Change ExcelWriter modes to {"w","a"} (#358)

* Change ExcelWriter modes to {"w","a")

* New test for ExcelWriter append mode

* Simplify test

Author: cameronb-ma

pandas-stubs/io/excel/_base.pyi

@@ -107,7 +107,7 @@ class ExcelWriter:
         engine: Literal["auto", "openpyxl", "pyxlsb", "odf"] | None = ...,
         date_format: str | None = ...,
         datetime_format: str | None = ...,
-        mode: Literal["w", "writer"] = ...,
+        mode: Literal["w", "a"] = ...,
         storage_options: StorageOptions = ...,
         if_sheet_exists: Literal["error", "new", "replace", "overlay"] | None = ...,
         engine_kwargs: dict[str, Any] | None = ...,

tests/test_io.py

@@ -672,6 +672,14 @@ def test_excel_writer():
         check(assert_type(ef.close(), None), type(None))
 
 
+def test_excel_writer_append_mode():
+    with ensure_clean(".xlsx") as path:
+        with pd.ExcelWriter(path, mode="w") as ew:
+            DF.to_excel(ew, sheet_name="A")
+        with pd.ExcelWriter(path, mode="a") as ew:
+            pass
+
+
 def test_to_string():
     check(assert_type(DF.to_string(), str), str)
     with ensure_clean() as path: