BUG: groupby sum, mean, var should always be floats

doc/source/whatsnew/v1.3.0.rst

@@ -333,6 +333,31 @@ values as measured by ``np.allclose``. Now no such casting occurs.
 
     df.groupby('key').agg(lambda x: x.sum())
 
+``float`` result for :meth:`.GroupBy.mean`, :meth:`.GroupBy.median`, and :meth:`.GroupBy.var`
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Previously, these methods could result in different dtypes depending on the input values.
+Now, these methods will always return a float dtype. (:issue:`41137`)
+
+.. ipython:: python
+
+    df = pd.DataFrame({'a': [True], 'b': [1], 'c': [1.0]})
+
+*pandas 1.2.x*
+
+.. code-block:: ipython
+
+    In [5]: df.groupby(df.index).mean()
+    Out[5]:
+            a  b    c
+    0    True  1  1.0
+
+*pandas 1.3.0*
+
+.. ipython:: python
+
+    df.groupby(df.index).mean()
+
 Try operating inplace when setting values with ``loc`` and ``iloc``
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 

pandas/core/groupby/groupby.py

@@ -1600,12 +1600,12 @@ def mean(self, numeric_only: bool = True):
         Groupby two columns and return the mean of the remaining column.
 
         >>> df.groupby(['A', 'B']).mean()
-               C
+                 C
         A B
-        1 2.0  2
-          4.0  1
-        2 3.0  1
-          5.0  2
+        1 2.0  2.0
+          4.0  1.0
+        2 3.0  1.0
+          5.0  2.0
 
         Groupby one column and return the mean of only particular column in
         the group.

pandas/core/groupby/grouper.py

@@ -150,8 +150,8 @@ class Grouper:
     >>> df.groupby(pd.Grouper(key="Animal")).mean()
             Speed
     Animal
-    Falcon    200
-    Parrot     10
+    Falcon  200.0
+    Parrot   10.0
 
     Specify a resample operation on the column 'Publish date'
 

pandas/core/groupby/ops.py

@@ -49,6 +49,7 @@
     is_categorical_dtype,
     is_complex_dtype,
     is_datetime64_any_dtype,
+    is_float_dtype,
     is_integer_dtype,
     is_numeric_dtype,
     is_sparse,
@@ -304,10 +305,13 @@ def _get_result_dtype(self, dtype: DtypeObj) -> DtypeObj:
                 return np.dtype(np.int64)
             elif isinstance(dtype, (BooleanDtype, _IntegerDtype)):
                 return Int64Dtype()
-        elif how in ["mean", "median", "var"] and isinstance(
-            dtype, (BooleanDtype, _IntegerDtype)
-        ):
-            return Float64Dtype()
+        elif how in ["mean", "median", "var"]:
+            if isinstance(dtype, (BooleanDtype, _IntegerDtype)):
+                return Float64Dtype()
+            elif is_float_dtype(dtype):
+                return dtype
+            elif is_numeric_dtype(dtype):
+                return np.dtype(np.float64)
         return dtype
 
     def uses_mask(self) -> bool:

pandas/tests/extension/base/groupby.py

@@ -25,7 +25,7 @@ def test_groupby_extension_agg(self, as_index, data_for_grouping):
         _, index = pd.factorize(data_for_grouping, sort=True)
 
         index = pd.Index(index, name="B")
-        expected = pd.Series([3, 1, 4], index=index, name="A")
+        expected = pd.Series([3.0, 1.0, 4.0], index=index, name="A")
         if as_index:
             self.assert_series_equal(result, expected)
         else:
@@ -54,7 +54,7 @@ def test_groupby_extension_no_sort(self, data_for_grouping):
         _, index = pd.factorize(data_for_grouping, sort=False)
 
         index = pd.Index(index, name="B")
-        expected = pd.Series([1, 3, 4], index=index, name="A")
+        expected = pd.Series([1.0, 3.0, 4.0], index=index, name="A")
         self.assert_series_equal(result, expected)
 
     def test_groupby_extension_transform(self, data_for_grouping):

pandas/tests/extension/test_boolean.py

@@ -272,7 +272,7 @@ def test_groupby_extension_agg(self, as_index, data_for_grouping):
         _, index = pd.factorize(data_for_grouping, sort=True)
 
         index = pd.Index(index, name="B")
-        expected = pd.Series([3, 1], index=index, name="A")
+        expected = pd.Series([3.0, 1.0], index=index, name="A")
         if as_index:
             self.assert_series_equal(result, expected)
         else:
@@ -301,7 +301,7 @@ def test_groupby_extension_no_sort(self, data_for_grouping):
         _, index = pd.factorize(data_for_grouping, sort=False)
 
