BUG: Series.tolist to return None in place of pd.NA

doc/source/whatsnew/v2.0.0.rst

@@ -1245,6 +1245,7 @@ I/O
 - Fixed memory leak which stemmed from the initialization of the internal JSON module (:issue:`49222`)
 - Fixed issue where :func:`json_normalize` would incorrectly remove leading characters from column names that matched the ``sep`` argument (:issue:`49861`)
 - Bug in :func:`read_csv` unnecessarily overflowing for extension array dtype when containing ``NA`` (:issue:`32134`)
+- Bug in :meth:`Series.tolist` not converting ``NA`` to ``None`` (:issue:`29738`)
 - Bug in :meth:`DataFrame.to_dict` not converting ``NA`` to ``None`` (:issue:`50795`)
 - Bug in :meth:`DataFrame.to_json` where it would segfault when failing to encode a string (:issue:`50307`)
 - Bug in :func:`read_xml` where file-like objects failed when iterparse is used (:issue:`50641`)

pandas/core/arrays/arrow/array.py

@@ -838,6 +838,10 @@ def take(
                 indices_array[indices_array < 0] += len(self._data)
             return type(self)(self._data.take(indices_array))
 
+    @doc(ExtensionArray.tolist)
+    def tolist(self) -> list:
+        return self._data.to_pylist()
+
     @doc(ExtensionArray.to_numpy)
     def to_numpy(
         self,

pandas/core/arrays/masked.py

@@ -435,11 +435,13 @@ def to_numpy(
         return data
 
     @doc(ExtensionArray.tolist)
-    def tolist(self):
+    def tolist(self) -> list:
         if self.ndim > 1:
             return [x.tolist() for x in self]
-        dtype = None if self._hasna else self._data.dtype
-        return self.to_numpy(dtype=dtype).tolist()
+        if self._hasna:
+            # pd.NA -> None (python native types)
+            return self.to_numpy(na_value=None).tolist()
+        return self._data.tolist()
 
     @overload
     def astype(self, dtype: npt.DTypeLike, copy: bool = ...) -> np.ndarray:

pandas/core/arrays/string_.py

@@ -223,12 +223,6 @@ class BaseStringArray(ExtensionArray):
     Mixin class for StringArray, ArrowStringArray.
     """
 
-    @doc(ExtensionArray.tolist)
-    def tolist(self):
-        if self.ndim > 1:
-            return [x.tolist() for x in self]
-        return list(self.to_numpy())
-
 
 class StringArray(BaseStringArray, PandasArray):
     """
@@ -514,6 +508,15 @@ def searchsorted(
             )
         return super().searchsorted(value=value, side=side, sorter=sorter)
 
+    @doc(ExtensionArray.tolist)
+    def tolist(self) -> list:
+        if self.ndim > 1:
+            return [x.tolist() for x in self]
+        if self._hasna:
+            # pd.NA -> None (python native types)
+            return self.to_numpy(na_value=None).tolist()
+        return self._ndarray.tolist()
+
     def _cmp_method(self, other, op):
         from pandas.arrays import BooleanArray
 

pandas/tests/extension/test_arrow.py

@@ -245,6 +245,12 @@ def test_astype_str(self, data, request):
             )
         super().test_astype_str(data)
 
+    def test_tolist(self, data):
+        result = pd.Series(data).tolist()
+        expected = [None if pd.isna(v) else v for v in data]
+        assert isinstance(result, list)
+        assert result == expected
+
 
 class TestConstructors(base.BaseConstructorsTests):
     def test_from_dtype(self, data, request):
@@ -782,6 +788,12 @@ class TestBaseInterface(base.BaseInterfaceTests):
     def test_view(self, data):
         super().test_view(data)
 
+    def test_tolist(self, data):
+        result = pd.Series(data).tolist()
+        expected = [None if pd.isna(v) else v for v in data]
+        assert isinstance(result, list)
+        assert result == expected
+
 
 class TestBaseMissing(base.BaseMissingTests):
     def test_dropna_array(self, data_missing):

pandas/tests/extension/test_boolean.py

@@ -82,7 +82,11 @@ class TestDtype(base.BaseDtypeTests):
 
 
 class TestInterface(base.BaseInterfaceTests):
-    pass
+    def test_tolist(self, data):
+        result = pd.Series(data).tolist()
+        expected = [None if pd.isna(v) else v for v in data]
+        assert isinstance(result, list)
+        assert result == expected
 
 
 class TestConstructors(base.BaseConstructorsTests):
@@ -229,7 +233,11 @@ def test_argmin_argmax(self, data_for_sorting, data_missing_for_sorting):
 
 
 class TestCasting(base.BaseCastingTests):
-    pass
+    def test_tolist(self, data):
+        result = pd.Series(data).tolist()
+        expected = [None if pd.isna(v) else v for v in data]
+        assert isinstance(result, list)
+        assert result == expected
 
 
 class TestGroupby(base.BaseGroupbyTests):

pandas/tests/extension/test_floating.py

@@ -145,7 +145,11 @@ def _compare_other(self, s, data, op, other):
 
 
 class TestInterface(base.BaseInterfaceTests):
-    pass
+    def test_tolist(self, data):
+        result = pd.Series(data).tolist()
+        expected = [None if pd.isna(v) else v for v in data]
+        assert isinstance(result, list)
+        assert result == expected
 
 
 class TestConstructors(base.BaseConstructorsTests):
@@ -177,7 +181,11 @@ class TestMethods(base.BaseMethodsTests):
 
 
 class TestCasting(base.BaseCastingTests):
-    pass
+    def test_tolist(self, data):
+        result = pd.Series(data).tolist()
+        expected = [None if pd.isna(v) else v for v in data]
+        assert isinstance(result, list)
+        assert result == expected
 
 
 class TestGroupby(base.BaseGroupbyTests):

pandas/tests/extension/test_integer.py

@@ -169,7 +169,11 @@ def _compare_other(self, s, data, op, other):
 
 
 class TestInterface(base.BaseInterfaceTests):
-    pass
+    def test_tolist(self, data):
+        result = pd.Series(data).tolist()
+        expected = [None if pd.isna(v) else v for v in data]
+        assert isinstance(result, list)
+        assert result == expected
 
 
 class TestConstructors(base.BaseConstructorsTests):
@@ -205,7 +209,11 @@ class TestMethods(base.BaseMethodsTests):
 
 
 class TestCasting(base.BaseCastingTests):
-    pass
+    def test_tolist(self, data):
+        result = pd.Series(data).tolist()
+        expected = [None if pd.isna(v) else v for v in data]
+        assert isinstance(result, list)
+        assert result == expected
 
 
 class TestGroupby(base.BaseGroupbyTests):