class_docstr: Test the filehandler when channel 3a is missing.
class_docstr_tok: ["Test","the","filehandler","when","channel","3a","is","missing","."]
docstr: Test that loading a missing channel raises a keyerror.
docstr_tok: ["Test","that","loading","a","missing","channel","raises","a","keyerror","."]
code: def test_loading_missing_channels_returns_none(self):
      """Test that loading a missing channel raises a keyerror."""
      with tempfile.TemporaryFile() as tmpfile:
          self._header.tofile(tmpfile)
          tmpfile.seek(22016, 0)
          self._data.tofile(tmpfile)
  
          fh = AVHRRAAPPL1BFile(
              tmpfile, self.filename_info, self.filetype_info
          )
          info = {}
          key = make_dataid(name="3a", calibration="reflectance")
          assert fh.get_dataset(key, info) is None
  
imports: import datetime
  import os
  import tempfile
  import unittest
  from contextlib import suppress
  from unittest import mock
  import numpy
  from satpy.readers.aapp_l1b import _HEADERTYPE, _SCANTYPE, AVHRRAAPPL1BFile
  from satpy.tests.utils import make_dataid
label_desc: Use image node_id 2 for calling the TestAAPPL1BChannel3AMissing obj's underlying member method code with example usage: obj.test_loading_missing_channels_returns_none() without return types