Add test to check exception when writing in append mode

Author: roberthdevries

pandas/tests/io/excel/test_odswriter.py

@@ -0,0 +1,17 @@
+import pytest
+
+import pandas._testing as tm
+
+from pandas.io.excel import ExcelWriter
+
+odf = pytest.importorskip("odf")
+
+pytestmark = pytest.mark.parametrize("ext", [".ods"])
+
+
+def test_write_append_mode_raises(ext):
+    msg = "Append mode is not supported with odf!"
+
+    with tm.ensure_clean(ext) as f:
+        with pytest.raises(ValueError, match=msg):
+            ExcelWriter(f, engine="odf", mode="a")