Add test for .metpy.units loading thru variable.data

tests/test_xarray.py

@@ -3,6 +3,7 @@
 # SPDX-License-Identifier: BSD-3-Clause
 """Test the operation of MetPy's XArray accessors."""
 from collections import OrderedDict
+from unittest.mock import patch, PropertyMock
 
 import numpy as np
 import pyproj
@@ -115,6 +116,13 @@ def test_units(test_var):
     assert test_var.metpy.units == units.kelvin
 
 
+def test_units_data(test_var):
+    """Test units property fetching does not touch variable.data."""
+    with patch.object(xr.Variable, 'data', new_callable=PropertyMock) as mock_data_property:
+        test_var.metpy.units
+        mock_data_property.assert_not_called()
+
+
 def test_units_percent():
     """Test that '%' is handled as 'percent'."""
     test_var_percent = xr.open_dataset(