code: def test_callable_filename():
      rs = [
          [0, 1],
      ]
      comm = create_communicator("naive")
      if comm.size < 2:
          pytest.skip()
  
      def filename_fun(t):
          return "deadbeef-{.updater.iteration}".format(t)
  
      snapshot = extensions.snapshot(filename=filename_fun)
  
      trainer = mock.MagicMock()
      filename = "{}.{}".format(filename_fun(trainer), comm.rank)
  
      replica_sets = rs
      mn_snapshot = multi_node_snapshot(comm, snapshot, replica_sets)
      if comm.rank == 0:
          assert mn_snapshot.is_master
          assert filename == mn_snapshot.snapshot.filename(trainer)
      elif comm.rank == 1:
          assert not mn_snapshot.is_master
      elif comm.rank == 2:
          assert mn_snapshot.is_master
          assert filename == mn_snapshot.snapshot.filename(trainer)
      else:
          assert not mn_snapshot.is_master
  
      comm.finalize()
  
imports: import mock
  import tempfile
  import pytest
  import chainer
  import chainer.links
  import chainer.functions
  from chainer.training import extensions
  from chainer.training import StandardUpdater
  from chainer.training import Trainer
  import chainermn
  from chainermn import create_communicator
  from chainermn.extensions import multi_node_snapshot
  from chainermn.extensions import _multi_node_snapshot
label_desc: Use image node_id 3 for calling a global function with example usage: test_callable_filename() and returns: str