[Numpy-discussion] subclassing ndaray
oliphant.travis at ieee.org
Fri Feb 24 17:59:03 CST 2006
Colin J. Williams wrote:
> I have a subclass Bar, a 1-dim array which has some methods and some
> attributes. One of the attributes is a view of the Bar to permit
> different shaping.
The ndarray handles sub-classing a little-bit differently. All array
constructors go through the same C-code call which can create
sub-classes as well. (it's what gets called by ndarray.__new__).
If there is a parent object, then additionally, the
method is called right-after creation of the sub-class. This is where
attributes should be finalized. But, care must be taken in this code so
that a recursion is not setup
If this mechanism is not sufficient for you, then you need to use a
container class (for this reason UserArray has been resurrected to serve
as a default container class model---it needs more testing, however).
The problem __array_finalize__ helps fix is how to get subclasses to
work well without having to re-define every single special method like
For the most part it seems to work, but I suppose it creates a few
surprises if you are not aware of what is going on.
The most important thing to remember is that attributes are not
automatically carried over to new instances because new instances can be
created without every calling __new__ or __init__.
I'm sure this mechanism can be improved upon and I welcome suggestions.
> Suppose that 'a' is an instance of 'Bar', which has a method 'show'
> and a view attribute 'v'.
> a ^ 15 returns a Bar instance, with its methods but without the
> I am attempt to change this, Bar has a method __xor__, see below:
> def __xor__(self, other):
> ''' Exclusive or: __xor__(x, y) => x ^ y . '''
> << this loops to the recursion limit
> result= ArrayType.__xor__(self, other)
> n= self.n
> result.n= n
> result.rowSize= self.rowSize
> result.show= self.show
> result.v= _n.reshape(result.view(), (n*n, n*n))
> return result
Look at the __array_finalize__ method in defmatrix.py for ideas about
how it can be used.
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