BUG: Fix StringArray use_inf_as_na bug

doc/source/whatsnew/v1.1.0.rst

@@ -635,8 +635,8 @@ Sparse
 ExtensionArray
 ^^^^^^^^^^^^^^
 
-- Fixed bug where :meth:`Serires.value_counts` would raise on empty input of ``Int64`` dtype (:issue:`33317`)
--
+- Fixed bug where :meth:`Series.value_counts` would raise on empty input of ``Int64`` dtype (:issue:`33317`)
+- Fixed bug where :meth:`StringArray.isna` would return ``False`` for NA values when ``pandas.options.mode.use_inf_as_na`` was set to ``True`` (:issue:`33655`)
 
 
 Other

pandas/_libs/missing.pyx

@@ -140,6 +140,7 @@ def isnaobj_old(arr: ndarray) -> ndarray:
      - INF
      - NEGINF
      - NaT
+     - NA
 
     Parameters
     ----------
@@ -160,7 +161,7 @@ def isnaobj_old(arr: ndarray) -> ndarray:
     result = np.zeros(n, dtype=np.uint8)
     for i in range(n):
         val = arr[i]
-        result[i] = val is NaT or _check_none_nan_inf_neginf(val)
+        result[i] = checknull(val) or val == INF or val == NEGINF
     return result.view(np.bool_)
 
 

pandas/core/dtypes/missing.py

@@ -231,7 +231,11 @@ def _isna_ndarraylike(obj):
 
 
 def _isna_ndarraylike_old(obj):
-    values = getattr(obj, "_values", obj)
+    if not isinstance(obj, np.ndarray):
+        values = obj.to_numpy()
+    else:
+        values = obj
+
     dtype = values.dtype
 
     if is_string_dtype(dtype):

pandas/tests/arrays/string_/test_string.py

@@ -277,3 +277,25 @@ def test_value_counts_na():
     result = arr.value_counts(dropna=True)
     expected = pd.Series([2, 1], index=["a", "b"], dtype="Int64")
     tm.assert_series_equal(result, expected)
+
+
+@pytest.mark.parametrize(
+    "values, expected",
+    [
+        (pd.array(["a", "b", "c"]), np.array([False, False, False])),
+        (pd.array(["a", "b", None]), np.array([False, False, True])),
+    ],
+)
+def test_use_na_as_inf(values, expected):
+    # https://github.com/pandas-dev/pandas/issues/33655
+    with pd.option_context("mode.use_inf_as_na", True):
+        result = values.isna()
+        tm.assert_numpy_array_equal(result, expected)
+
+        result = pd.Series(values).isna()
+        expected = pd.Series(expected)
+        tm.assert_series_equal(result, expected)
+
+        result = pd.DataFrame(values).isna()
+        expected = pd.DataFrame(expected)
+        tm.assert_frame_equal(result, expected)