Implement some reductions for string Series

doc/source/whatsnew/v1.0.2.rst

@@ -28,6 +28,11 @@ Fixed regressions
 Bug fixes
 ~~~~~~~~~
 
+**ExtensionArray**
+
+- Fixed issue where taking the minimum or maximum of a ``StringArray`` or ``Series`` with ``StringDtype`` type would raise. (:issue:`31746`)
+-
+
 **Categorical**
 
 - Fixed bug where :meth:`Categorical.from_codes` improperly raised a ``ValueError`` when passed nullable integer codes. (:issue:`31779`)

pandas/core/arrays/string_.py

@@ -4,6 +4,7 @@
 import numpy as np
 
 from pandas._libs import lib, missing as libmissing
+from pandas.compat.numpy import function as nv
 
 from pandas.core.dtypes.base import ExtensionDtype
 from pandas.core.dtypes.common import pandas_dtype
@@ -12,7 +13,7 @@
 from pandas.core.dtypes.inference import is_array_like
 
 from pandas import compat
-from pandas.core import ops
+from pandas.core import nanops, ops
 from pandas.core.arrays import PandasArray
 from pandas.core.construction import extract_array
 from pandas.core.indexers import check_array_indexer
@@ -274,8 +275,21 @@ def astype(self, dtype, copy=True):
         return super().astype(dtype, copy)
 
     def _reduce(self, name, skipna=True, **kwargs):
+        if name in ["min", "max"]:
+            return getattr(self, name)(skipna=skipna, **kwargs)
+
         raise TypeError(f"Cannot perform reduction '{name}' with string dtype")
 
+    def min(self, axis=None, out=None, keepdims=False, skipna=True):
+        nv.validate_min((), dict(out=out, keepdims=keepdims))
+        result = nanops.nanmin(self._ndarray, axis=axis, skipna=skipna)
+        return libmissing.NA if isna(result) else result
+
+    def max(self, axis=None, out=None, keepdims=False, skipna=True):
+        nv.validate_max((), dict(out=out, keepdims=keepdims))
+        result = nanops.nanmax(self._ndarray, axis=axis, skipna=skipna)
+        return libmissing.NA if isna(result) else result
+
     def value_counts(self, dropna=False):
         from pandas import value_counts
 

pandas/core/nanops.py

@@ -854,6 +854,8 @@ def reduction(
         mask: Optional[np.ndarray] = None,
     ) -> Dtype:
 
+        na_mask = isna(values)
+
         values, mask, dtype, dtype_max, fill_value = _get_values(
             values, skipna, fill_value_typ=fill_value_typ, mask=mask
         )
@@ -864,6 +866,12 @@ def reduction(
                 result.fill(np.nan)
             except (AttributeError, TypeError, ValueError):
                 result = np.nan
+        elif is_object_dtype(dtype) and values.ndim == 1 and na_mask.any():
+            # Need to explicitly mask NA values for object dtypes
+            if skipna:
+                result = getattr(values[~na_mask], meth)(axis)
+            else:
+                result = np.nan
         else:
             result = getattr(values, meth)(axis)
 

pandas/tests/arrays/string_/test_string.py

@@ -269,3 +269,26 @@ 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("func", ["min", "max"])
+@pytest.mark.parametrize("skipna", [True, False])
+def test_reduction(func, skipna):
+    s = pd.Series(["x", "y", "z"], dtype="string")
+    result = getattr(s, func)(skipna=skipna)
+    expected = "x" if func == "min" else "z"
+
+    assert result == expected
+
+
+@pytest.mark.parametrize("func", ["min", "max"])
+@pytest.mark.parametrize("skipna", [True, False])
+def test_reduction_with_na(func, skipna):
+    s = pd.Series([pd.NA, "y", "z"], dtype="string")
+    result = getattr(s, func)(skipna=skipna)
+
+    if skipna:
+        expected = "y" if func == "min" else "z"
+        assert result == expected
+    else:
+        assert result is pd.NA

pandas/tests/extension/base/reduce.py

@@ -25,6 +25,12 @@ class BaseNoReduceTests(BaseReduceTests):
 
     @pytest.mark.parametrize("skipna", [True, False])
     def test_reduce_series_numeric(self, data, all_numeric_reductions, skipna):
+        if isinstance(data, pd.arrays.StringArray) and all_numeric_reductions in [
+            "min",
+            "max",
+        ]:
+            pytest.skip("These reductions are implemented")
+
         op_name = all_numeric_reductions
         s = pd.Series(data)
 

pandas/tests/frame/test_apply.py

@@ -1406,8 +1406,8 @@ def test_apply_datetime_tz_issue(self):
     @pytest.mark.parametrize("method", ["min", "max", "sum"])
     def test_consistency_of_aggregates_of_columns_with_missing_values(self, df, method):
         # GH 16832
-        none_in_first_column_result = getattr(df[["A", "B"]], method)()
-        none_in_second_column_result = getattr(df[["B", "A"]], method)()
+        none_in_first_column_result = getattr(df[["A", "B"]], method)().sort_index()
+        none_in_second_column_result = getattr(df[["B", "A"]], method)().sort_index()
 
         tm.assert_series_equal(
             none_in_first_column_result, none_in_second_column_result