[Numpy-discussion] Renaming a field of an object array

[Brent Pedersen]

On Wed, Feb 4, 2009 at 2:50 PM, Pierre GM <pgmdevlist at gmail.com> wrote:
> All,
> I'm a tad puzzled by the following behavior (I'm trying to correct a
> bug in genfromtxt):
>
> I'm creating an empty structured ndarray, using np.object as dtype.
>
>  >>> a = np.empty(1,dtype=[('',np.object)])
> array([(None,)],
>       dtype=[('f0', '|O4')])
>
> Now, I'd like to rename the field:
>  >>> a.view([('NAME',np.object)])
> TypeError: Cannot change data-type for object array.
>
> I understand why I can't change the *type* of the field, but not why I
> can't change its name that way. What would be an option that wouldn't
> involve creating a new array ?
> Thx in advance.
>
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>

hi, i was looking at this as well. the code in arrayobject.c doesnt
match the error string. i changed the code to do what the error string
says and seems thing to work.
i think the if-block below it should also use xor (not changed in this
patch), but i'm not a c programmer so i may be missing something
obvious.

svn diff numpy/core/src/arrayobject.c
Index: numpy/core/src/arrayobject.c
===================================================================
--- numpy/core/src/arrayobject.c        (revision 6338)
+++ numpy/core/src/arrayobject.c        (working copy)
@@ -6506,9 +6506,16 @@
         PyErr_SetString(PyExc_TypeError, "invalid data-type for array");
         return -1;
     }
-    if (PyDataType_FLAGCHK(newtype, NPY_ITEM_HASOBJECT) ||
-        PyDataType_FLAGCHK(newtype, NPY_ITEM_IS_POINTER) ||
-        PyDataType_FLAGCHK(self->descr, NPY_ITEM_HASOBJECT) ||
+    if (PyDataType_FLAGCHK(newtype, NPY_ITEM_HASOBJECT) ^
+            PyDataType_FLAGCHK(self->descr, NPY_ITEM_HASOBJECT)) {
+        PyErr_SetString(PyExc_TypeError,                      \
+                        "Cannot change data-type for object " \
+                        "array.");
+        Py_DECREF(newtype);
+        return -1;
+    }
+
+    if (PyDataType_FLAGCHK(newtype, NPY_ITEM_IS_POINTER) ||
         PyDataType_FLAGCHK(self->descr, NPY_ITEM_IS_POINTER)) {
         PyErr_SetString(PyExc_TypeError,                      \
                         "Cannot change data-type for object " \