         index = pd.Index(index, name="B")
-        expected = pd.Series([1, 3], index=index, name="A")
+        expected = pd.Series([1.0, 3.0], index=index, name="A")
         self.assert_series_equal(result, expected)
 
     def test_groupby_extension_transform(self, data_for_grouping):

pandas/tests/groupby/aggregate/test_aggregate.py

@@ -512,7 +512,9 @@ def test_uint64_type_handling(dtype, how):
     expected = df.groupby("y").agg({"x": how})
     df.x = df.x.astype(dtype)
     result = df.groupby("y").agg({"x": how})
-    result.x = result.x.astype(np.int64)
+    if how not in ("mean", "median"):
+        # mean and median always result in floats
+        result.x = result.x.astype(np.int64)
     tm.assert_frame_equal(result, expected, check_exact=True)
 
 

pandas/tests/groupby/aggregate/test_cython.py

@@ -196,9 +196,6 @@ def test_cython_agg_empty_buckets(op, targop, observed):
 
     g = df.groupby(pd.cut(df[0], grps), observed=observed)
     expected = g.agg(lambda x: targop(x))
-    if observed and op not in ("min", "max"):
-        # TODO: GH 41137
-        expected = expected.astype("int64")
     tm.assert_frame_equal(result, expected)
 
 

pandas/tests/groupby/test_categorical.py

@@ -285,8 +285,6 @@ def test_apply(ordered):
     result = grouped.apply(lambda x: np.mean(x))
     tm.assert_frame_equal(result, expected)
 
-    # we coerce back to ints
-    expected = expected.astype("int")
     result = grouped.mean()
     tm.assert_frame_equal(result, expected)
 
@@ -371,7 +369,7 @@ def test_observed(observed, using_array_manager):
     result = groups_double_key.agg("mean")
     expected = DataFrame(
         {
-            "val": [10, 30, 20, 40],
+            "val": [10.0, 30.0, 20.0, 40.0],
             "cat": Categorical(
                 ["a", "a", "b", "b"], categories=["a", "b", "c"], ordered=True
             ),
@@ -418,7 +416,9 @@ def test_observed_codes_remap(observed):
     groups_double_key = df.groupby([values, "C2"], observed=observed)
 
     idx = MultiIndex.from_arrays([values, [1, 2, 3, 4]], names=["cat", "C2"])
-    expected = DataFrame({"C1": [3, 3, 4, 5], "C3": [10, 100, 200, 34]}, index=idx)
+    expected = DataFrame(
+        {"C1": [3.0, 3.0, 4.0, 5.0], "C3": [10.0, 100.0, 200.0, 34.0]}, index=idx
+    )
     if not observed:
         expected = cartesian_product_for_groupers(
             expected, [values.values, [1, 2, 3, 4]], ["cat", "C2"]
@@ -1515,7 +1515,9 @@ def test_read_only_category_no_sort():
     df = DataFrame(
         {"a": [1, 3, 5, 7], "b": Categorical([1, 1, 2, 2], categories=Index(cats))}
     )
-    expected = DataFrame(data={"a": [2, 6]}, index=CategoricalIndex([1, 2], name="b"))
+    expected = DataFrame(
+        data={"a": [2.0, 6.0]}, index=CategoricalIndex([1, 2], name="b")
+    )
     result = df.groupby("b", sort=False).mean()
     tm.assert_frame_equal(result, expected)
 

pandas/tests/groupby/test_groupby.py

@@ -1257,7 +1257,7 @@ def test_groupby_keys_same_size_as_index():
     )
     df = DataFrame([["A", 10], ["B", 15]], columns=["metric", "values"], index=index)
     result = df.groupby([Grouper(level=0, freq=freq), "metric"]).mean()
-    expected = df.set_index([df.index, "metric"])
+    expected = df.set_index([df.index, "metric"]).astype(float)
 
     tm.assert_frame_equal(result, expected)
 
@@ -1350,7 +1350,7 @@ def test_groupby_2d_malformed():
     d["ones"] = [1, 1]
     d["label"] = ["l1", "l2"]
     tmp = d.groupby(["group"]).mean()
-    res_values = np.array([[0, 1], [0, 1]], dtype=np.int64)
+    res_values = np.array([[0.0, 1.0], [0.0, 1.0]])
     tm.assert_index_equal(tmp.columns, Index(["zeros", "ones"]))
     tm.assert_numpy_array_equal(tmp.values, res_values)
 
