From 47c9ee7b32d8bde2ff8cf50288a8787b11d512cb Mon Sep 17 00:00:00 2001
From: Matthew Roeschke <10647082+mroeschke@users.noreply.github.com>
Date: Fri, 6 Jan 2023 15:08:28 -0800
Subject: [PATCH] DEPR: Enforce certain DataFrame reductions w/ axis=None to
 return scalars (#50593)

---
 doc/source/whatsnew/v2.0.0.rst                |  1 +
 pandas/core/frame.py                          | 10 ++--
 pandas/core/generic.py                        | 51 ++++++++-----------
 pandas/tests/frame/test_reductions.py         | 31 +++++------
 pandas/tests/groupby/test_categorical.py      |  8 +--
 pandas/tests/groupby/test_function.py         | 16 ++----
 .../tests/groupby/transform/test_transform.py | 17 ++-----
 7 files changed, 54 insertions(+), 80 deletions(-)

diff --git a/doc/source/whatsnew/v2.0.0.rst b/doc/source/whatsnew/v2.0.0.rst
index 3b049f3059666..e44ae2cb40826 100644
--- a/doc/source/whatsnew/v2.0.0.rst
+++ b/doc/source/whatsnew/v2.0.0.rst
@@ -715,6 +715,7 @@ Removal of prior version deprecations/changes
 - Changed behavior of :meth:`Series.fillna` and :meth:`DataFrame.fillna` with ``timedelta64[ns]`` dtype and an incompatible ``fill_value``; this now casts to ``object`` dtype instead of raising, consistent with the behavior with other dtypes (:issue:`45746`)
 - Change the default argument of ``regex`` for :meth:`Series.str.replace` from ``True`` to ``False``. Additionally, a single character ``pat`` with ``regex=True`` is now treated as a regular expression instead of a string literal. (:issue:`36695`, :issue:`24804`)
 - Changed behavior of :meth:`DataFrame.any` and :meth:`DataFrame.all` with ``bool_only=True``; object-dtype columns with all-bool values will no longer be included, manually cast to ``bool`` dtype first (:issue:`46188`)
+- Changed behavior of :meth:`DataFrame.max`, :class:`DataFrame.min`, :class:`DataFrame.mean`, :class:`DataFrame.median`, :class:`DataFrame.skew`, :class:`DataFrame.kurt` with ``axis=None`` to return a scalar applying the aggregation across both axes (:issue:`45072`)
 - Changed behavior of comparison of a :class:`Timestamp` with a ``datetime.date`` object; these now compare as un-equal and raise on inequality comparisons, matching the ``datetime.datetime`` behavior (:issue:`36131`)
 - Changed behavior of comparison of ``NaT`` with a ``datetime.date`` object; these now raise on inequality comparisons (:issue:`39196`)
 - Enforced deprecation of silently dropping columns that raised a ``TypeError`` in :class:`Series.transform` and :class:`DataFrame.transform` when used with a list or dictionary (:issue:`43740`)
diff --git a/pandas/core/frame.py b/pandas/core/frame.py
index fed92dc80a99b..6491081c54592 100644
--- a/pandas/core/frame.py
+++ b/pandas/core/frame.py
@@ -10357,9 +10357,8 @@ def _reduce(
         assert filter_type is None or filter_type == "bool", filter_type
         out_dtype = "bool" if filter_type == "bool" else None
 
-        # TODO: Make other agg func handle axis=None properly GH#21597
-        axis = self._get_axis_number(axis)
-        assert axis in [0, 1]
+        if axis is not None:
+            axis = self._get_axis_number(axis)
 
         def func(values: np.ndarray):
             # We only use this in the case that operates on self.values
@@ -10410,7 +10409,7 @@ def _get_data() -> DataFrame:
 
             return out
 
-        assert not numeric_only and axis == 1
+        assert not numeric_only and axis in (1, None)
 
         data = self
         values = data.values
@@ -10426,6 +10425,9 @@ def _get_data() -> DataFrame:
                     # try to coerce to the original dtypes item by item if we can
                     pass
 
+        if axis is None:
+            return result
+
         labels = self._get_agg_axis(axis)
         result = self._constructor_sliced(result, index=labels)
         return result
diff --git a/pandas/core/generic.py b/pandas/core/generic.py
index 3cade2568d921..561422c868e91 100644
--- a/pandas/core/generic.py
+++ b/pandas/core/generic.py
@@ -10944,7 +10944,7 @@ def _stat_function(
         self,
         name: str,
         func,
-        axis: Axis | None | lib.NoDefault = None,
+        axis: Axis | None = 0,
         skipna: bool_t = True,
         numeric_only: bool_t = False,
         **kwargs,
@@ -10956,30 +10956,13 @@ def _stat_function(
 
         validate_bool_kwarg(skipna, "skipna", none_allowed=False)
 
-        if axis is None and self.ndim > 1:
-            # user must have explicitly passed axis=None
-            # GH#21597
-            warnings.warn(
-                f"In a future version, DataFrame.{name}(axis=None) will return a "
-                f"scalar {name} over the entire DataFrame. To retain the old "
-                f"behavior, use 'frame.{name}(axis=0)' or just 'frame.{name}()'",
-                FutureWarning,
-                stacklevel=find_stack_level(),
-            )
-
-        if axis is lib.no_default:
-            axis = None
-
-        if axis is None:
-            axis = self._stat_axis_number
-
         return self._reduce(
             func, name=name, axis=axis, skipna=skipna, numeric_only=numeric_only
         )
 
     def min(
         self,
-        axis: Axis | None | lib.NoDefault = lib.no_default,
+        axis: Axis | None = 0,
         skipna: bool_t = True,
         numeric_only: bool_t = False,
         **kwargs,
@@ -10995,7 +10978,7 @@ def min(
 
     def max(
         self,
-        axis: Axis | None | lib.NoDefault = lib.no_default,
+        axis: Axis | None = 0,
         skipna: bool_t = True,
         numeric_only: bool_t = False,
         **kwargs,
@@ -11011,7 +10994,7 @@ def max(
 
     def mean(
         self,
-        axis: Axis | None | lib.NoDefault = lib.no_default,
+        axis: Axis | None = 0,
         skipna: bool_t = True,
         numeric_only: bool_t = False,
         **kwargs,
@@ -11022,7 +11005,7 @@ def mean(
 
     def median(
         self,
-        axis: Axis | None | lib.NoDefault = lib.no_default,
+        axis: Axis | None = 0,
         skipna: bool_t = True,
         numeric_only: bool_t = False,
         **kwargs,
@@ -11033,7 +11016,7 @@ def median(
 
     def skew(
         self,
-        axis: Axis | None | lib.NoDefault = lib.no_default,
+        axis: Axis | None = 0,
         skipna: bool_t = True,
         numeric_only: bool_t = False,
         **kwargs,
@@ -11044,7 +11027,7 @@ def skew(
 
     def kurt(
         self,
-        axis: Axis | None | lib.NoDefault = lib.no_default,
+        axis: Axis | None = 0,
         skipna: bool_t = True,
         numeric_only: bool_t = False,
         **kwargs,
@@ -11366,7 +11349,7 @@ def prod(
         )
         def mean(
             self,
-            axis: AxisInt | None | lib.NoDefault = lib.no_default,
+            axis: AxisInt | None = 0,
             skipna: bool_t = True,
             numeric_only: bool_t = False,
             **kwargs,
@@ -11387,7 +11370,7 @@ def mean(
         )
         def skew(
             self,
-            axis: AxisInt | None | lib.NoDefault = lib.no_default,
+            axis: AxisInt | None = 0,
             skipna: bool_t = True,
             numeric_only: bool_t = False,
             **kwargs,
@@ -11411,7 +11394,7 @@ def skew(
         )
         def kurt(
             self,
-            axis: Axis | None | lib.NoDefault = lib.no_default,
+            axis: Axis | None = 0,
             skipna: bool_t = True,
             numeric_only: bool_t = False,
             **kwargs,
@@ -11433,7 +11416,7 @@ def kurt(
         )
         def median(
             self,
-            axis: AxisInt | None | lib.NoDefault = lib.no_default,
+            axis: AxisInt | None = 0,
             skipna: bool_t = True,
             numeric_only: bool_t = False,
             **kwargs,
@@ -11456,7 +11439,7 @@ def median(
         )
         def max(
             self,
-            axis: AxisInt | None | lib.NoDefault = lib.no_default,
+            axis: AxisInt | None = 0,
             skipna: bool_t = True,
             numeric_only: bool_t = False,
             **kwargs,
@@ -11479,7 +11462,7 @@ def max(
         )
         def min(
             self,
-            axis: AxisInt | None | lib.NoDefault = lib.no_default,
+            axis: AxisInt | None = 0,
             skipna: bool_t = True,
             numeric_only: bool_t = False,
             **kwargs,
@@ -11708,6 +11691,12 @@ def _doc_params(cls):
 axis : {axis_descr}
     Axis for the function to be applied on.
     For `Series` this parameter is unused and defaults to 0.
+
+    For DataFrames, specifying ``axis=None`` will apply the aggregation
+    across both axes.
+
+    .. versionadded:: 2.0.0
+
 skipna : bool, default True
     Exclude NA/null values when computing the result.
 numeric_only : bool, default False
@@ -11719,7 +11708,7 @@ def _doc_params(cls):
 
 Returns
 -------
-{name1} or {name2} (if level specified)\
+{name1} or scalar\
 {see_also}\
 {examples}
 """
diff --git a/pandas/tests/frame/test_reductions.py b/pandas/tests/frame/test_reductions.py
index 2e0aa5fd0cf40..f1d176e59373f 100644
--- a/pandas/tests/frame/test_reductions.py
+++ b/pandas/tests/frame/test_reductions.py
@@ -1488,7 +1488,6 @@ def test_median_categorical_dtype_nuisance_column(self):
         # TODO: np.median(df, axis=0) gives np.array([2.0, 2.0]) instead
         #  of expected.values
 
-    @pytest.mark.filterwarnings("ignore:.*will return a scalar.*:FutureWarning")
     @pytest.mark.parametrize("method", ["min", "max"])
     def test_min_max_categorical_dtype_non_ordered_nuisance_column(self, method):
         # GH#28949 DataFrame.min should behave like Series.min
@@ -1510,7 +1509,7 @@ def test_min_max_categorical_dtype_non_ordered_nuisance_column(self, method):
             getattr(df, method)()
 
         with pytest.raises(TypeError, match="is not ordered for operation"):
-            getattr(np, method)(df)
+            getattr(np, method)(df, axis=0)
 
         # same thing, but with an additional non-categorical column
         df["B"] = df["A"].astype(object)
@@ -1518,7 +1517,7 @@ def test_min_max_categorical_dtype_non_ordered_nuisance_column(self, method):
             getattr(df, method)()
 
         with pytest.raises(TypeError, match="is not ordered for operation"):
-            getattr(np, method)(df)
+            getattr(np, method)(df, axis=0)
 
 
 def test_sum_timedelta64_skipna_false(using_array_manager, request):
@@ -1600,20 +1599,22 @@ def test_prod_sum_min_count_mixed_object():
 
 
 @pytest.mark.parametrize("method", ["min", "max", "mean", "median", "skew", "kurt"])
-def test_reduction_axis_none_deprecation(method):
-    # GH#21597 deprecate axis=None defaulting to axis=0 so that we can change it
-    #  to reducing over all axes.
+def test_reduction_axis_none_returns_scalar(method):
+    # GH#21597 As of 2.0, axis=None reduces over all axes.
 
     df = DataFrame(np.random.randn(4, 4))
-    meth = getattr(df, method)
-
-    msg = f"scalar {method} over the entire DataFrame"
-    with tm.assert_produces_warning(FutureWarning, match=msg):
-        res = meth(axis=None)
-    with tm.assert_produces_warning(None):
-        expected = meth()
-    tm.assert_series_equal(res, expected)
-    tm.assert_series_equal(res, meth(axis=0))
+
+    result = getattr(df, method)(axis=None)
+    np_arr = df.to_numpy()
+    if method in {"skew", "kurt"}:
+        comp_mod = pytest.importorskip("scipy.stats")
+        if method == "kurt":
+            method = "kurtosis"
+        expected = getattr(comp_mod, method)(np_arr, bias=False, axis=None)
+        tm.assert_almost_equal(result, expected)
+    else:
+        expected = getattr(np, method)(np_arr, axis=None)
+        assert result == expected
 
 
 @pytest.mark.parametrize(
diff --git a/pandas/tests/groupby/test_categorical.py b/pandas/tests/groupby/test_categorical.py
index d9181830925f7..0fabdf84e5e86 100644
--- a/pandas/tests/groupby/test_categorical.py
+++ b/pandas/tests/groupby/test_categorical.py
@@ -147,11 +147,7 @@ def f(x):
     tm.assert_frame_equal(df.groupby(c, observed=False).transform(sum), df[["a"]])
 
     gbc = df.groupby(c, observed=False)
-    with tm.assert_produces_warning(
-        FutureWarning, match="scalar max", check_stacklevel=False
-    ):
-        # stacklevel is thrown off (i think) bc the stack goes through numpy C code
-        result = gbc.transform(lambda xs: np.max(xs))
+    result = gbc.transform(lambda xs: np.max(xs, axis=0))
     tm.assert_frame_equal(result, df[["a"]])
 
     with tm.assert_produces_warning(None):
@@ -295,7 +291,7 @@ def test_apply(ordered):
     idx = MultiIndex.from_arrays([missing, dense], names=["missing", "dense"])
     expected = DataFrame([0, 1, 2.0], index=idx, columns=["values"])
 
