Turn a string into a real index, otherwise return the index.

def _get_index(self, dim, index):
    """Turn a string into a real index, otherwise return the index."""
    if isinstance(index, six.string_types):
      try:
        return self._index_names[dim][index]
      except KeyError:
        raise KeyError("Name '%s' is invalid for axis %s." % (index, dim))
      except TypeError:
        raise TypeError(
            "Trying to access an unnamed axis %s by name: '%s'" % (dim, index))
    else:
      return index