Fix StringArray.astype for category dtype

pandas/core/arrays/string_.py

@@ -36,10 +36,12 @@
 from pandas.core import ops
 from pandas.core.array_algos import masked_reductions
 from pandas.core.arrays import (
+    Categorical,
     FloatingArray,
     IntegerArray,
     PandasArray,
 )
+from pandas.core.arrays.categorical import CategoricalDtype
 from pandas.core.arrays.floating import FloatingDtype
 from pandas.core.arrays.integer import _IntegerDtype
 from pandas.core.construction import extract_array
@@ -307,7 +309,6 @@ def __setitem__(self, key, value):
 
     def astype(self, dtype, copy=True):
         dtype = pandas_dtype(dtype)
-
         if is_dtype_equal(dtype, self.dtype):
             if copy:
                 return self.copy()
@@ -327,6 +328,8 @@ def astype(self, dtype, copy=True):
             arr[mask] = "0"
             values = arr.astype(dtype.numpy_dtype)
             return FloatingArray(values, mask, copy=False)
+        elif isinstance(dtype, CategoricalDtype):
+            return Categorical(self, dtype=dtype, copy=copy)
         elif np.issubdtype(dtype, np.floating):
             arr = self._ndarray.copy()
             mask = self.isna()

pandas/tests/series/methods/test_astype.py

@@ -470,6 +470,13 @@ def test_astype_categories_raises(self):
         with pytest.raises(TypeError, match="got an unexpected"):
             s.astype("category", categories=["a", "b"], ordered=True)
 
+    def test_astype_str_to_categorical(self):
+        # GH-40351
+        s = Series(["A", np.NaN], dtype="string")
+        result = s.astype("category")
+        expected = Series(["A", np.NaN], dtype="category")
+        tm.assert_series_equal(result, expected)
+
     @pytest.mark.parametrize("items", [["a", "b", "c", "a"], [1, 2, 3, 1]])
     def test_astype_from_categorical(self, items):
         ser = Series(items)