code: def retrieve_series(self, *args, **kwargs):
      instance = Instance()
      instance["SOPInstanceUID"] = SOPInstanceUID()
      return [instance]
  
imports: import os
  import tempfile
  import unittest
  from unittest.mock import patch
  from dicomweb_client import DICOMwebClient
label_desc: Use image node_id 2 for calling the MockDICOMwebClient obj's underlying member method code with example usage: obj.retrieve_series() and returns: list