[Numpy-discussion] Attempting to sub-class NumArray

Colin J. Williams cjw at sympatico.ca
Sat Sep 6 08:26:04 CDT 2003


Todd Miller wrote:

>I think the problem you are having is caused by the assumption that
>instance initialization is equivalent to an unordered assignment of all
>instance attributes.  That's not true because the private attributes are
>interrelated;  assigning "shape" affects "_shape" and "_strides",
>_itemsize must be known before _strides can be computed from shape, and
>so on. 
>
It would be helpful if you could let us know what assignment ordering 
would be appropriate, or, if there were information about the 
interactions, perhaps we could work these things out for ourselves.

As we've discussed before, there is little information available on 
private objects and their roles.  Even to have the source code of some 
of the C code might be useful.  I wasn't able to spot the relevant C 
code among the CVS directories.

However, the assumption would appear to apply to the crude 
initialization of 'special'.  The problem which is being reported seems 
to be connected with the function setshape in generic.py.  This is far 
from the unordered assignment, although I recognize that ther might be 
some connection.

Colin W.

>
>On Fri, 2003-09-05 at 20:32, Colin J. Williams wrote:
>  
>
>>Todd,
>>
>>Thanks again for your prompt response.
>>Todd Miller wrote:
>>    
>>
>>>Hi Colin,
>>>
>>>I haven't looked at this in detail,  but special.__init__() is making me
>>>queasy.  
>>>      
>>>
>>We don't call __init__ directly, but only when we construct the
>>instance, a,  of the class special.
>>    
>>
>>>Why doesn't it call NumArray.__init__()?
>>>      
>>>
>>Because we have data to install in the newly created a.  We go off to
>>the factory function, numarray.array, to create the array, which we
>>then specialize.
>>
>>I don't see how NumArray.__init__ would help.  This cumbersome
>>procedure seems to work OK in most respects, but fails to display
>>properly.
>>
>>I don't follow the code sufficiently well to be sure, but my hunch is
>>that the problem lies with setShape.  It fails to update _strides.
>>
>>Colin W.
>>    
>>
>>>Todd
>>>
>>>On Fri, 2003-09-05 at 15:03, Colin J. Williams wrote:
>>>  
>>>      
>>>
>>>>When ravel() is used on an instance of NumArray, the _strides attribute 
>>>>is recalculated correctly.
>>>>When ravel() is used on an instance of special, a sub-class of NumArray, 
>>>>the _strides attribute keeps its old value.  This appears to cause the 
>>>>display problem which is reported.
>>>>
>>>>The essentials of the code used are below.
>>>>
>>>>To investigate this problem, two print statements were added to the 
>>>>method setshape in generic.py:
>>>>
>>>>            if newnelements == nelements:
>>>>                self._shape = tuple(shape)
>>>>                print 'In setShape self._shape:', self._shape
>>>>                self._strides = self._stridesFromShape()
>>>>                print 'In setShape self._strides:', self._strides
>>>>            else:
>>>>                raise ValueError("New shape is not consistent with the
>>>>    old shape")
>>>>
>>>>It seems that only the first of these print statements is executed with 
>>>>special, but both are executed with NumArray.  In the second case, the 
>>>>output is not in the expected sequence.
>>>>
>>>>I would appreciate any suggestion as to a workaround.
>>>>
>>>>Colin W,
>>>>
>>>>Grief when attempting to sub-class NumArray
>>>>
>>>>Code used:
>>>>
>>>># Try to sub-class
>>>>class special(N.NumArray):
>>>>  def __init__(self, data= None, shape= None, eType= None):
>>>>##                         eType= _nt.Any or AnyType not acceptable %%
>>>>    arr= N.array(sequence= data, shape= shape, type= eType)
>>>>    for attr in dir(arr):
>>>>##      This is a longwinded way of setting up special
>>>>##      There must be a better way                     %%
>>>>##      Perhaps we should use generic._view
>>>>##      It's not documented                 TRY IT LATER
>>>>##      There is no need to transfer methods!
>>>>      if attr[0] == '_' and attr[1] != '_':
>>>>        print attr
>>>>        exec 'self.' + attr + '= ' 'arr.' + attr
>>>>        exec 'print self.' + attr
>>>>    pass
>>>>
>>>>a= special(data= [1, 2, 3, 4], shape= (2, 2))
>>>>a= N.array([1, 2, 3, 4], shape= (2, 2))
>>>>print 'In ts a._strides:', a._strides
>>>>b= _gen.ravel(a)
>>>>print 'In ts b._strides:', b._strides       #  <<< Unchanged with 
>>>>special, OK with NumArray
>>>>print 'b:', b
>>>>
>>>>
>>>>
>>>>USING THE SUBCLASS special
>>>> >C:\Progra~1\Python23\pythonw -u ts.py
>>>>In ts a._strides: (8, 4)
>>>>In setShape self._shape: (4,)
>>>>In ts b._strides: (8, 4)
>>>>b: In setShape self._shape: (4,)
>>>>Traceback (most recent call last):
>>>>  File "ts.py", line 37, in ?
>>>>    print 'b:', b
>>>>  File 
>>>>"C:\PROGRA~1\Python23\lib\site-packages\numarray\numarraycore.py", line 
>>>>618, in __str__
>>>>    MAX_LINE_WIDTH, PRECISION, SUPPRESS_SMALL, ' ', "")
>>>>  File "C:\PROGRA~1\Python23\lib\site-packages\numarray\arrayprint.py", 
>>>>line 176, in array2string
>>>>    separator, prefix)
>>>>  File "C:\PROGRA~1\Python23\lib\site-packages\numarray\arrayprint.py", 
>>>>line 138, in _array2string
>>>>    max_str_len = max(len(str(max_reduce(data))),
>>>>libnumarray.error: maximum_Int32_reduce: access beyond buffer. offset=27 
>>>>buffersize=16
>>>> >Exit code: 1
>>>>__________________________________________________________
>>>>
>>>># USING NumArray
>>>>
>>>>In setShape self._shape: (2, 2)
>>>>In ts a._strides: (8, 4)
>>>>In setShape self._shape: (4,)
>>>>In ts b._strides: (4,)
>>>>b: In setShape self._shape: (4,)
>>>>[1 2 3 4]
>>>> >Exit code: 0
>>>>
>>>>
>>>>
>>>>
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>>>>        
>>>>






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