Nice error message for empty buffer and new test case (#517)

Fixes #419

Author: matthias-wende-frequenz

src/frequenz/sdk/timeseries/_moving_window.py

@@ -305,6 +305,8 @@ def __getitem__(self, key: SupportsIndex | datetime | slice) -> float | ArrayLik
             A float if the key is a number or a timestamp.
             an numpy array if the key is a slice.
         """
+        if len(self._buffer) == 0:
+            raise IndexError("The buffer is empty.")
         if isinstance(key, slice):
             if isinstance(key.start, int) or isinstance(key.stop, int):
                 if key.start is None or key.stop is None:
@@ -327,6 +329,7 @@ def __getitem__(self, key: SupportsIndex | datetime | slice) -> float | ArrayLik
             _logger.debug("Returning value at time %s ", key)
             return self._buffer[self._buffer.datetime_to_index(key)]
         elif isinstance(key, SupportsIndex):
+            _logger.debug("Returning value at index %s ", key)
             return self._buffer[key]
 
         raise TypeError(

tests/timeseries/test_moving_window.py

@@ -111,6 +111,17 @@ async def test_access_window_by_ts_slice() -> None:
     assert np.array_equal(window[time_start:time_end], np.array([3.0, 4.0]))  # type: ignore
 
 
+async def test_access_empty_window() -> None:
+    """Test accessing an empty window, should throw IndexError"""
+    window, _ = init_moving_window(timedelta(seconds=5))
+    try:
+        window[42]
+    except IndexError as index_error:
+        assert str(index_error) == "The buffer is empty."
+    else:
+        assert False
+
+
 async def test_window_size() -> None:
     """Test the size of the window."""
     window, sender = init_moving_window(timedelta(seconds=5))