test.py

from itertools import zip_longest

class ListSlices:
    """A basic template for how to implement slice support for list-like
    objects.
    
    This exploits the fact that the semantics of range() are the same as
    list slices.
    """

    def __getitem__(self, item, value):
        if isinstance(item, slice):
            indices = item.indices(len(self))
            return [self[i] for i in range(*indices)]

        # Non-slice case
        raise NotImplementedError

    def __setitem__(self, item, value):
        if isinstance(item, slice):
            len_self = len(self)
            len_value = len(value)

            indices = item.indices(len_self)

            # Slice length
            len_slice = (indices[1] - indices[0]) // indices[2]

            if len_slice > len_value and indices[2] != 1:
                # Mimicking default Python list behaviour
                e = ("attempt to assign sequence of size {} to extended "
                     "slice of size {}").format(len_value, len_slice)
                raise ValueError(e)

            # NOTE:
            # vi is the index of the unpacked value
            # v is the value we are unpacking
            # i is the range of the indicies
            for (vi, v), i in zip_longest(enumerate(value), range(*indices)):
                if i is None:
                    # i will be None if the range is exhausted before we run
                    # out of values to unpack, so make a provision for that.
                    # This mimics Python's default behaviour.
                    i = vi + indices[1]

                if i >= len_self:
                    # If we're about to exceed array length, append onto
                    # ourselves. If you don't implement an append() operation,
                    # you'll want to change this to something equivalent (or
                    # raise an error).
                    self.append(v)
                else:
                    self[i] = v

            return

        # Non-slice case
        raise NotImplementedError