Implement some reductions for string Series

doc/source/whatsnew/v1.0.2.rst

@@ -59,6 +59,10 @@ Previously indexing with a nullable Boolean array containing ``NA`` would raise
 Bug fixes
 ~~~~~~~~~
 
+**ExtensionArray**
+
+- Fixed issue where taking ``min`` or ``max`` of a ``StringArray`` or ``Series`` with ``StringDtype`` type would raise. (:issue:`31746`)
+
 **Datetimelike**
 
 - Bug in :meth:`DataFrame.reindex` and :meth:`Series.reindex` when reindexing with a tz-aware index (:issue:`26683`)
@@ -74,6 +78,10 @@ Bug fixes
 - Using ``pd.NA`` with :meth:`DataFrame.to_json` now correctly outputs a null value instead of an empty object (:issue:`31615`)
 - Fixed bug in parquet roundtrip with nullable unsigned integer dtypes (:issue:`31896`).
 
+**Reduction**
+
+- Fixed bug where :meth:`Series.min` and :meth:`Series.max` would raise a ``TypeError`` in the presence of ``NaN`` for object dtype. (:issue:`18588`)
+
 **Experimental dtypes**
 
 - Fix bug in :meth:`DataFrame.convert_dtypes` for columns that were already using the ``"string"`` dtype (:issue:`31731`).

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, mask=isna(self))
+        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, mask=isna(self))
+        return libmissing.NA if isna(result) else result
+
     def value_counts(self, dropna=False):
         from pandas import value_counts
 

pandas/core/nanops.py

@@ -228,7 +228,9 @@ def _maybe_get_mask(
             # Boolean data cannot contain nulls, so signal via mask being None
             return None
 
-        if skipna:
+        if skipna or is_object_dtype(values.dtype):
+            # The masking in nanminmax does not work for object dtype, so always
+            # compute a mask for use later when skipna is False
             mask = isna(values)
 
     return mask
@@ -865,6 +867,17 @@ def reduction(
                 result.fill(np.nan)
             except (AttributeError, TypeError, ValueError):
                 result = np.nan
+        elif (
+            is_object_dtype(dtype)
+            and values.ndim == 1
+            and mask is not None
+            and mask.any()
+        ):
+            # Need to explicitly mask NA values for object dtypes
+            if skipna:
+                result = getattr(values[~mask], meth)(axis)
+            else:
+                result = np.nan
         else:
             result = getattr(values, meth)(axis)
 

pandas/tests/arrays/string_/test_string.py

@@ -269,3 +269,31 @@ 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])
+@pytest.mark.parametrize("box_in_series", [True, False])
+def test_reduction_with_na(func, skipna, box_in_series):
+    data = pd.array([pd.NA, "y", "z"], dtype="string")
+
+    if box_in_series:
+        data = pd.Series(data)
+
+    result = getattr(data, func)(skipna=skipna)
+
+    if skipna:
+        expected = "y" if func == "min" else "z"
+        assert result == expected
+    else:
+        assert result is pd.NA

pandas/tests/extension/test_string.py

@@ -77,7 +77,20 @@ class TestMissing(base.BaseMissingTests):
 
 
 class TestNoReduce(base.BaseNoReduceTests):
-    pass
+    @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",
+        ]:
+            # Tested in pandas/tests/arrays/string_/test_string.py
+            pytest.skip("These reductions are implemented")
+
+        op_name = all_numeric_reductions
+        s = pd.Series(data)
+
+        with pytest.raises(TypeError):
+            getattr(s, op_name)(skipna=skipna)
 
 
 class TestMethods(base.BaseMethodsTests):

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

pandas/tests/reductions/test_reductions.py

@@ -172,6 +172,19 @@ def test_numpy_reduction_with_tz_aware_dtype(self, tz_aware_fixture, func):
         result = getattr(np, func)(expected, expected)
         tm.assert_series_equal(result, expected)
 
+    @pytest.mark.parametrize("func", ["min", "max"])
+    @pytest.mark.parametrize("skipna", [True, False])
+    def test_reduce_object_with_na(self, func, skipna):
+        # https://github.com/pandas-dev/pandas/issues/18588
+        s = pd.Series(np.array(["a", "b", np.nan], dtype=object))
+        result = getattr(s, func)(skipna=skipna)
+
+        if skipna:
+            expected = "a" if func == "min" else "b"
+            assert result == expected
+        else:
+            assert result is np.nan
+
 
 class TestIndexReductions:
     # Note: the name TestIndexReductions indicates these tests