArg_ParseTupleAndKeywords(args, kwargs, "OO", kwlist, &input, &s)) goto error; if (PySequence_Check(s) == 0) goto error; if ((array = (PyArrayObject *) PyArray_FromAny(input, NULL, 1, 1, NPY_CARRAY | NPY_UPDATEIFCOPY, NULL)) == NULL) goto error; n = PyArray_DIM(array,0); if (PyArray_ISCOMPLEX(array)) { complex* data = (complex*)PyArray_DATA(array); for (i = 0; i < PySequence_Size(s); i++) { item = PySequence_GetItem(s, i); do_something_complex(data, n, PyInt_AsSsize_t(item)); Py_DECREF(item); } } else if (PyArray_ISFLOAT(array)) { double* data = (double*)PyArray_DATA(array); for (i = 0; i < PySequence_Size(s); i++) { item = PySequence_GetItem(s, i); do_something_double(data, n, PyInt_AsSsize_t(item)); Py_DECREF(item); } } else goto error; Py_DECREF(array); Py_RETURN_NONE; error: Py_XDECREF(array); return NULL; }