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