-    result = grouped.apply(lambda x: np.mean(x))
+    result = grouped.apply(lambda x: np.mean(x, axis=0))
     tm.assert_frame_equal(result, expected)
 
     result = grouped.mean()
diff --git a/pandas/tests/groupby/test_function.py b/pandas/tests/groupby/test_function.py
index bb15783f4607f..875037b390883 100644
--- a/pandas/tests/groupby/test_function.py
+++ b/pandas/tests/groupby/test_function.py
@@ -80,20 +80,12 @@ def test_builtins_apply(keys, f):
     assert_msg = f"invalid frame shape: {result.shape} (expected ({ngroups}, 3))"
     assert result.shape == (ngroups, 3), assert_msg
 
-    npfunc = getattr(np, fname)  # numpy's equivalent function
-    if f in [max, min]:
-        warn = FutureWarning
-    else:
-        warn = None
-    msg = "scalar (max|min) over the entire DataFrame"
-    with tm.assert_produces_warning(warn, match=msg, check_stacklevel=False):
-        # stacklevel can be thrown off because (i think) the stack
-        #  goes through some of numpy's C code.
-        expected = gb.apply(npfunc)
+    npfunc = lambda x: getattr(np, fname)(x, axis=0)  # numpy's equivalent function
+    expected = gb.apply(npfunc)
     tm.assert_frame_equal(result, expected)
 
     with tm.assert_produces_warning(None):
-        expected2 = gb.apply(lambda x: npfunc(x, axis=0))
+        expected2 = gb.apply(lambda x: npfunc(x))
     tm.assert_frame_equal(result, expected2)
 
     if f != sum:
@@ -101,7 +93,7 @@ def test_builtins_apply(keys, f):
         expected.set_index(keys, inplace=True, drop=False)
         tm.assert_frame_equal(result, expected, check_dtype=False)
 
-    tm.assert_series_equal(getattr(result, fname)(), getattr(df, fname)())
+    tm.assert_series_equal(getattr(result, fname)(axis=0), getattr(df, fname)(axis=0))
 
 
 class TestNumericOnly:
diff --git a/pandas/tests/groupby/transform/test_transform.py b/pandas/tests/groupby/transform/test_transform.py
index 8f1d52c2ea03d..cb3b7def39c52 100644
--- a/pandas/tests/groupby/transform/test_transform.py
+++ b/pandas/tests/groupby/transform/test_transform.py
@@ -57,7 +57,7 @@ def test_transform():
     tm.assert_frame_equal(result, expected)
 
     def demean(arr):
-        return arr - arr.mean()
+        return arr - arr.mean(axis=0)
 
     people = DataFrame(
         np.random.randn(5, 5),
@@ -144,7 +144,7 @@ def test_transform_broadcast(tsframe, ts):
     result = grouped.transform(np.mean)
     tm.assert_index_equal(result.index, tsframe.index)
     for _, gp in grouped:
-        agged = gp.mean()
+        agged = gp.mean(axis=0)
         res = result.reindex(gp.index)
         for col in tsframe:
             assert_fp_equal(res[col], agged[col])
@@ -214,7 +214,7 @@ def test_transform_axis_ts(tsframe):
     ts = tso
     grouped = ts.groupby(lambda x: x.weekday(), group_keys=False)
     result = ts - grouped.transform("mean")
-    expected = grouped.apply(lambda x: x - x.mean())
+    expected = grouped.apply(lambda x: x - x.mean(axis=0))
     tm.assert_frame_equal(result, expected)
 
     ts = ts.T
@@ -227,7 +227,7 @@ def test_transform_axis_ts(tsframe):
     ts = tso.iloc[[1, 0] + list(range(2, len(base)))]
     grouped = ts.groupby(lambda x: x.weekday(), group_keys=False)
     result = ts - grouped.transform("mean")
-    expected = grouped.apply(lambda x: x - x.mean())
+    expected = grouped.apply(lambda x: x - x.mean(axis=0))
     tm.assert_frame_equal(result, expected)
 
     ts = ts.T
@@ -477,16 +477,9 @@ def test_transform_coercion():
 
     expected = g.transform(np.mean)
 
-    # in 2.0 np.mean on a DataFrame is equivalent to frame.mean(axis=None)
-    #  which not gives a scalar instead of Series
-    with tm.assert_produces_warning(FutureWarning, check_stacklevel=False):
-        result = g.transform(lambda x: np.mean(x))
+    result = g.transform(lambda x: np.mean(x, axis=0))
     tm.assert_frame_equal(result, expected)
 
-    with tm.assert_produces_warning(None):
-        result2 = g.transform(lambda x: np.mean(x, axis=0))
-    tm.assert_frame_equal(result2, expected)
-
 
 def test_groupby_transform_with_int():