How to use the mne.io.tag.find_tag function in mne

res['rr'] = tag.data.astype(np.float)  # XXX : double because of mayavi bug
    if res['rr'].shape[0] != res['np']:
        raise ValueError('Vertex information is incorrect')

    tag = find_tag(fid, this, FIFF.FIFF_MNE_SOURCE_SPACE_NORMALS)
    if tag is None:
        tag = find_tag(fid, this, FIFF.FIFF_BEM_SURF_NORMALS)
    if tag is None:
        res['nn'] = None
    else:
        res['nn'] = tag.data.copy()
        if res['nn'].shape[0] != res['np']:
            raise ValueError('Vertex normal information is incorrect')

    tag = find_tag(fid, this, FIFF.FIFF_BEM_SURF_TRIANGLES)
    if tag is None:
        raise ValueError('Triangulation not found')

    res['tris'] = tag.data - 1  # index start at 0 in Python
    if res['tris'].shape[0] != res['ntri']:
        raise ValueError('Triangulation information is incorrect')

    return res

if tag is None:
        raise ValueError('Number of triangles not found')
    res['ntri'] = int(tag.data)

    tag = find_tag(fid, this, FIFF.FIFF_MNE_COORD_FRAME)
    if tag is None:
        tag = find_tag(fid, this, FIFF.FIFF_BEM_COORD_FRAME)
        if tag is None:
            res['coord_frame'] = def_coord_frame
        else:
            res['coord_frame'] = tag.data
    else:
        res['coord_frame'] = tag.data

    # Vertices, normals, and triangles
    tag = find_tag(fid, this, FIFF.FIFF_BEM_SURF_NODES)
    if tag is None:
        raise ValueError('Vertex data not found')

    res['rr'] = tag.data.astype(np.float)  # XXX : double because of mayavi bug
    if res['rr'].shape[0] != res['np']:
        raise ValueError('Vertex information is incorrect')

    tag = find_tag(fid, this, FIFF.FIFF_MNE_SOURCE_SPACE_NORMALS)
    if tag is None:
        tag = find_tag(fid, this, FIFF.FIFF_BEM_SURF_NORMALS)
    if tag is None:
        res['nn'] = None
    else:
        res['nn'] = tag.data.copy()
        if res['nn'].shape[0] != res['np']:
            raise ValueError('Vertex normal information is incorrect')

inv['units'] = unit_dict.get(int(getattr(tag, 'data', -1)), None)

        #
        #   The actual source orientation vectors
        #
        tag = find_tag(fid, invs, FIFF.FIFF_MNE_INVERSE_SOURCE_ORIENTATIONS)
        if tag is None:
            raise Exception('Source orientation information not found')

        inv['source_nn'] = tag.data
        logger.info('    [done]')
        #
        #   The SVD decomposition...
        #
        logger.info('    Reading inverse operator decomposition...')
        tag = find_tag(fid, invs, FIFF.FIFF_MNE_INVERSE_SING)
        if tag is None:
            raise Exception('Singular values not found')

        inv['sing'] = tag.data
        inv['nchan'] = len(inv['sing'])
        #
        #   The eigenleads and eigenfields
        #
        inv['eigen_leads_weighted'] = False
        inv['eigen_leads'] = _read_named_matrix(
            fid, invs, FIFF.FIFF_MNE_INVERSE_LEADS, transpose=True)
        if inv['eigen_leads'] is None:
            inv['eigen_leads_weighted'] = True
            inv['eigen_leads'] = _read_named_matrix(
                fid, invs, FIFF.FIFF_MNE_INVERSE_LEADS_WEIGHTED,
                transpose=True)

res['vertno'] = None
    else:
        res['nuse'] = int(tag.data)
        tag = find_tag(fid, this, FIFF.FIFF_MNE_SOURCE_SPACE_SELECTION)
        if tag is None:
            raise ValueError('Source selection information missing')

        res['inuse'] = tag.data.astype(np.int).T
        if len(res['inuse']) != res['np']:
            raise ValueError('Incorrect number of entries in source space '
                             'selection')

        res['vertno'] = np.where(res['inuse'])[0]

