code: def _mock_dem_retrieve(tmpdir, url):
      rmock_obj = mock.patch("satpy.modifiers._crefl.retrieve")
      rmock = rmock_obj.start()
      dem_fn = str(tmpdir.join(url))
      rmock.return_value = dem_fn
      return rmock_obj, dem_fn
  
imports: from contextlib import contextmanager
  from datetime import datetime
  from unittest import mock
  import numpy
  import pytest
  import xarray
  from dask import array
  from pyresample.geometry import AreaDefinition
  from satpy.tests.utils import assert_maximum_dask_computes
label_desc: Use image node_id 4 for calling a global function with example usage: _mock_dem_retrieve(tmpdir, url) and returns: rmock_obj,  dem_fn