@@ -2114,7 +2114,7 @@ def test_groupby_crash_on_nunique(axis):
 
 def test_groupby_list_level():
     # GH 9790
-    expected = DataFrame(np.arange(0, 9).reshape(3, 3))
+    expected = DataFrame(np.arange(0, 9).reshape(3, 3), dtype=float)
     result = expected.groupby(level=[0]).mean()
     tm.assert_frame_equal(result, expected)
 

pandas/tests/io/formats/test_to_csv.py

@@ -274,7 +274,7 @@ def test_to_csv_date_format(self):
         df_sec["B"] = 0
         df_sec["C"] = 1
 
-        expected_rows = ["A,B,C", "2013-01-01,0,1"]
+        expected_rows = ["A,B,C", "2013-01-01,0,1.0"]
         expected_ymd_sec = tm.convert_rows_list_to_csv_str(expected_rows)
 
         df_sec_grouped = df_sec.groupby([pd.Grouper(key="A", freq="1h"), "B"])

pandas/tests/resample/test_datetime_index.py

@@ -1064,7 +1064,7 @@ def test_nanosecond_resample_error():
     result = r.agg("mean")
 
     exp_indx = date_range(start=pd.to_datetime(exp_start), periods=10, freq="100n")
-    exp = Series(range(len(exp_indx)), index=exp_indx)
+    exp = Series(range(len(exp_indx)), index=exp_indx, dtype=float)
 
     tm.assert_series_equal(result, exp)
 
@@ -1636,15 +1636,15 @@ def test_resample_with_nat():
     index_1s = DatetimeIndex(
         ["1970-01-01 00:00:00", "1970-01-01 00:00:01", "1970-01-01 00:00:02"]
     )
-    frame_1s = DataFrame([3, 7, 11], index=index_1s)
+    frame_1s = DataFrame([3.0, 7.0, 11.0], index=index_1s)
     tm.assert_frame_equal(frame.resample("1s").mean(), frame_1s)
 
     index_2s = DatetimeIndex(["1970-01-01 00:00:00", "1970-01-01 00:00:02"])
-    frame_2s = DataFrame([5, 11], index=index_2s)
+    frame_2s = DataFrame([5.0, 11.0], index=index_2s)
     tm.assert_frame_equal(frame.resample("2s").mean(), frame_2s)
 
     index_3s = DatetimeIndex(["1970-01-01 00:00:00"])
-    frame_3s = DataFrame([7], index=index_3s)
+    frame_3s = DataFrame([7.0], index=index_3s)
     tm.assert_frame_equal(frame.resample("3s").mean(), frame_3s)
 
     tm.assert_frame_equal(frame.resample("60s").mean(), frame_3s)
@@ -1687,7 +1687,7 @@ def f(data, add_arg):
 
     # Testing dataframe
     df = DataFrame({"A": 1, "B": 2}, index=date_range("2017", periods=10))
-    result = df.groupby("A").resample("D").agg(f, multiplier)
+    result = df.groupby("A").resample("D").agg(f, multiplier).astype(float)
     expected = df.groupby("A").resample("D").mean().multiply(multiplier)
     # TODO: GH 41137
     expected = expected.astype("float64")

pandas/tests/resample/test_period_index.py

@@ -269,7 +269,7 @@ def test_with_local_timezone_pytz(self):
         # Index is moved back a day with the timezone conversion from UTC to
         # Pacific
         expected_index = period_range(start=start, end=end, freq="D") - offsets.Day()
-        expected = Series(1, index=expected_index)
+        expected = Series(1.0, index=expected_index)
         tm.assert_series_equal(result, expected)
 
     def test_resample_with_pytz(self):
@@ -279,7 +279,7 @@ def test_resample_with_pytz(self):
         )
         result = s.resample("D").mean()
         expected = Series(
-            2,
+            2.0,
             index=pd.DatetimeIndex(
                 ["2017-01-01", "2017-01-02"], tz="US/Eastern", freq="D"
             ),
@@ -312,7 +312,7 @@ def test_with_local_timezone_dateutil(self):
         expected_index = (
             period_range(start=start, end=end, freq="D", name="idx") - offsets.Day()
         )
-        expected = Series(1, index=expected_index)
+        expected = Series(1.0, index=expected_index)
         tm.assert_series_equal(result, expected)
 
