Implement reductions from #24024 (#24484)

Author: jbrockmendel

Diff for: pandas/core/arrays/datetimelike.py

@@ -27,6 +27,7 @@
 from pandas.core.dtypes.generic import ABCDataFrame, ABCIndexClass, ABCSeries
 from pandas.core.dtypes.missing import isna
 
+from pandas.core import nanops
 from pandas.core.algorithms import checked_add_with_arr, take, unique1d
 import pandas.core.common as com
 
@@ -1381,6 +1382,71 @@ def _ensure_localized(self, arg, ambiguous='raise', nonexistent='raise',
                 )
         return arg
 
+    # --------------------------------------------------------------
+    # Reductions
+
+    def _reduce(self, name, axis=0, skipna=True, **kwargs):
+        op = getattr(self, name, None)
+        if op:
+            return op(axis=axis, skipna=skipna, **kwargs)
+        else:
+            raise TypeError("cannot perform {name} with type {dtype}"
+                            .format(name=name, dtype=self.dtype))
+            # TODO: use super(DatetimeLikeArrayMixin, self)._reduce
+            #  after we subclass ExtensionArray
+
+    def min(self, axis=None, skipna=True, *args, **kwargs):
+        """
+        Return the minimum value of the Array or minimum along
+        an axis.
+
+        See Also
+        --------
+        numpy.ndarray.min
+        Index.min : Return the minimum value in an Index.
+        Series.min : Return the minimum value in a Series.
+        """
+        nv.validate_min(args, kwargs)
+        nv.validate_minmax_axis(axis)
+
+        result = nanops.nanmin(self.asi8, skipna=skipna, mask=self.isna())
+        if isna(result):
+            # Period._from_ordinal does not handle np.nan gracefully
+            return NaT
+        return self._box_func(result)
+
+    def max(self, axis=None, skipna=True, *args, **kwargs):
+        """
+        Return the maximum value of the Array or maximum along
+        an axis.
+
+        See Also
+        --------
+        numpy.ndarray.max
+        Index.max : Return the maximum value in an Index.
+        Series.max : Return the maximum value in a Series.
+        """
+        # TODO: skipna is broken with max.
+        # See https://github.com/pandas-dev/pandas/issues/24265
+        nv.validate_max(args, kwargs)
+        nv.validate_minmax_axis(axis)
+
+        mask = self.isna()
+        if skipna:
+            values = self[~mask].asi8
+        elif mask.any():
+            return NaT
+        else:
+            values = self.asi8
+
+        if not len(values):
+            # short-circut for empty max / min
+            return NaT
+
+        result = nanops.nanmax(values, skipna=skipna)
+        # Don't have to worry about NA `result`, since no NA went in.
+        return self._box_func(result)
+
 
 DatetimeLikeArrayMixin._add_comparison_ops()
 

Diff for: pandas/tests/arrays/test_datetimelike.py

@@ -157,6 +157,13 @@ def test_scalar_from_string(self):
         result = arr._scalar_from_string(str(arr[0]))
         assert result == arr[0]
 
+    def test_reduce_invalid(self):
+        data = np.arange(10, dtype='i8') * 24 * 3600 * 10**9
+        arr = self.array_cls(data, freq='D')
+
+        with pytest.raises(TypeError, match='cannot perform'):
+            arr._reduce("not a method")
+
     def test_searchsorted(self):
         data = np.arange(10, dtype='i8') * 24 * 3600 * 10**9
         arr = self.array_cls(data, freq='D')

Diff for: pandas/tests/arrays/test_datetimes.py

@@ -126,3 +126,41 @@ def test_tz_dtype_matches(self):
         result, _, _ = sequence_to_dt64ns(
             arr, dtype=DatetimeTZDtype(tz="US/Central"))
         tm.assert_numpy_array_equal(arr._data, result)
+
+
+class TestReductions(object):
+
+    @pytest.mark.parametrize("tz", [None, "US/Central"])
+    def test_min_max(self, tz):
+        arr = DatetimeArray._from_sequence([
+            '2000-01-03',
+            '2000-01-03',
+            'NaT',
+            '2000-01-02',
+            '2000-01-05',
+            '2000-01-04',
+        ], tz=tz)
+
+        result = arr.min()
+        expected = pd.Timestamp('2000-01-02', tz=tz)
+        assert result == expected
+
+        result = arr.max()
+        expected = pd.Timestamp('2000-01-05', tz=tz)
+        assert result == expected
+
+        result = arr.min(skipna=False)
+        assert result is pd.NaT
+
+        result = arr.max(skipna=False)
+        assert result is pd.NaT
+
+    @pytest.mark.parametrize("tz", [None, "US/Central"])
+    @pytest.mark.parametrize('skipna', [True, False])
+    def test_min_max_empty(self, skipna, tz):
+        arr = DatetimeArray._from_sequence([], tz=tz)
+        result = arr.min(skipna=skipna)
+        assert result is pd.NaT
+
+        result = arr.max(skipna=skipna)
+        assert result is pd.NaT

Diff for: pandas/tests/arrays/test_period.py

@@ -261,3 +261,42 @@ def test_repr_large():
         "Length: 1000, dtype: period[D]"
     )
     assert result == expected
+
+
+# ----------------------------------------------------------------------------
+# Reductions
+
+class TestReductions(object):
+
+    def test_min_max(self):
+        arr = period_array([
+            '2000-01-03',
+            '2000-01-03',
+            'NaT',
+            '2000-01-02',
+            '2000-01-05',
+            '2000-01-04',
+        ], freq='D')
+
+        result = arr.min()
+        expected = pd.Period('2000-01-02', freq='D')
+        assert result == expected
+
+        result = arr.max()
+        expected = pd.Period('2000-01-05', freq='D')
+        assert result == expected
+
+        result = arr.min(skipna=False)
+        assert result is pd.NaT
+
+        result = arr.max(skipna=False)
+        assert result is pd.NaT
+
+    @pytest.mark.parametrize('skipna', [True, False])
+    def test_min_max_empty(self, skipna):
+        arr = period_array([], freq='D')
+        result = arr.min(skipna=skipna)
+        assert result is pd.NaT
+
+        result = arr.max(skipna=skipna)
+        assert result is pd.NaT

Diff for: pandas/tests/arrays/test_timedeltas.py

@@ -93,3 +93,34 @@ def test_setitem_clears_freq(self):
         a = TimedeltaArray(pd.timedelta_range('1H', periods=2, freq='H'))
         a[0] = pd.Timedelta("1H")
         assert a.freq is None
+
+
+class TestReductions(object):
+
+    def test_min_max(self):
+        arr = TimedeltaArray._from_sequence([
+            '3H', '3H', 'NaT', '2H', '5H', '4H',
+        ])
+
+        result = arr.min()
+        expected = pd.Timedelta('2H')
+        assert result == expected
+
+        result = arr.max()
+        expected = pd.Timedelta('5H')
+        assert result == expected
+
+        result = arr.min(skipna=False)
+        assert result is pd.NaT
+
+        result = arr.max(skipna=False)
+        assert result is pd.NaT
+
+    @pytest.mark.parametrize('skipna', [True, False])
+    def test_min_max_empty(self, skipna):
+        arr = TimedeltaArray._from_sequence([])
+        result = arr.min(skipna=skipna)
+        assert result is pd.NaT
+
+        result = arr.max(skipna=skipna)
+        assert result is pd.NaT