DEPR: argmin/argmax with all-NA or skipa=False

doc/source/whatsnew/v2.1.0.rst

@@ -368,6 +368,7 @@ Deprecations
 - Deprecated the ``method`` and ``limit`` keywords in :meth:`DataFrame.replace` and :meth:`Series.replace` (:issue:`33302`)
 - Deprecated the use of non-supported datetime64 and timedelta64 resolutions with :func:`pandas.array`. Supported resolutions are: "s", "ms", "us", "ns" resolutions (:issue:`53058`)
 - Deprecated values "pad", "ffill", "bfill", "backfill" for :meth:`Series.interpolate` and :meth:`DataFrame.interpolate`, use ``obj.ffill()`` or ``obj.bfill()`` instead (:issue:`53581`)
+- Deprecated the behavior of :meth:`Index.argmax`, :meth:`Index.argmin`, :meth:`Series.argmax`, :meth:`Series.argmin` with either all-NAs and skipna=True or any-NAs and skipna=False; in a future version this will raise ``ValueError`` (:issue:`33941`, :issue:`33942`)
 -
 
 .. ---------------------------------------------------------------------------

pandas/core/base.py

@@ -14,6 +14,7 @@
     final,
     overload,
 )
+import warnings
 
 import numpy as np
 
@@ -36,6 +37,7 @@
     cache_readonly,
     doc,
 )
+from pandas.util._exceptions import find_stack_level
 
 from pandas.core.dtypes.cast import can_hold_element
 from pandas.core.dtypes.common import (
@@ -735,15 +737,29 @@ def argmax(
 
         if isinstance(delegate, ExtensionArray):
             if not skipna and delegate.isna().any():
+                warnings.warn(
+                    f"The behavior of {type(self).__name__}.argmax/argmin "
+                    "with skipna=False and NAs, or with all-NAs is deprecated. "
+                    "In a future version this will raise ValueError.",
+                    FutureWarning,
+                    stacklevel=find_stack_level(),
+                )
                 return -1
             else:
                 return delegate.argmax()
         else:
+            result = nanops.nanargmax(delegate, skipna=skipna)
+            if result == -1:
+                warnings.warn(
+                    f"The behavior of {type(self).__name__}.argmax/argmin "
+                    "with skipna=False and NAs, or with all-NAs is deprecated. "
+                    "In a future version this will raise ValueError.",
+                    FutureWarning,
+                    stacklevel=find_stack_level(),
+                )
             # error: Incompatible return value type (got "Union[int, ndarray]", expected
             # "int")
-            return nanops.nanargmax(  # type: ignore[return-value]
-                delegate, skipna=skipna
-            )
+            return result  # type: ignore[return-value]
 
     @doc(argmax, op="min", oppose="max", value="smallest")
     def argmin(
@@ -755,15 +771,29 @@ def argmin(
 
         if isinstance(delegate, ExtensionArray):
             if not skipna and delegate.isna().any():
+                warnings.warn(
+                    f"The behavior of {type(self).__name__}.argmax/argmin "
+                    "with skipna=False and NAs, or with all-NAs is deprecated. "
+                    "In a future version this will raise ValueError.",
+                    FutureWarning,
+                    stacklevel=find_stack_level(),
+                )
                 return -1
             else:
                 return delegate.argmin()
         else:
+            result = nanops.nanargmin(delegate, skipna=skipna)
+            if result == -1:
+                warnings.warn(
+                    f"The behavior of {type(self).__name__}.argmax/argmin "
+                    "with skipna=False and NAs, or with all-NAs is deprecated. "
+                    "In a future version this will raise ValueError.",
+                    FutureWarning,
+                    stacklevel=find_stack_level(),
+                )
             # error: Incompatible return value type (got "Union[int, ndarray]", expected
             # "int")
-            return nanops.nanargmin(  # type: ignore[return-value]
-                delegate, skipna=skipna
-            )
+            return result  # type: ignore[return-value]
 
     def tolist(self):
         """

pandas/core/indexes/base.py

@@ -7271,6 +7271,13 @@ def argmin(self, axis=None, skipna: bool = True, *args, **kwargs) -> int:
             # Take advantage of cache
             mask = self._isnan
             if not skipna or mask.all():
+                warnings.warn(
+                    f"The behavior of {type(self).__name__}.argmax/argmin "
+                    "with skipna=False and NAs, or with all-NAs is deprecated. "
+                    "In a future version this will raise ValueError.",
+                    FutureWarning,
+                    stacklevel=find_stack_level(),
+                )
                 return -1
         return super().argmin(skipna=skipna)
 
@@ -7283,6 +7290,13 @@ def argmax(self, axis=None, skipna: bool = True, *args, **kwargs) -> int:
             # Take advantage of cache
             mask = self._isnan
             if not skipna or mask.all():
+                warnings.warn(
+                    f"The behavior of {type(self).__name__}.argmax/argmin "
+                    "with skipna=False and NAs, or with all-NAs is deprecated. "
+                    "In a future version this will raise ValueError.",
+                    FutureWarning,
+                    stacklevel=find_stack_level(),
+                )
                 return -1
         return super().argmax(skipna=skipna)
 

