ENH: support argmin/max, idxmin/max with object dtype (#54109)

Author: jbrockmendel

Diff for: doc/source/whatsnew/v2.1.0.rst

@@ -171,6 +171,7 @@ Other enhancements
 - Many read/to_* functions, such as :meth:`DataFrame.to_pickle` and :func:`read_csv`, support forwarding compression arguments to lzma.LZMAFile (:issue:`52979`)
 - Performance improvement in :func:`concat` with homogeneous ``np.float64`` or ``np.float32`` dtypes (:issue:`52685`)
 - Performance improvement in :meth:`DataFrame.filter` when ``items`` is given (:issue:`52941`)
+- Reductions :meth:`Series.argmax`, :meth:`Series.argmin`, :meth:`Series.idxmax`, :meth:`Series.idxmin`, :meth:`Index.argmax`, :meth:`Index.argmin`, :meth:`DataFrame.idxmax`, :meth:`DataFrame.idxmin` are now supported for object-dtype objects (:issue:`4279`, :issue:`18021`, :issue:`40685`, :issue:`43697`)
 -
 
 .. ---------------------------------------------------------------------------

Diff for: pandas/core/nanops.py

@@ -1100,7 +1100,6 @@ def reduction(
 nanmax = _nanminmax("max", fill_value_typ="-inf")
 
 
-@disallow("O")
 def nanargmax(
     values: np.ndarray,
     *,
@@ -1146,7 +1145,6 @@ def nanargmax(
     return result
 
 
-@disallow("O")
 def nanargmin(
     values: np.ndarray,
     *,

Diff for: pandas/tests/frame/test_reductions.py

@@ -987,13 +987,12 @@ def test_idxmin_empty(self, index, skipna, axis):
     @pytest.mark.parametrize("numeric_only", [True, False])
     def test_idxmin_numeric_only(self, numeric_only):
         df = DataFrame({"a": [2, 3, 1], "b": [2, 1, 1], "c": list("xyx")})
+        result = df.idxmin(numeric_only=numeric_only)
         if numeric_only:
-            result = df.idxmin(numeric_only=numeric_only)
             expected = Series([2, 1], index=["a", "b"])
-            tm.assert_series_equal(result, expected)
         else:
-            with pytest.raises(TypeError, match="not allowed for this dtype"):
-                df.idxmin(numeric_only=numeric_only)
+            expected = Series([2, 1, 0], index=["a", "b", "c"])
+        tm.assert_series_equal(result, expected)
 
     def test_idxmin_axis_2(self, float_frame):
         frame = float_frame
@@ -1027,13 +1026,12 @@ def test_idxmax_empty(self, index, skipna, axis):
     @pytest.mark.parametrize("numeric_only", [True, False])
     def test_idxmax_numeric_only(self, numeric_only):
         df = DataFrame({"a": [2, 3, 1], "b": [2, 1, 1], "c": list("xyx")})
+        result = df.idxmax(numeric_only=numeric_only)
         if numeric_only:
-            result = df.idxmax(numeric_only=numeric_only)
             expected = Series([1, 0], index=["a", "b"])
-            tm.assert_series_equal(result, expected)
         else:
-            with pytest.raises(TypeError, match="not allowed for this dtype"):
-                df.idxmin(numeric_only=numeric_only)
+            expected = Series([1, 0, 1], index=["a", "b", "c"])
+        tm.assert_series_equal(result, expected)
 
     def test_idxmax_axis_2(self, float_frame):
         frame = float_frame

Diff for: pandas/tests/groupby/aggregate/test_aggregate.py

@@ -235,7 +235,7 @@ def test_agg_str_with_kwarg_axis_1_raises(df, reduction_func):
     warn_msg = f"DataFrameGroupBy.{reduction_func} with axis=1 is deprecated"
     if reduction_func in ("idxmax", "idxmin"):
         error = TypeError
-        msg = "reduction operation '.*' not allowed for this dtype"
+        msg = "'[<>]' not supported between instances of 'float' and 'str'"
         warn = FutureWarning
     else:
         error = ValueError

Diff for: pandas/tests/groupby/test_function.py

@@ -534,7 +534,7 @@ def test_idxmin_idxmax_axis1():
     df["E"] = date_range("2016-01-01", periods=10)
     gb2 = df.groupby("A")
 
-    msg = "reduction operation 'argmax' not allowed for this dtype"
+    msg = "'>' not supported between instances of 'Timestamp' and 'float'"
     with pytest.raises(TypeError, match=msg):
         with tm.assert_produces_warning(FutureWarning, match=warn_msg):
             gb2.idxmax(axis=1)
@@ -1467,7 +1467,7 @@ def test_numeric_only(kernel, has_arg, numeric_only, keys):
     ):
         result = method(*args, **kwargs)
         assert "b" in result.columns
-    elif has_arg or kernel in ("idxmax", "idxmin"):
+    elif has_arg:
         assert numeric_only is not True
         # kernels that are successful on any dtype were above; this will fail
 
@@ -1486,6 +1486,10 @@ def test_numeric_only(kernel, has_arg, numeric_only, keys):
                 re.escape(f"agg function failed [how->{kernel},dtype->object]"),
             ]
         )
+        if kernel == "idxmin":
+            msg = "'<' not supported between instances of 'type' and 'type'"
+        elif kernel == "idxmax":
+            msg = "'>' not supported between instances of 'type' and 'type'"
         with pytest.raises(exception, match=msg):
             method(*args, **kwargs)
     elif not has_arg and numeric_only is not lib.no_default:
@@ -1529,8 +1533,6 @@ def test_deprecate_numeric_only_series(dtype, groupby_func, request):
         "cummin",
         "cumprod",
         "cumsum",
-        "idxmax",
-        "idxmin",
         "quantile",
     )
     # ops that give an object result on object input
@@ -1556,9 +1558,7 @@ def test_deprecate_numeric_only_series(dtype, groupby_func, request):
     # Test default behavior; kernels that fail may be enabled in the future but kernels
     # that succeed should not be allowed to fail (without deprecation, at least)
     if groupby_func in fails_on_numeric_object and dtype is object:
-        if groupby_func in ("idxmax", "idxmin"):
-            msg = "not allowed for this dtype"
-        elif groupby_func == "quantile":
+        if groupby_func == "quantile":
             msg = "cannot be performed against 'object' dtypes"
         else:
             msg = "is not supported for object dtype"

