code: def _create_fake_dem_file(dem_fn, var_name, fill_value):
      from pyhdf.SD import SD, SDC
  
      h = SD(dem_fn, SDC.WRITE | SDC.CREATE)
      dem_var = h.create(var_name, SDC.INT16, (10, 10))
      dem_var[:] = np.zeros((10, 10), dtype=np.int16)
      if fill_value is not None:
          dem_var.setfillvalue(fill_value)
      h.end()
  
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 5 for calling a global function with example usage: _create_fake_dem_file(dem_fn, var_name, fill_value) without return types