how do I write a scliceable class?

[Ernest Adrogué]

Hi,

Thanks a lot for your comments. I think I've got enough
information to make a decision now.

13/02/10 @ 15:16 (+0100), thus spake Peter Otten:
> Ernest Adrogué wrote:
> 
> > I'm designing a container class that supports slicing.
> > The problem is that I don't really know how to do it.
> > 
> > class MyClass(object):
> >         def __init__(self, input_data):
> >                 self._data = transform_input(input_data)
> >         def __getitem__(self, key):
> >                 if isinstance(key, slice):
> >                         # return a slice of self
> >                         pass
> >                 else:
> >                         # return a scalar value
> >                         return self._data[key]
> > 
> > The question is how to return a slice of self.
> > First I need to create a new instance... but how? I can't
> > use MyClass(self._data[key]) because the __init__ method
> > expects a different kind of input data.
> > 
> > Another option is
> > 
> > out = MyClass.__new__(MyClass)
> > out._data = self._data[key]
> > return out
> > 
> > But then the __init__ method is not called, which is
> > undesirable because subclasses of this class might need
> > to set some custom settings in their __init__ method.
> > 
> > So what is there to do? Any suggestion?
> 
> Either 
> 
> (1) make transform_input() idempotent, i. e. ensure that
> 
> transform_input(transform_input(data)) == transform_input(data)
> 
> and construct the slice with MyClass(self._data[key])
> 
> or 
> 
> (2) require it to be invertible with
> 
> inverse_transform_input(transform_input(data)) == data
> 
> and make the slice with MyClass(inverse_transform_input(self._data[key]))
> 
> Just stating the obvious...
> 
> Peter
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