Diff for: pandas/tests/groupby/test_raises.py

@@ -157,8 +157,8 @@ def test_groupby_raises_string(
         "ffill": (None, ""),
         "fillna": (None, ""),
         "first": (None, ""),
-        "idxmax": (TypeError, "'argmax' not allowed for this dtype"),
-        "idxmin": (TypeError, "'argmin' not allowed for this dtype"),
+        "idxmax": (None, ""),
+        "idxmin": (None, ""),
         "last": (None, ""),
         "max": (None, ""),
         "mean": (

Diff for: pandas/tests/reductions/test_reductions.py

@@ -2,6 +2,7 @@
     datetime,
     timedelta,
 )
+from decimal import Decimal
 
 import numpy as np
 import pytest
@@ -1070,27 +1071,89 @@ def test_timedelta64_analytics(self):
             (Series(["foo", "foo", "bar", "bar", None, np.nan, "baz"]), TypeError),
         ],
     )
-    def test_assert_idxminmax_raises(self, test_input, error_type):
+    def test_assert_idxminmax_empty_raises(self, test_input, error_type):
         """
         Cases where ``Series.argmax`` and related should raise an exception
         """
-        msg = (
-            "reduction operation 'argmin' not allowed for this dtype|"
-            "attempt to get argmin of an empty sequence"
-        )
-        with pytest.raises(error_type, match=msg):
+        test_input = Series([], dtype="float64")
+        msg = "attempt to get argmin of an empty sequence"
+        with pytest.raises(ValueError, match=msg):
             test_input.idxmin()
-        with pytest.raises(error_type, match=msg):
+        with pytest.raises(ValueError, match=msg):
             test_input.idxmin(skipna=False)
-        msg = (
-            "reduction operation 'argmax' not allowed for this dtype|"
-            "attempt to get argmax of an empty sequence"
-        )
-        with pytest.raises(error_type, match=msg):
+        msg = "attempt to get argmax of an empty sequence"
+        with pytest.raises(ValueError, match=msg):
             test_input.idxmax()
-        with pytest.raises(error_type, match=msg):
+        with pytest.raises(ValueError, match=msg):
             test_input.idxmax(skipna=False)
 
+    def test_idxminmax_object_dtype(self):
+        # pre-2.1 object-dtype was disallowed for argmin/max
+        ser = Series(["foo", "bar", "baz"])
+        assert ser.idxmax() == 0
+        assert ser.idxmax(skipna=False) == 0
+        assert ser.idxmin() == 1
+        assert ser.idxmin(skipna=False) == 1
+
+        ser2 = Series([(1,), (2,)])
+        assert ser2.idxmax() == 1
+        assert ser2.idxmax(skipna=False) == 1
+        assert ser2.idxmin() == 0
+        assert ser2.idxmin(skipna=False) == 0
+
+        # attempting to compare np.nan with string raises
+        ser3 = Series(["foo", "foo", "bar", "bar", None, np.nan, "baz"])
+        msg = "'>' not supported between instances of 'float' and 'str'"
+        with pytest.raises(TypeError, match=msg):
+            ser3.idxmax()
+        with pytest.raises(TypeError, match=msg):
+            ser3.idxmax(skipna=False)
+        msg = "'<' not supported between instances of 'float' and 'str'"
+        with pytest.raises(TypeError, match=msg):
+            ser3.idxmin()
+        with pytest.raises(TypeError, match=msg):
+            ser3.idxmin(skipna=False)
+
+    def test_idxminmax_object_frame(self):
+        # GH#4279
+        df = DataFrame([["zimm", 2.5], ["biff", 1.0], ["bid", 12.0]])
+        res = df.idxmax()
+        exp = Series([0, 2])
+        tm.assert_series_equal(res, exp)
+
+    def test_idxminmax_object_tuples(self):
+        # GH#43697
+        ser = Series([(1, 3), (2, 2), (3, 1)])
+        assert ser.idxmax() == 2
+        assert ser.idxmin() == 0
+        assert ser.idxmax(skipna=False) == 2
+        assert ser.idxmin(skipna=False) == 0
+
+    def test_idxminmax_object_decimals(self):
+        # GH#40685
+        df = DataFrame(
+            {
+                "idx": [0, 1],
+                "x": [Decimal("8.68"), Decimal("42.23")],
+                "y": [Decimal("7.11"), Decimal("79.61")],
+            }
+        )
+        res = df.idxmax()
+        exp = Series({"idx": 1, "x": 1, "y": 1})
+        tm.assert_series_equal(res, exp)
+
+        res2 = df.idxmin()
+        exp2 = exp - 1
+        tm.assert_series_equal(res2, exp2)
+
+    def test_argminmax_object_ints(self):
+        # GH#18021
+        ser = Series([0, 1], dtype="object")
+        assert ser.argmax() == 1
+        assert ser.argmin() == 0
+        assert ser.argmax(skipna=False) == 1
+        assert ser.argmin(skipna=False) == 0
+
     def test_idxminmax_with_inf(self):
         # For numeric data with NA and Inf (GH #13595)
         s = Series([0, -np.inf, np.inf, np.nan])