[Numpy-discussion] how add new attribute to a numpy array object ?
haase at msg.ucsf.edu
Sun Jul 23 14:19:04 CDT 2006
Kevin Jacobs <jacobs at bioinformed.com> wrote:
> On 7/22/06, *Sebastian Haase* <haase at msg.ucsf.edu
> <mailto:haase at msg.ucsf.edu>> wrote:
> Normally I could do
> class B(N.ndarray):
> a.__class__ = B
> BUT I get this error:
> #>>> a.__class__ = B
> Traceback (most recent call last):
> File "<input>", line 1, in ?
> TypeError: __class__ assignment: only for heap types
> What is a "heap type" ? Why ? How can I do what I want ?
> Assigning to __class__ makes sense for objects that allocate a
> dictionary for storage of attributes or have slots allocated to hold the
> values. The heap type error is due to a missing flag in the class
> definition and could be corrected. However, it may not be the best
> thing to do. Calling B(array) is certainly safer, although a bit more
Thanks for the replies.
Googling for this I was surprised myself that it IS legal (and done) to
assign to obj.__class__.
Kevin, I tried what you suggested first -- I think in C++ it would be
called "using the copy-constructor".
But I get an error - something like:
"__new__() needs at least 3 arguments"
In other words: (maybe?) in Python there is not always a
"copy-constructor" (in fact there is no constructor overloading at all ...)
So if there is "just a missing flag" - it would be great if this could
be put in.
It turns out that the "assigning to __class__"-scheme worked for the
ndarray subclass "memmap" (i.e. I was sub classing from memmap and
then I could assing origMemmapObj.__class__ = myClass)
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