code: def test_save_checkpoint_formatting(self, mock_save):
      torchmodel = Mock()
      optim = Mock()
      state = {
          torchbearer.SELF: Trial(torchmodel, optim, None, []),
          torchbearer.METRICS: {},
          torchbearer.EPOCH: 2,
      }
  
      file_format = "test_file_{epoch}.pt"
      check = _Checkpointer(file_format)
      check.save_checkpoint(state)
      self.assertEqual(mock_save.call_count, 1)
  
      self.assertTrue(mock_save.call_args[0][1] == "test_file_2.pt")
  
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 4 for calling the TestCheckpointer obj's underlying member method code with example usage: obj.test_save_checkpoint_formatting(mock_save) without return types