On Mon, Oct 24, 2011 at 5:54 PM, David Voong <voong.david@gmail.com> wrote:

Hi guys,

I have a question regarding subclassing of the numpy.matrix class.

I read through the wiki page, http://docs.scipy.org/doc/numpy/user/basics.subclassing.html

and tried to subclass numpy.matrix, I find that if I override the __finalize_array__ method I have problems using the sum method and get the following error,

Traceback (most recent call last): File "test.py", line 61, in <module> print (a * b).sum() File "/afs/ cern.ch/user/d/dvoong/programs/lib/python2.6/site-packages/numpy/matrixlib/defmatrix.py", line 435, in sum return N.ndarray.sum(self, axis, dtype, out)._align(axis) File "/afs/ cern.ch/user/d/dvoong/programs/lib/python2.6/site-packages/numpy/matrixlib/defmatrix.py", line 370, in _align return self[0,0] File "/afs/ cern.ch/user/d/dvoong/programs/lib/python2.6/site-packages/numpy/matrixlib/defmatrix.py", line 305, in __getitem__ out = N.ndarray.__getitem__(self, index) IndexError: 0-d arrays can only use a single () or a list of newaxes (and a single ...) as an index

Hi, Thanks for asking this - I'm also trying to subclass np.matrix and running into similar problems; I never generally need to sum my vectors so this wasn't a problem I had noticed thus far. Anyway, for np.matrix, there are definitely particular issues beyond what is described on the array subclassing wiki. I think I have a workaround, based on struggling with my own subclass. This is really a hack since I'm not sure how some parts of matrix actually work, so if someone has a better solution please speak up! You didn't give details on the actual subclass, but I can recreate the error with the following minimal example (testing with Numpy 1.6.1 inside EPD 7.1): class MatSubClass1(np.matrix): def __new__(cls, input_array): obj = np.asarray(input_array).view(cls) return obj def __array_finalize__(self, obj): pass def __array_wrap__(self, out_arr, context=None): return np.ndarray.__array_wrap__(self, out_arr, context) In [2]: m1 = MatSubClass1( [[2,0],[1,1]] ) In [3]: m1.sum() ... IndexError: 0-d arrays can only use a single () or a list of newaxes (and a single ...) as an index The problem is that __array_finalize__ of the matrix class that needs to get called, to preserve dimensions (matrix should always have 2 dimensions). You can't just add the matrix __array_finalize__ because the initial call happens when you create the object, in which case obj is a ndarray object, not a matrix. So, check to see obj is a matrix first before calling it. In addition, there is some undocumented _getitem attribute inside matrix, and I do not know what it does. If you just set that attribute during __new__, you get something that seems to work: class MatSubClass2(np.matrix): def __new__(cls, input_array): obj = np.asarray(input_array).view(cls) obj._getitem = False return obj def __array_finalize__(self, obj): if isinstance(obj, np.matrix): np.matrix.__array_finalize__(self, obj) def __array_wrap__(self, out_arr, context=None): return np.ndarray.__array_wrap__(self, out_arr, context) In [4]: m2 = MatSubClass2( [[2,0],[1,1]] ) In [5]: m2.sum(), m2.sum(0), m2.sum(1) Out[5]: (4, matrix([[3, 1]]), matrix([[2], [2]])) HTH, Aronne