use 'auto' instead of None

Author: jorisvandenbossche

pandas/core/arrays/string_.py

@@ -141,13 +141,15 @@ def __init__(
         if storage is None:
             if na_value is not libmissing.NA:
                 storage = get_option("mode.string_storage")
-                if storage is None:
+                if storage == "auto":
                     if HAS_PYARROW:
                         storage = "pyarrow"
                     else:
                         storage = "python"
             else:
-                storage = get_option("mode.string_storage") or "python"
+                storage = get_option("mode.string_storage")
+                if storage == "auto":
+                    storage = "python"
 
         if storage == "pyarrow_numpy":
             # TODO raise a deprecation warning

pandas/core/config_init.py

@@ -457,7 +457,7 @@ def is_terminal() -> bool:
 
 
 def is_valid_string_storage(value: Any) -> None:
-    legal_values = [None, "python", "pyarrow"]
+    legal_values = ["auto", "python", "pyarrow"]
     if value not in legal_values:
         msg = "Value must be one of python|pyarrow"
         if value == "pyarrow_numpy":
@@ -472,7 +472,7 @@ def is_valid_string_storage(value: Any) -> None:
 with cf.config_prefix("mode"):
     cf.register_option(
         "string_storage",
-        None,
+        "auto",
         string_storage_doc,
         # validator=is_one_of_factory(["python", "pyarrow"]),
         validator=is_valid_string_storage,