[Tutor] creating a subclass from superclass without __init__

[Steven D'Aprano]

On 25/08/12 06:03, Matt Gregory wrote:

> There are two classes of interest in GDAL - a Dataset which typically
>describes a geospatial raster file and Band which described a single band
>  from the Dataset. gdal.Band just raises an AttributeError within __init__,
>but a gdal.Band instance can be created using
>gdal.Dataset.GetRasterBand(bandNum).

What a nasty, horrible, pointless restriction. Is there any reason why you
need to follow it?


> I had wanted to create additional methods on gdal.Band that would allow
>some GIS type operations (e.g. overloading __add__), thus I thought I would
>  need a subclass of gdal.Band. So I was unsure of how to get an instance of
>my subclass when the only way I knew how to create an instance of the
>superclass (gdal.Band) was through the call to GetRasterBand().


class Band(object):
     def __new__(cls, *args):
         return super(Band, cls).__new__(cls, *args)
     def __init__(cls, *args):
         # Silly booby-trap.
         raise AttributeError('not only is this a pointless restriction '
             'on how you create instances, but this is the wrong '
             'exception type too!')

def GetRasterBand(*args):
     return Band.__new__(Band, *args)

class MyBand(Band):
     def __init__(self, *args):
         # Override the silly booby trap.
         pass
     def __add__(self, other):
         return 'something...'
     __rand__ = __add__



Works for me.



> Does that make any sense in relation to my contrived example? In my
>  example, I didn't really want to create a Spam instance from the
> SubSpam __new__, I just didn't know how to return a SubSpam instance
>  that was forced to go through SpamMaker.make_spam().

I don't think you can, but I don't think you need to.



-- 
Steven