code: def test_best_only(self):
      self.assertTrue(
          isinstance(ModelCheckpoint(save_best_only=True), Best)
      )
  
imports: from unittest import TestCase
  from mock import patch, Mock
  import torchbearer
  from torchbearer import Trial
  from torchbearer.callbacks.checkpointers import _Checkpointer, ModelCheckpoint, MostRecent, Interval, Best
  import warnings
label_desc: Use image node_id 1 for calling the TestModelCheckpoint obj's underlying member method code with example usage: obj.test_best_only() without return types