[Numpy-discussion] my derived ndarray class object loses its attribute after a transpose()
Fri Nov 23 10:07:44 CST 2007
On Nov 23, 2007 5:01 PM, Sebastian Haase <email@example.com> wrote:
> On Nov 23, 2007 4:43 PM, Sebastian Haase <firstname.lastname@example.org> wrote:
> > On Nov 23, 2007 3:37 PM, Pierre GM <email@example.com> wrote:
> > > On Friday 23 November 2007 03:25:37 Sebastian Haase wrote:
> > > > Hi,
> > > > this question might habe been answered before:
> > > > I have my own ndarray-derived class. I did this, so that I can add
> > > > another "custom attribute" -- let's say
> > > > arr.filename
> > >
> > > Sebastian,
> > > Could you post the __new__ and __array_finalize__ for your class ? Are you
> > > sure you update the attribute in __array_finalize__ ?
> > > And this page may come handy:
> > > http://www.scipy.org/Subclasses
> > Aah - I guess I did not do my homework ;-)
> > Here is my code:
> > class ndarray_inMrcFile(N.memmap):
> > pass
> > data = self.data
> > data.__class__ = ndarray_inMrcFile
> > Two points:
> > 1.) Please don't get distracted by the way how I change the class type
> > "after the fact". I get the data as a memmaped slice from a file. Then
> > I do some (potentially not entirely clean) acrobatic to have this
> > slice change it's class-type so that I can attach the original memmap
> > (+ other attributes, such as filename) onto the ndarray
> > 2.) I guess the """class ndarray_inMrcFile(N.memmap):
> > pass""" construct is to simplistic ....
> > Could someone suggest a *minimal* definition, so that my attributes
> > would be preserved ?
> > A few comments about the wiki page:
> > 1) The example does not show the printed info, such as "__new__
> > received %s", in the example session
> > 2) I don't understand, why in the example the "info" attribute is set
> > in "__new__()", even though the text above says:
> > """However, we need to keep in mind that any attribute that we define
> > in the __new__ method will be shared among all the instances. If we
> > want instance-specific attributes, we still need some specific
> > initialization. We cannot use the __init__ method, as it won't be
> > called. That's where __array_finalize__ comes to play."""
> > 3) the text about "The __array_finalize__ method" should at least
> > once say that it is defined as "def __array_finalize__(self,obj)" --
> > otherwise I could only guess where the "obj" comes from.
> > 4) related to the text after: "In other terms, __array_finalize__ is called"
> > How do I know if a function or method actually invokes __new__ ;-)
> > ? Would I have to study the numpy source ?
> > 5) For the "__array_finalize__ method" there a text that says:
> > """Subclasses inherit a default implementation of this method that
> > does nothing""" --- but how about "__new__()" : what happens if you
> > don't define that (either) ?
> > (Hope these comments are helpful)
> > Thanks for the help,
> > Sebastian
> First try seems to show that just changing my class def to:
> class ndarray_inMrcFile(N.memmap):
> def __array_finalize__(self,obj):
> self.Mrc = getattr(obj, 'Mrc', None)
> Seems to add the wanted attribute back into result of transpose().
> However now I get (many!) exceptions like:
> Exception exceptions.AttributeError: "'ndarray_inMrcFile' object has no attribut
> e '_mmap'" in <bound method ndarray_inMrcFile.__del__ of
> ndarray_inMrcFile([ 6,....
> Do I need to call super.__array_finalize__(obj) first ?
This seems to work without any problem now:
self.Mrc = getattr(obj, 'Mrc', None)
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