[Numpy-discussion] my derived ndarray class object loses its attribute after a transpose()

[Sebastian Haase]

On Nov 23, 2007 5:01 PM, Sebastian Haase <haase at msg.ucsf.edu> wrote:
>
> On Nov 23, 2007 4:43 PM, Sebastian Haase <haase at msg.ucsf.edu> wrote:
> > On Nov 23, 2007 3:37 PM, Pierre GM <pgmdevlist at gmail.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 ?
>
> -Sebastian
>
This seems to work without any problem now:
        class ndarray_inMrcFile(N.ndarray):
            def __array_finalize__(self,obj):
                self.Mrc = getattr(obj, 'Mrc', None)

Comments?

-Sebastian