    #   Use triangulation
    tag1 = find_tag(fid, this, FIFF.FIFF_MNE_SOURCE_SPACE_NUSE_TRI)
    tag2 = find_tag(fid, this, FIFF.FIFF_MNE_SOURCE_SPACE_USE_TRIANGLES)
    if tag1 is None or tag2 is None:
        res['nuse_tri'] = 0
        res['use_tris'] = None
    else:
        res['nuse_tri'] = tag1.data
        res['use_tris'] = tag2.data - 1  # index start at 0 in Python

    #   Patch-related information
    tag1 = find_tag(fid, this, FIFF.FIFF_MNE_SOURCE_SPACE_NEAREST)
    tag2 = find_tag(fid, this, FIFF.FIFF_MNE_SOURCE_SPACE_NEAREST_DIST)

    if tag1 is None or tag2 is None:
        res['nearest'] = None
        res['nearest_dist'] = None
    else:

def _read_one_source_space(fid, this):
    """Read one source space."""
    FIFF_BEM_SURF_NTRI = 3104
    FIFF_BEM_SURF_TRIANGLES = 3106

    res = dict()

    tag = find_tag(fid, this, FIFF.FIFF_MNE_SOURCE_SPACE_ID)
    if tag is None:
        res['id'] = int(FIFF.FIFFV_MNE_SURF_UNKNOWN)
    else:
        res['id'] = int(tag.data)

    tag = find_tag(fid, this, FIFF.FIFF_MNE_SOURCE_SPACE_TYPE)
    if tag is None:
        raise ValueError('Unknown source space type')
    else:
        src_type = int(tag.data)
        if src_type == FIFF.FIFFV_MNE_SPACE_SURFACE:
            res['type'] = 'surf'
        elif src_type == FIFF.FIFFV_MNE_SPACE_VOLUME:
            res['type'] = 'vol'
        elif src_type == FIFF.FIFFV_MNE_SPACE_DISCRETE:
            res['type'] = 'discrete'
        else:
            raise ValueError('Unknown source space type (%d)' % src_type)

    if res['type'] == 'vol':

        tag = find_tag(fid, this, FIFF.FIFF_MNE_SOURCE_SPACE_VOXEL_DIMS)

tag = find_tag(fid, this, FIFF.FIFF_MNE_SOURCE_SPACE_NNEIGHBORS)
        if tag is not None:
            nneighbors = tag.data
            tag = find_tag(fid, this, FIFF.FIFF_MNE_SOURCE_SPACE_NEIGHBORS)
            offset = 0
            neighbors = []
            for n in nneighbors:
                neighbors.append(tag.data[offset:offset + n])
                offset += n
            res['neighbor_vert'] = neighbors

        tag = find_tag(fid, this, FIFF.FIFF_COMMENT)
        if tag is not None:
            res['seg_name'] = tag.data

    tag = find_tag(fid, this, FIFF.FIFF_MNE_SOURCE_SPACE_NPOINTS)
    if tag is None:
        raise ValueError('Number of vertices not found')

    res['np'] = int(tag.data)

    tag = find_tag(fid, this, FIFF_BEM_SURF_NTRI)
    if tag is None:
        tag = find_tag(fid, this, FIFF.FIFF_MNE_SOURCE_SPACE_NTRI)
        if tag is None:
            res['ntri'] = 0
        else:
            res['ntri'] = int(tag.data)
    else:
        res['ntri'] = tag.data

    tag = find_tag(fid, this, FIFF.FIFF_MNE_COORD_FRAME)

Returns
    -------
    info : instance of Info
        The measurement info.
    """
    # This function assumes fid is being used as a context manager
    info = Info()

    # Information from the MRI file
    parent_mri = dir_tree_find(tree, FIFF.FIFFB_MNE_PARENT_MRI_FILE)
    if len(parent_mri) == 0:
        raise ValueError('No parent MEG information found in operator')
    parent_mri = parent_mri[0]

    tag = find_tag(fid, parent_mri, FIFF.FIFF_MNE_FILE_NAME)
    info['mri_file'] = tag.data if tag is not None else None
    tag = find_tag(fid, parent_mri, FIFF.FIFF_PARENT_FILE_ID)
    info['mri_id'] = tag.data if tag is not None else None

