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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def _validate_indices(self, indices=None):
 """Return a valid list of indices. `indices` may be a string or a list of strings. If `indices` is not supplied, returns the default_indices. """

        if indices is None:
            indices = self.default_indices
        if isinstance(indices, six.string_types):
            indices = [indices]
        return indices