class DatasetMixin(object):

    """Default implementation of dataset indexing.

    DatasetMixin provides the :meth:`__getitem__` operator. The default
    implementation uses :meth:`get_exmample` to extract each example, and
    combines the results into a list. This mixin makes it easy to implement a
    new dataset that does not support efficient slicing.

    Dataset implementation using DatasetMixin still has to provide the
    :meth:`__len__` operator explicitly.

    """
    def __getitem__(self, index):
        """Returns an example or a sequence of examples.

        It implements the standard Python indexing. It uses the
        :meth:`get_example` method by default, but it may be overridden by the
        implementation to, for example, improve the slicing performance.

        """
        if isinstance(index, slice):
            current, stop, step = index.indices(len(self))
            ret = []
            while current < stop and step > 0 or current > stop and step < 0:
                ret.append(self.get_example(current))
                current += step
            return ret
        else:
            return self.get_example(index)

    def __len__(self):
        """Returns the number of data points."""
        raise NotImplementedError

    def get_example(self, i):
        """Returns the i-th example.

        Implementations should override it. It should raise :class:`IndexError`
        if the index is invalid.

        Args:
            i (int): The index of the example.

        Returns:
            The i-th example.

        """
        raise NotImplementedError
