REF (string): remove _str_na_value (pandas-dev#59515)

* REF (string): remove _str_na_value

* mypy fixup

Author: jbrockmendel

pandas/core/arrays/numpy_.py

@@ -557,7 +557,3 @@ def _wrap_ndarray_result(self, result: np.ndarray):
 
             return TimedeltaArray._simple_new(result, dtype=result.dtype)
         return type(self)(result)
-
-    # ------------------------------------------------------------------------
-    # String methods interface
-    _str_na_value = np.nan

pandas/core/arrays/string_.py

@@ -838,12 +838,6 @@ def _cmp_method(self, other, op):
 
     _arith_method = _cmp_method
 
-    # ------------------------------------------------------------------------
-    # String methods interface
-    # error: Incompatible types in assignment (expression has type "NAType",
-    # base class "NumpyExtensionArray" defined the type as "float")
-    _str_na_value = libmissing.NA  # type: ignore[assignment]
-
 
 class StringArrayNumpySemantics(StringArray):
     _storage = "python"
@@ -874,7 +868,3 @@ def _from_backing_data(self, arr: np.ndarray) -> StringArrayNumpySemantics:
         # need to override NumpyExtensionArray._from_backing_data to ensure
         # we always preserve the dtype
         return NDArrayBacked._from_backing_data(self, arr)
-
-    # ------------------------------------------------------------------------
-    # String methods interface
-    _str_na_value = np.nan

pandas/core/arrays/string_arrow.py

@@ -279,10 +279,6 @@ def _data(self):
     # ------------------------------------------------------------------------
     # String methods interface
 
-    # error: Incompatible types in assignment (expression has type "NAType",
-    # base class "ObjectStringArrayMixin" defined the type as "float")
-    _str_na_value = libmissing.NA  # type: ignore[assignment]
-
     _str_map = BaseStringArray._str_map
 
     def _str_contains(

pandas/core/strings/object_array.py

@@ -37,8 +37,6 @@ class ObjectStringArrayMixin(BaseStringArrayMethods):
     String Methods operating on object-dtype ndarrays.
     """
 
-    _str_na_value = np.nan
-
     def __len__(self) -> int:
         # For typing, _str_map relies on the object being sized.
         raise NotImplementedError
@@ -56,7 +54,7 @@ def _str_map(
         na_value : Scalar, optional
             The value to set for NA values. Might also be used for the
             fill value if the callable `f` raises an exception.
-            This defaults to ``self._str_na_value`` which is ``np.nan``
+            This defaults to ``self.dtype.na_value`` which is ``np.nan``
             for object-dtype and Categorical and ``pd.NA`` for StringArray.
         dtype : Dtype, optional
             The dtype of the result array.
@@ -66,7 +64,7 @@ def _str_map(
         if dtype is None:
             dtype = np.dtype("object")
         if na_value is None:
-            na_value = self._str_na_value
+            na_value = self.dtype.na_value  # type: ignore[attr-defined]
 
         if not len(self):
             return np.array([], dtype=dtype)
@@ -270,7 +268,7 @@ def f(x):
                 return x.get(i)
             elif len(x) > i >= -len(x):
                 return x[i]
-            return self._str_na_value
+            return self.dtype.na_value  # type: ignore[attr-defined]
 
         return self._str_map(f)
 
@@ -473,7 +471,7 @@ def _str_removesuffix(self, suffix: str) -> Series:
 
     def _str_extract(self, pat: str, flags: int = 0, expand: bool = True):
         regex = re.compile(pat, flags=flags)
-        na_value = self._str_na_value
+        na_value = self.dtype.na_value  # type: ignore[attr-defined]
 
         if not expand: