API: membership checks on ExtensionArray containing NA values

doc/source/whatsnew/v1.2.0.rst

@@ -752,6 +752,7 @@ ExtensionArray
 - Fixed bug when applying a NumPy ufunc with multiple outputs to an :class:`.IntegerArray` returning None (:issue:`36913`)
 - Fixed an inconsistency in :class:`.PeriodArray`'s ``__init__`` signature to those of :class:`.DatetimeArray` and :class:`.TimedeltaArray` (:issue:`37289`)
 - Reductions for :class:`.BooleanArray`, :class:`.Categorical`, :class:`.DatetimeArray`, :class:`.FloatingArray`, :class:`.IntegerArray`, :class:`.PeriodArray`, :class:`.TimedeltaArray`, and :class:`.PandasArray` are now keyword-only methods (:issue:`37541`)
+- Fixed a bug where a  ``TypeError`` was wrongly raised if a membership check was made on an ``ExtensionArray`` containing nan-like values (:issue:`37867`)
 
 Other
 ^^^^^

pandas/core/arrays/base.py

@@ -37,6 +37,7 @@
     is_array_like,
     is_dtype_equal,
     is_list_like,
+    is_scalar,
     pandas_dtype,
 )
 from pandas.core.dtypes.dtypes import ExtensionDtype
@@ -354,6 +355,23 @@ def __iter__(self):
         for i in range(len(self)):
             yield self[i]
 
+    def __contains__(self, item) -> bool:
+        """
+        Return for `item in self`.
+        """
+        # GH37867
+        # 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 is_scalar(item) and isna(item):
+            if not self._can_hold_na:
+                return False
+            elif item is self.dtype.na_value or isinstance(item, self.dtype.type):
+                return self.isna().any()
+            else:
+                return False
+        else:
+            return (item == self).any()
+
     def __eq__(self, other: Any) -> ArrayLike:
         """
         Return for `self == other` (element-wise equality).

pandas/tests/extension/arrow/test_bool.py

@@ -50,6 +50,10 @@ def test_view(self, data):
         # __setitem__ does not work, so we only have a smoke-test
         data.view()
 
+    @pytest.mark.xfail(raises=AssertionError, reason="Not implemented yet")
+    def test_contains(self, data, data_missing, nulls_fixture):
+        super().test_contains(data, data_missing, nulls_fixture)
+
 
 class TestConstructors(BaseArrowTests, base.BaseConstructorsTests):
     def test_from_dtype(self, data):

pandas/tests/extension/base/interface.py

@@ -29,6 +29,29 @@ def test_can_hold_na_valid(self, data):
         # GH-20761
         assert data._can_hold_na is True
 
+    def test_contains(self, data, data_missing, nulls_fixture):
+        # GH-37867
+        # Tests for membership checks. Membership checks for nan-likes is tricky and
+        # the settled on rule is: `nan_like in arr` is True if nan_like is
+        # arr.dtype.na_value and arr.isna().any() is True. Else the check returns False.
+
+        na_value = data.dtype.na_value
+        # ensure data without missing values
+        data = data[~data.isna()]
+
+        # first elements are non-missing
+        assert data[0] in data
+        assert data_missing[0] in data_missing
+
+        # check the presence of na_value
+        assert na_value in data_missing
+        assert na_value not in data
+
+        if nulls_fixture is not na_value:
+            # the data can never contain other nan-likes than na_value
+            assert nulls_fixture not in data
+            assert nulls_fixture not in data_missing
+
     def test_memory_usage(self, data):
         s = pd.Series(data)
         result = s.memory_usage(index=False)

pandas/tests/extension/decimal/array.py

@@ -155,6 +155,14 @@ def __setitem__(self, key, value):
     def __len__(self) -> int:
         return len(self._data)
 
+    def __contains__(self, item) -> bool:
+        if not isinstance(item, decimal.Decimal):
+            return False
+        elif item.is_nan():
+            return self.isna().any()
+        else:
+            return super().__contains__(item)
+
     @property
     def nbytes(self) -> int:
         n = len(self)

pandas/tests/extension/json/test_json.py

@@ -143,6 +143,13 @@ def test_custom_asserts(self):
         with pytest.raises(AssertionError, match=msg):
             self.assert_frame_equal(a.to_frame(), b.to_frame())
 
+    @pytest.mark.xfail(
+        reason="comparison method not implemented for JSONArray (GH-37867)"
+    )
+    def test_contains(self, data):
+        # GH-37867
+        super().test_contains(data)
+
 
 class TestConstructors(BaseJSON, base.BaseConstructorsTests):
     @pytest.mark.skip(reason="not implemented constructor from dtype")

pandas/tests/extension/test_categorical.py

@@ -87,6 +87,28 @@ def test_memory_usage(self, data):
         # Is this deliberate?
         super().test_memory_usage(data)
 
+    def test_contains(self, data, data_missing, nulls_fixture):
+        # GH-37867
+        # na value handling in Categorical.__contains__ is deprecated.
+        # See base.BaseInterFaceTests.test_contains for more details.
+
+        na_value = data.dtype.na_value
+        # ensure data without missing values
+        data = data[~data.isna()]
+
+        # first elements are non-missing
+        assert data[0] in data
+        assert data_missing[0] in data_missing
+
+        # check the presence of na_value
+        assert na_value in data_missing
+        assert na_value not in data
+
+        # Categoricals can contain other nan-likes than na_value
+        if nulls_fixture is not na_value:
+            assert nulls_fixture not in data
+            assert nulls_fixture in data_missing  # this line differs from super method
+
 
 class TestConstructors(base.BaseConstructorsTests):
     pass