code: def list_labels(self):
      if self.list_labels is None:
          return ()
      return tuple(np.unique(self.list_labels))
  
imports: from __future__ import absolute_import, print_function
  import numpy
  from scipy import ndimage
  from niftynet.utilities.util_common import MorphologyOps, CacheFunctionOutput
label_desc: Use image node_id 33 for calling the PairwiseMeasures obj's underlying member method code with example usage: obj.list_labels() and returns: tuple, 