Test coverage fix

Author: adamreeve

nptdms/test/test_tdms_file.py

@@ -482,6 +482,20 @@ def test_read_with_index_file(test_file, expected_data):
         compare_arrays(channel_obj.data, expected_channel_data)
 
 
+def test_read_index_file_only():
+    """ Test reading the index file directly
+    """
+    test_file, expected_data = scenarios.single_segment_with_two_channels().values
+    with test_file.get_tempfile_with_index() as tdms_file_path:
+        with TdmsFile.open(tdms_file_path + "_index") as tdms_file:
+            for ((group, channel), expected_channel_data) in expected_data.items():
+                channel_obj = tdms_file[group][channel]
+                assert len(channel_obj) == len(expected_channel_data)
+                with pytest.raises(RuntimeError) as exc_info:
+                    channel_obj[:]
+                assert "Data cannot be read from index file only" in str(exc_info.value)
+
+
 @pytest.mark.skipif(sys.version_info < (3, 4), reason="pathlib only available in stdlib since 3.4")
 def test_read_file_passed_as_pathlib_path():
     """ Test reading a file when using a pathlib Path object