DOC: add storage and na_value to StringDtype reference page

doc/source/reference/arrays.rst

@@ -637,6 +637,8 @@ with a bool :class:`numpy.ndarray`.
       DatetimeTZDtype.tz
       PeriodDtype.freq
       IntervalDtype.subtype
+      StringDtype.storage
+      StringDtype.na_value
 
 *********
 Utilities

doc/source/user_guide/text.rst

@@ -753,7 +753,10 @@ Differences in behavior will be primarily due to the kind of NA value.
 The four :class:`StringDtype` variants
 ======================================
 
-There are four :class:`StringDtype` variants that are available to users.
+There are four :class:`StringDtype` variants that are available to users,
+controlled by the ``storage`` and ``na_value`` parameters of :class:`StringDtype`.
+At runtime, these can be checked via the :attr:`StringDtype.storage`
+and :attr:`StringDtype.na_value` attributes.
 
 Python storage with ``np.nan`` values
 -------------------------------------

pandas/core/arrays/string_.py

@@ -119,7 +119,8 @@ class StringDtype(StorageExtensionDtype):
 
     Attributes
     ----------
-    None
+    storage
+    na_value
 
     Methods
     -------
@@ -149,8 +150,24 @@ def name(self) -> str:  # type: ignore[override]
     # follows NumPy semantics, which uses nan.
     @property
     def na_value(self) -> libmissing.NAType | float:  # type: ignore[override]
+        """
+        The missing value representation for this dtype.
+
+        This value indicates which missing value semantics are used by this dtype.
+        Returns ``np.nan`` for the default string dtype with NumPy semantics,
+        and ``pd.NA`` for the opt-in string dtype with pandas NA semantics.
+        """
         return self._na_value
 
+    @property
+    def storage(self) -> str:
+        """
+        The storage backend for this dtype.
+
+        Can be either "pyarrow" or "python".
+        """
+        return self._storage
+
     _metadata = ("storage", "_na_value")  # type: ignore[assignment]
 
     def __init__(
@@ -185,7 +202,7 @@ def __init__(
         elif na_value is not libmissing.NA:
             raise ValueError(f"'na_value' must be np.nan or pd.NA, got {na_value}")
 
-        self.storage = cast(str, storage)
+        self._storage = cast(str, storage)
         self._na_value = na_value
 
     def __repr__(self) -> str:
@@ -211,7 +228,7 @@ def __eq__(self, other: object) -> bool:
 
     def __setstate__(self, state: MutableMapping[str, Any]) -> None:
         # back-compat for pandas < 2.3, where na_value did not yet exist
-        self.storage = state.pop("storage", "python")
+        self._storage = state.pop("storage", "python")
         self._na_value = state.pop("_na_value", libmissing.NA)
 
     def __hash__(self) -> int:
@@ -306,7 +323,7 @@ def _get_common_dtype(self, dtypes: list[DtypeObj]) -> DtypeObj | None:
             # if both python and pyarrow storage -> priority to pyarrow
             storage = "pyarrow"
         else:
-            storage = next(iter(storages))  # type: ignore[assignment]
+            storage = next(iter(storages))
 
         na_value: libmissing.NAType | float
         if len(na_values) == 2:

pandas/core/dtypes/base.py

@@ -458,8 +458,8 @@ class StorageExtensionDtype(ExtensionDtype):
     name: str
     _metadata = ("storage",)
 
-    def __init__(self, storage: str | None = None) -> None:
-        self.storage = storage
+    def __init__(self, storage: str) -> None:
+        self._storage = storage
 
     def __repr__(self) -> str:
         return f"{self.name}[{self.storage}]"
@@ -480,6 +480,10 @@ def __hash__(self) -> int:
     def na_value(self) -> libmissing.NAType:
         return libmissing.NA
 
+    @property
+    def storage(self) -> str:
+        return self._storage
+
 
 @set_module("pandas.api.extensions")
 def register_extension_dtype(cls: type_t[ExtensionDtypeT]) -> type_t[ExtensionDtypeT]: