ENH: Basis for a StringDtype using Arrow

pandas/_libs/lib.pyx

@@ -636,7 +636,7 @@ cpdef ndarray[object] ensure_string_array(
     ----------
     arr : array-like
         The values to be converted to str, if needed.
-    na_value : Any
+    na_value : Any, default np.nan
         The value to use for na. For example, np.nan or pd.NA.
     convert_na_value : bool, default True
         If False, existing na values will be used unchanged in the new array.

pandas/core/arrays/base.py

@@ -468,14 +468,19 @@ def astype(self, dtype, copy=True):
             NumPy ndarray with 'dtype' for its dtype.
         """
         from pandas.core.arrays.string_ import StringDtype
+        from pandas.core.arrays.string_arrow import ArrowStringDtype
 
         dtype = pandas_dtype(dtype)
         if is_dtype_equal(dtype, self.dtype):
             if not copy:
                 return self
             else:
                 return self.copy()
-        if isinstance(dtype, StringDtype):  # allow conversion to StringArrays
+
+        # FIXME: Really hard-code here?
+        if isinstance(
+            dtype, (ArrowStringDtype, StringDtype)
+        ):  # allow conversion to StringArrays
             return dtype.construct_array_type()._from_sequence(self, copy=False)
 
         return np.array(self, dtype=dtype, copy=copy)