BUG: reductions for nullable dtypes should return pd.NA for skipna=False (#30971)

Author: jorisvandenbossche

doc/source/whatsnew/v1.0.0.rst

@@ -483,6 +483,25 @@ Use :meth:`arrays.IntegerArray.to_numpy` with an explicit ``na_value`` instead.
 
    a.to_numpy(dtype="float", na_value=np.nan)
 
+**Reductions can return ``pd.NA``**
+
+When performing a reduction such as a sum with ``skipna=False``, the result
+will now be ``pd.NA`` instead of ``np.nan`` in presence of missing values
+(:issue:`30958`).
+
+*pandas 0.25.x*
+
+.. code-block:: python
+
+    >>> pd.Series(a).sum(skipna=False)
+    nan
+
+*pandas 1.0.0*
+
+.. ipython:: python
+
+   pd.Series(a).sum(skipna=False)
+
 **value_counts returns a nullable integer dtype**
 
 :meth:`Series.value_counts` with a nullable integer dtype now returns a nullable

pandas/core/arrays/boolean.py

@@ -670,13 +670,15 @@ def _reduce(self, name, skipna=True, **kwargs):
         mask = self._mask
 
         # coerce to a nan-aware float if needed
-        if mask.any():
-            data = self._data.astype("float64")
-            data[mask] = np.nan
+        if self._hasna:
+            data = self.to_numpy("float64", na_value=np.nan)
 
         op = getattr(nanops, "nan" + name)
         result = op(data, axis=0, skipna=skipna, mask=mask, **kwargs)
 
+        if np.isnan(result):
+            return libmissing.NA
+
         # if we have numeric op that would result in an int, coerce to int if possible
         if name in ["sum", "prod"] and notna(result):
             int_result = np.int64(result)

pandas/core/arrays/integer.py

@@ -21,7 +21,7 @@
     is_scalar,
 )
 from pandas.core.dtypes.dtypes import register_extension_dtype
-from pandas.core.dtypes.missing import isna, notna
+from pandas.core.dtypes.missing import isna
 
 from pandas.core import nanops, ops
 from pandas.core.ops import invalid_comparison
@@ -549,21 +549,23 @@ def _reduce(self, name, skipna=True, **kwargs):
         mask = self._mask
 
         # coerce to a nan-aware float if needed
-        if mask.any():
-            data = self._data.astype("float64")
-            # We explicitly use NaN within reductions.
-            data[mask] = np.nan
+        # (we explicitly use NaN within reductions)
+        if self._hasna:
+            data = self.to_numpy("float64", na_value=np.nan)
 
         op = getattr(nanops, "nan" + name)
         result = op(data, axis=0, skipna=skipna, mask=mask, **kwargs)
 
+        if np.isnan(result):
+            return libmissing.NA
+
         # if we have a boolean op, don't coerce
         if name in ["any", "all"]:
             pass
 
         # if we have a preservable numeric op,
         # provide coercion back to an integer type if possible
-        elif name in ["sum", "min", "max", "prod"] and notna(result):
+        elif name in ["sum", "min", "max", "prod"]:
             int_result = int(result)
             if int_result == result:
                 result = int_result

pandas/tests/extension/test_boolean.py

@@ -327,7 +327,9 @@ def check_reduce(self, s, op_name, skipna):
         result = getattr(s, op_name)(skipna=skipna)
         expected = getattr(s.astype("float64"), op_name)(skipna=skipna)
         # override parent function to cast to bool for min/max
-        if op_name in ("min", "max") and not pd.isna(expected):
+        if np.isnan(expected):
+            expected = pd.NA
+        elif op_name in ("min", "max"):
             expected = bool(expected)
         tm.assert_almost_equal(result, expected)
 

pandas/tests/extension/test_integer.py

@@ -19,6 +19,7 @@
 from pandas.core.dtypes.common import is_extension_array_dtype
 
 import pandas as pd
+import pandas._testing as tm
 from pandas.core.arrays import integer_array
 from pandas.core.arrays.integer import (
     Int8Dtype,
@@ -233,7 +234,14 @@ class TestGroupby(base.BaseGroupbyTests):
 
 
 class TestNumericReduce(base.BaseNumericReduceTests):
-    pass
+    def check_reduce(self, s, op_name, skipna):
+        # overwrite to ensure pd.NA is tested instead of np.nan
+        # https://github.com/pandas-dev/pandas/issues/30958
+        result = getattr(s, op_name)(skipna=skipna)
+        expected = getattr(s.astype("float64"), op_name)(skipna=skipna)
+        if np.isnan(expected):
+            expected = pd.NA
+        tm.assert_almost_equal(result, expected)
 
 
 class TestBooleanReduce(base.BaseBooleanReduceTests):