     def test_resample_nonexistent_time_bin_edge(self):
@@ -777,8 +777,8 @@ def test_upsampling_ohlc(self, freq, period_mult, kind):
         "freq, expected_values",
         [
             ("1s", [3, np.NaN, 7, 11]),
-            ("2s", [3, int((7 + 11) / 2)]),
-            ("3s", [int((3 + 7) / 2), 11]),
+            ("2s", [3, (7 + 11) / 2]),
+            ("3s", [(3 + 7) / 2, 11]),
         ],
     )
     def test_resample_with_nat(self, periods, values, freq, expected_values):
@@ -798,7 +798,7 @@ def test_resample_with_only_nat(self):
         pi = PeriodIndex([pd.NaT] * 3, freq="S")
         frame = DataFrame([2, 3, 5], index=pi, columns=["a"])
         expected_index = PeriodIndex(data=[], freq=pi.freq)
-        expected = DataFrame(index=expected_index, columns=["a"], dtype="int64")
+        expected = DataFrame(index=expected_index, columns=["a"], dtype="float64")
         result = frame.resample("1s").mean()
         tm.assert_frame_equal(result, expected)
 

pandas/tests/resample/test_timedelta.py

@@ -77,7 +77,7 @@ def test_resample_timedelta_idempotency():
     index = timedelta_range("0", periods=9, freq="10L")
     series = Series(range(9), index=index)
     result = series.resample("10L").mean()
-    expected = series
+    expected = series.astype(float)
     tm.assert_series_equal(result, expected)
 
 

pandas/tests/reshape/test_pivot.py

@@ -240,13 +240,13 @@ def test_pivot_with_non_observable_dropna(self, dropna):
                     categories=["low", "high"],
                     ordered=True,
                 ),
-                "B": range(5),
+                "B": [0.0, 1.0, 2.0, 3.0, 4.0],
             }
         )
 
         result = df.pivot_table(index="A", values="B", dropna=dropna)
         expected = DataFrame(
-            {"B": [2, 3]},
+            {"B": [2.0, 3.0]},
             index=Index(
                 Categorical.from_codes(
                     [0, 1], categories=["low", "high"], ordered=True
@@ -279,6 +279,8 @@ def test_pivot_with_non_observable_dropna(self, dropna):
                 name="A",
             ),
         )
+        if not dropna:
+            expected["B"] = expected["B"].astype(float)
 
         tm.assert_frame_equal(result, expected)
 
@@ -287,6 +289,8 @@ def test_pivot_with_interval_index(self, interval_values, dropna):
         df = DataFrame({"A": interval_values, "B": 1})
         result = df.pivot_table(index="A", values="B", dropna=dropna)
         expected = DataFrame({"B": 1}, index=Index(interval_values.unique(), name="A"))
+        if not dropna:
+            expected = expected.astype(float)
         tm.assert_frame_equal(result, expected)
 
     def test_pivot_with_interval_index_margins(self):
@@ -388,10 +392,7 @@ def test_pivot_preserve_dtypes(self, columns, values):
         )
 
         result = dict(df_res.dtypes)
-        expected = {
-            col: np.dtype("O") if col[0].startswith("b") else np.dtype("float64")
-            for col in df_res
-        }
+        expected = {col: np.dtype("float64") for col in df_res}
         assert result == expected
 
     def test_pivot_no_values(self):
@@ -1711,8 +1712,13 @@ def test_pivot_table_margins_name_with_aggfunc_list(self):
         expected = DataFrame(table.values, index=ix, columns=cols)
         tm.assert_frame_equal(table, expected)
 
-    @pytest.mark.xfail(reason="GH#17035 (np.mean of ints is casted back to ints)")
-    def test_categorical_margins(self, observed):
+    def test_categorical_margins(self, observed, request):
+        if observed:
+            request.node.add_marker(
+                pytest.mark.xfail(
+                    reason="GH#17035 (np.mean of ints is casted back to ints)"
+                )
+            )
         # GH 10989
         df = DataFrame(
             {"x": np.arange(8), "y": np.arange(8) // 4, "z": np.arange(8) % 2}
@@ -1725,8 +1731,13 @@ def test_categorical_margins(self, observed):
         table = df.pivot_table("x", "y", "z", dropna=observed, margins=True)
         tm.assert_frame_equal(table, expected)
 
-    @pytest.mark.xfail(reason="GH#17035 (np.mean of ints is casted back to ints)")
-    def test_categorical_margins_category(self, observed):
+    def test_categorical_margins_category(self, observed, request):
+        if observed:
+            request.node.add_marker(
+                pytest.mark.xfail(
+                    reason="GH#17035 (np.mean of ints is casted back to ints)"
+                )
+            )
         df = DataFrame(
             {"x": np.arange(8), "y": np.arange(8) // 4, "z": np.arange(8) % 2}
         )