'Get the dtype to use to store data from disk.'

@property
def _dtype(self):

    if (self._dtype_ is not None):
        return self._dtype_
    dtype = None
    for (raw_extra, filename) in zip(self._raw_extras, self._filenames):
        for this in raw_extra:
            if (this['ent'] is not None):
                with _fiff_get_fid(filename) as fid:
                    fid.seek(this['ent'].pos, 0)
                    tag = read_tag_info(fid)
                    if (tag is not None):
                        if (tag.type in (FIFF.FIFFT_COMPLEX_FLOAT, FIFF.FIFFT_COMPLEX_DOUBLE)):
                            dtype = np.complex128
                        else:
                            dtype = np.float64
                if (dtype is not None):
                    break
        if (dtype is not None):
            break
    if (dtype is None):
        raise RuntimeError('bug    in    reading')
    self._dtype_ = dtype
    return dtype
