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 > -- > http://mail.python.org/mailman/listinfo/python-list -- http://mail.python.org/mailman/listinfo/python-list
