Support negative integer indexing into channels (#224)

nptdms/tdms.py

@@ -719,8 +719,11 @@ def _read_slice(self, start, stop, step):
             return read_data[::step]
 
     def _read_at_index(self, index):
+        orig_index = index
+        if index < 0:
+            index = self._length + index
         if index < 0 or index >= self._length:
-            raise IndexError("Index {0} is outside of the channel bounds [0, {1}]".format(index, self._length - 1))
+            raise IndexError("Index {0} is outside of the channel bounds [0, {1}]".format(orig_index, self._length - 1))
 
         if self._cached_chunk is not None:
             # Check if we've already read and cached the chunk containing this index

nptdms/test/test_tdms_file.py

@@ -284,6 +284,18 @@ def test_indexing_channel_with_integer(index):
                 assert channel_object[index] == expected_channel_data[index]
 
 
+@given(index=strategies.integers(-8, -1))
+def test_indexing_channel_with_negative_integer(index):
+    """ Test indexing into a channel with a negative integer index
+    """
+    test_file, expected_data = scenarios.chunked_segment().values
+    with test_file.get_tempfile() as temp_file:
+        with TdmsFile.open(temp_file.file) as tdms_file:
+            for ((group, channel), expected_channel_data) in expected_data.items():
+                channel_object = tdms_file[group][channel]
+                assert channel_object[index] == expected_channel_data[index]
+
+
 def test_indexing_channel_with_integer_and_caching():
     """ Test indexing into a channel with an integer index, reusing the same file to test caching
     """