NA in arr should return True if arr contains NA

Author: topper-123

pandas/core/arrays/base.py

@@ -358,7 +358,7 @@ def __contains__(self, item) -> bool:
         # comparisons of any item to pd.NA always return pd.NA, so e.g. "a" in [pd.NA]
         # would raise a TypeError. The implementation below works around that.
         if isna(item):
-            return False
+            return isna(self).any() if self._can_hold_na else False
         else:
             return (item == self).any()
 

pandas/tests/arrays/categorical/test_operators.py

@@ -406,4 +406,9 @@ def test_contains(self, ordered):
         cat = Categorical([np.nan, "a"], ordered=ordered)
         assert "a" in cat
         assert "x" not in cat
-        assert pd.NA not in cat
+        assert pd.NA in cat
+
+        cat = cat[::-1]
+        assert "a" in cat
+        assert "x" not in cat
+        assert pd.NA in cat

pandas/tests/arrays/string_/test_string.py

@@ -358,4 +358,9 @@ def test_contains():
     arr = pd.arrays.StringArray(np.array(["a", pd.NA]))
     assert "a" in arr
     assert "x" not in arr
-    assert pd.NA not in arr
+    assert pd.NA in arr
+
+    arr = arr[::-1]
+    assert "a" in arr
+    assert "x" not in arr
+    assert pd.NA in arr