CLN/TST: delegate StringArray.fillna() to parent class + add tests

pandas/core/arrays/string_.py

@@ -254,19 +254,8 @@ def _values_for_factorize(self):
         arr[mask] = -1
         return arr, -1
 
-    def __setitem__(self, key, value):
-        value = extract_array(value, extract_numpy=True)
-        if isinstance(value, type(self)):
-            # extract_array doesn't extract PandasArray subclasses
-            value = value._ndarray
-
-        key = check_array_indexer(self, key)
-        scalar_key = lib.is_scalar(key)
+    def _validate_setitem_value(self, value):
         scalar_value = lib.is_scalar(value)
-        if scalar_key and not scalar_value:
-            raise ValueError("setting an array element with a sequence.")
-
-        # validate new items
         if scalar_value:
             if isna(value):
                 value = StringDtype.na_value
@@ -279,11 +268,27 @@ def __setitem__(self, key, value):
                 value = np.asarray(value, dtype=object)
             if len(value) and not lib.is_string_array(value, skipna=True):
                 raise ValueError("Must provide strings.")
+        return value
+
+    def __setitem__(self, key, value):
+        value = extract_array(value, extract_numpy=True)
+        if isinstance(value, type(self)):
+            # extract_array doesn't extract PandasArray subclasses
+            value = value._ndarray
+
+        key = check_array_indexer(self, key)
+        scalar_key = lib.is_scalar(key)
+        scalar_value = lib.is_scalar(value)
+        if scalar_key and not scalar_value:
+            raise ValueError("setting an array element with a sequence.")
+
+        value = self._validate_setitem_value(value)
 
         super().__setitem__(key, value)
 
     def fillna(self, value=None, method=None, limit=None):
-        # TODO: validate dtype
+        if value is not None:
+            value = self._validate_setitem_value(value)
         return super().fillna(value, method, limit)
 
     def astype(self, dtype, copy=True):

pandas/tests/arrays/string_/test_string.py

@@ -422,6 +422,24 @@ def test_reduce_missing(skipna, dtype):
         assert pd.isna(result)
 
 
+def test_fillna_args():
+    # GH 37987
+
+    arr = pd.array(["a", pd.NA], dtype="string")
+
+    res = arr.fillna(value="b")
+    expected = pd.array(["a", "b"], dtype="string")
+    tm.assert_extension_array_equal(res, expected)
+
+    res = arr.fillna(value=np.str_("b"))
+    expected = pd.array(["a", "b"], dtype="string")
+    tm.assert_extension_array_equal(res, expected)
+
+    msg = "Cannot set non-string value '1' into a StringArray."
+    with pytest.raises(ValueError, match=msg):
+        arr.fillna(value=1)
+
+
 @td.skip_if_no("pyarrow", min_version="0.15.0")
 def test_arrow_array(dtype):
     # protocol added in 0.15.0