pandas/core/series.py

@@ -2530,7 +2530,11 @@ def idxmin(self, axis: Axis = 0, skipna: bool = True, *args, **kwargs) -> Hashab
         nan
         """
         axis = self._get_axis_number(axis)
-        i = self.argmin(axis, skipna, *args, **kwargs)
+        with warnings.catch_warnings():
+            # ignore warning produced by argmin since we will issue a different
+            #  warning for idxmin
+            warnings.simplefilter("ignore")
+            i = self.argmin(axis, skipna, *args, **kwargs)
         if i == -1:
             # GH#43587 give correct NA value for Index.
             return self.index._na_value
@@ -2601,7 +2605,11 @@ def idxmax(self, axis: Axis = 0, skipna: bool = True, *args, **kwargs) -> Hashab
         nan
         """
         axis = self._get_axis_number(axis)
-        i = self.argmax(axis, skipna, *args, **kwargs)
+        with warnings.catch_warnings():
+            # ignore warning produced by argmax since we will issue a different
+            #  warning for argmax
+            warnings.simplefilter("ignore")
+            i = self.argmax(axis, skipna, *args, **kwargs)
         if i == -1:
             # GH#43587 give correct NA value for Index.
             return self.index._na_value

pandas/tests/extension/base/methods.py

@@ -160,8 +160,13 @@ def test_argreduce_series(
         self, data_missing_for_sorting, op_name, skipna, expected
     ):
         # data_missing_for_sorting -> [B, NA, A] with A < B and NA missing.
+        warn = None
+        if op_name.startswith("arg") and expected == -1:
+            warn = FutureWarning
+        msg = "The behavior of Series.argmax/argmin"
         ser = pd.Series(data_missing_for_sorting)
-        result = getattr(ser, op_name)(skipna=skipna)
+        with tm.assert_produces_warning(warn, match=msg):
+            result = getattr(ser, op_name)(skipna=skipna)
         tm.assert_almost_equal(result, expected)
 
     def test_argmax_argmin_no_skipna_notimplemented(self, data_missing_for_sorting):

pandas/tests/reductions/test_reductions.py

@@ -113,10 +113,17 @@ def test_nanargminmax(self, opname, index_or_series):
         # GH#7261
         klass = index_or_series
         arg_op = "arg" + opname if klass is Index else "idx" + opname
+        warn = FutureWarning if klass is Index else None
 
         obj = klass([NaT, datetime(2011, 11, 1)])
         assert getattr(obj, arg_op)() == 1
-        result = getattr(obj, arg_op)(skipna=False)
+
+        msg = (
+            "The behavior of (DatetimeIndex|Series).argmax/argmin with "
+            "skipna=False and NAs"
+        )
+        with tm.assert_produces_warning(warn, match=msg):
+            result = getattr(obj, arg_op)(skipna=False)
         if klass is Series:
             assert np.isnan(result)
         else:
@@ -125,7 +132,8 @@ def test_nanargminmax(self, opname, index_or_series):
         obj = klass([NaT, datetime(2011, 11, 1), NaT])
         # check DatetimeIndex non-monotonic path
         assert getattr(obj, arg_op)() == 1
-        result = getattr(obj, arg_op)(skipna=False)
+        with tm.assert_produces_warning(warn, match=msg):
+            result = getattr(obj, arg_op)(skipna=False)
         if klass is Series:
             assert np.isnan(result)
         else:
@@ -155,26 +163,40 @@ def test_argminmax(self):
         obj = Index([np.nan, 1, np.nan, 2])
         assert obj.argmin() == 1
         assert obj.argmax() == 3
-        assert obj.argmin(skipna=False) == -1
-        assert obj.argmax(skipna=False) == -1
+        msg = "The behavior of Index.argmax/argmin with skipna=False and NAs"
+        with tm.assert_produces_warning(FutureWarning, match=msg):
+            assert obj.argmin(skipna=False) == -1
+        with tm.assert_produces_warning(FutureWarning, match=msg):
+            assert obj.argmax(skipna=False) == -1
 
         obj = Index([np.nan])
-        assert obj.argmin() == -1
-        assert obj.argmax() == -1
-        assert obj.argmin(skipna=False) == -1
-        assert obj.argmax(skipna=False) == -1
+        with tm.assert_produces_warning(FutureWarning, match=msg):
+            assert obj.argmin() == -1
+        with tm.assert_produces_warning(FutureWarning, match=msg):
+            assert obj.argmax() == -1
+        with tm.assert_produces_warning(FutureWarning, match=msg):
+            assert obj.argmin(skipna=False) == -1
+        with tm.assert_produces_warning(FutureWarning, match=msg):
+            assert obj.argmax(skipna=False) == -1
 
+        msg = "The behavior of DatetimeIndex.argmax/argmin with skipna=False and NAs"
         obj = Index([NaT, datetime(2011, 11, 1), datetime(2011, 11, 2), NaT])
         assert obj.argmin() == 1
         assert obj.argmax() == 2
-        assert obj.argmin(skipna=False) == -1
-        assert obj.argmax(skipna=False) == -1
+        with tm.assert_produces_warning(FutureWarning, match=msg):
+            assert obj.argmin(skipna=False) == -1
+        with tm.assert_produces_warning(FutureWarning, match=msg):
+            assert obj.argmax(skipna=False) == -1
 
         obj = Index([NaT])
-        assert obj.argmin() == -1
-        assert obj.argmax() == -1
-        assert obj.argmin(skipna=False) == -1
-        assert obj.argmax(skipna=False) == -1
+        with tm.assert_produces_warning(FutureWarning, match=msg):
+            assert obj.argmin() == -1
+        with tm.assert_produces_warning(FutureWarning, match=msg):
+            assert obj.argmax() == -1
+        with tm.assert_produces_warning(FutureWarning, match=msg):
+            assert obj.argmin(skipna=False) == -1
+        with tm.assert_produces_warning(FutureWarning, match=msg):
+            assert obj.argmax(skipna=False) == -1
 
     @pytest.mark.parametrize("op, expected_col", [["max", "a"], ["min", "b"]])
     def test_same_tz_min_max_axis_1(self, op, expected_col):