Stefan van der Walt wrote:
>On Wed, Oct 18, 2006 at 09:17:49PM -0400, Pierre GM wrote:
>
>
>>On Wednesday 18 October 2006 20:29, Stefan van der Walt wrote:
>>
>>
>>>A quick question on extending numpy arrays: is it possible to easily
>>>add an attribute to an ndarray?
>>>
>>>
>>It might be easier to create a subclass: pleasehave a look here:
>>http://projects.scipy.org/scipy/numpy/attachment/wiki/MaskedArray/test_subclasses.py
>>That's a tiny example of subclassing ndarrays, with some extra attributes.
>>(BTW, I'm not sure that's the most obvious place where to look: if it turns
>>out to be useful, I'll put it on the scipy wiki)
>>
>>
>
>Thanks very much, Pierre.
>
>If I understand correctly, the following should work:
>
>import numpy as N
>
>class InfoArray(N.ndarray):
> def __new__(info_arr_cls,arr,info={}):
> info_arr_cls.info = info
> return N.array(arr).view(info_arr_cls)
>
>When does __array_finalize__ get called, and is it always necessary to
>specify it?
>
>
Actually something as simple as
class InfoArray(N.ndarray):
pass
will allow you to add attributes to InfoArray.
I just learned about how to allow built-ins to have attributes assigned
to their instances. It's actually pretty easy because of Python
support for it --- but it comes at a cost. You have to add a dictionary
to the PyArrayObject structure, create that dictionary when the ndarray
is allocated, and set the tp_dictoffset in the TypeObject structure to
its location in PyArrayObject. It takes 4 lines of code with the cost of
creating a new dictionary for every ndarray.
I don't think the extra bytes for every ndarray object are worth it,
given how easy it is to sub-class and create your own ndarray that can
have attributes attached. What are others opinions.
-Travis
P.S. Here is the patch that adds it:
Index: numpy/core/include/numpy/ndarrayobject.h
===================================================================
--- numpy/core/include/numpy/ndarrayobject.h (revision 3366)
+++ numpy/core/include/numpy/ndarrayobject.h (working copy)
@@ -1172,6 +1172,7 @@
PyArray_Descr *descr; /* Pointer to type structure */
int flags; /* Flags describing array -- see below*/
PyObject *weakreflist; /* For weakreferences */
+ PyObject *instancedict; /* For instance attributes */
} PyArrayObject;
#define NPY_AO PyArrayObject
Index: numpy/core/src/arrayobject.c
===================================================================
--- numpy/core/src/arrayobject.c (revision 3366)
+++ numpy/core/src/arrayobject.c (working copy)
@@ -1906,6 +1906,8 @@
if (self->weakreflist != NULL)
PyObject_ClearWeakRefs((PyObject *)self);
+ Py_DECREF(self->instancedict);
+
if(self->base) {
/* UPDATEIFCOPY means that base points to an
array that should be updated with the contents
@@ -5305,6 +5307,7 @@
self->descr = descr;
self->base = (PyObject *)NULL;
self->weakreflist = (PyObject *)NULL;
+ self->instancedict = PyDict_New();
if (nd > 0) {
self->dimensions = PyDimMem_NEW(2*nd);
@@ -6689,7 +6692,7 @@
0, /* tp_dict */
0, /* tp_descr_get */
0, /* tp_descr_set */
- 0, /* tp_dictoffset */
+ offsetof(PyArrayObject, instancedict), /* tp_dictoffset */
(initproc)0, /* tp_init */
array_alloc, /* tp_alloc */
(newfunc)array_new, /* tp_new */
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