    # Information from the MEG file
    parent_meg = dir_tree_find(tree, FIFF.FIFFB_MNE_PARENT_MEAS_FILE)
    if len(parent_meg) == 0:
        raise ValueError('No parent MEG information found in operator')
    parent_meg = parent_meg[0]

    tag = find_tag(fid, parent_meg, FIFF.FIFF_MNE_FILE_NAME)
    info['meas_file'] = tag.data if tag is not None else None
    tag = find_tag(fid, parent_meg, FIFF.FIFF_PARENT_FILE_ID)
    info['meas_id'] = tag.data if tag is not None else None

    # Add channel information

raise ValueError('Vertex data not found')

    res['rr'] = tag.data.astype(np.float)  # double precision for mayavi
    if res['rr'].shape[0] != res['np']:
        raise ValueError('Vertex information is incorrect')

    tag = find_tag(fid, this, FIFF.FIFF_MNE_SOURCE_SPACE_NORMALS)
    if tag is None:
        raise ValueError('Vertex normals not found')

    res['nn'] = tag.data.copy()
    if res['nn'].shape[0] != res['np']:
        raise ValueError('Vertex normal information is incorrect')

    if res['ntri'] > 0:
        tag = find_tag(fid, this, FIFF_BEM_SURF_TRIANGLES)
        if tag is None:
            tag = find_tag(fid, this, FIFF.FIFF_MNE_SOURCE_SPACE_TRIANGLES)
            if tag is None:
                raise ValueError('Triangulation not found')
            else:
                res['tris'] = tag.data - 1  # index start at 0 in Python
        else:
            res['tris'] = tag.data - 1  # index start at 0 in Python

        if res['tris'].shape[0] != res['ntri']:
            raise ValueError('Triangulation information is incorrect')
    else:
        res['tris'] = None

    #   Which vertices are active
    tag = find_tag(fid, this, FIFF.FIFF_MNE_SOURCE_SPACE_NUSE)

return projs

    tag = find_tag(fid, nodes[0], FIFF.FIFF_NCHAN)
    if tag is not None:
        global_nchan = int(tag.data)

    items = dir_tree_find(nodes[0], FIFF.FIFFB_PROJ_ITEM)
    for item in items:
        #   Find all desired tags in one item
        tag = find_tag(fid, item, FIFF.FIFF_NCHAN)
        if tag is not None:
            nchan = int(tag.data)
        else:
            nchan = global_nchan

        tag = find_tag(fid, item, FIFF.FIFF_DESCRIPTION)
        if tag is not None:
            desc = tag.data
        else:
            tag = find_tag(fid, item, FIFF.FIFF_NAME)
            if tag is not None:
                desc = tag.data
            else:
                raise ValueError('Projection item description missing')

        # XXX : is this useful ?
        # tag = find_tag(fid, item, FIFF.FIFF_PROJ_ITEM_CH_NAME_LIST)
        # if tag is not None:
        #     namelist = tag.data
        # else:
        #     raise ValueError('Projection item channel list missing')

if tag is None:
        raise ValueError('Number of vertices not found')

    res['np'] = int(tag.data)

    tag = find_tag(fid, this, FIFF_BEM_SURF_NTRI)
    if tag is None:
        tag = find_tag(fid, this, FIFF.FIFF_MNE_SOURCE_SPACE_NTRI)
        if tag is None:
            res['ntri'] = 0
        else:
            res['ntri'] = int(tag.data)
    else:
        res['ntri'] = tag.data

    tag = find_tag(fid, this, FIFF.FIFF_MNE_COORD_FRAME)
    if tag is None:
        raise ValueError('Coordinate frame information not found')

    res['coord_frame'] = tag.data[0]

    #   Vertices, normals, and triangles
    tag = find_tag(fid, this, FIFF.FIFF_MNE_SOURCE_SPACE_POINTS)
    if tag is None:
        raise ValueError('Vertex data not found')

    res['rr'] = tag.data.astype(np.float)  # double precision for mayavi
    if res['rr'].shape[0] != res['np']:
        raise ValueError('Vertex information is incorrect')

    tag = find_tag(fid, this, FIFF.FIFF_MNE_SOURCE_SPACE_NORMALS)
    if tag is None: