BUG: groupby.size and groupby.transform('size') incorrect for axis=1

doc/source/whatsnew/v1.5.0.rst

@@ -371,7 +371,8 @@ Plotting
 Groupby/resample/rolling
 ^^^^^^^^^^^^^^^^^^^^^^^^
 - Bug in :meth:`DataFrame.resample` ignoring ``closed="right"`` on :class:`TimedeltaIndex` (:issue:`45414`)
-- Bug in :meth:`.DataFrameGroupBy.transform` fails when the input DataFrame has multiple columns (:issue:`27469`)
+- Bug in :meth:`.DataFrameGroupBy.transform` fails when ``func="size"`` and the input DataFrame has multiple columns (:issue:`27469`)
+- Bug in :meth:`.DataFrameGroupBy.size` and :meth:`.DataFrameGroupBy.transform` with ``func="size"`` produced incorrect results when ``axis=1`` (:issue:`45715`)
 
 Reshaping
 ^^^^^^^^^

pandas/conftest.py

@@ -229,6 +229,14 @@ def axis(request):
 axis_frame = axis
 
 
+@pytest.fixture(params=[1, "columns"], ids=lambda x: f"axis={repr(x)}")
+def axis_1(request):
+    """
+    Fixture for returning aliases of axis 1 of a DataFrame.
+    """
+    return request.param
+
+
 @pytest.fixture(params=[True, False, None])
 def observed(request):
     """

pandas/core/groupby/groupby.py

@@ -1665,15 +1665,15 @@ def _wrap_transform_fast_result(self, result: NDFrameT) -> NDFrameT:
 
         # for each col, reshape to size of original frame by take operation
         ids, _, _ = self.grouper.group_info
-        result = result.reindex(self.grouper.result_index, copy=False)
+        result = result.reindex(self.grouper.result_index, axis=self.axis, copy=False)
 
         if self.obj.ndim == 1:
             # i.e. SeriesGroupBy
             out = algorithms.take_nd(result._values, ids)
             output = obj._constructor(out, index=obj.index, name=obj.name)
         else:
-            output = result.take(ids, axis=0)
-            output.index = obj.index
+            output = result.take(ids, axis=self.axis)
+            output = output.set_axis(obj._get_axis(self.axis), axis=self.axis)
         return output
 
     # -----------------------------------------------------------------
@@ -2150,6 +2150,13 @@ def size(self) -> DataFrame | Series:
         """
         result = self.grouper.size()
 
+        if self.axis == 1:
+            return DataFrame(
+                data=np.tile(result.values, (self.obj.shape[0], 1)),
+                columns=result.index,
+                index=self.obj.index,
+            )
+
         # GH28330 preserve subclassed Series/DataFrames through calls
         if issubclass(self.obj._constructor, Series):
             result = self._obj_1d_constructor(result, name=self.obj.name)

pandas/tests/groupby/conftest.py

@@ -19,6 +19,11 @@ def as_index(request):
     return request.param
 
 
+@pytest.fixture(params=[True, False])
+def dropna(request):
+    return request.param
+
+
 @pytest.fixture
 def mframe(multiindex_dataframe_random_data):
     return multiindex_dataframe_random_data

pandas/tests/groupby/test_size.py

@@ -18,6 +18,29 @@ def test_size(df, by):
         assert result[key] == len(group)
 
 
+@pytest.mark.parametrize(
+    "by",
+    [
+        [0, 0, 0, 0],
+        [0, 1, 1, 1],
+        [1, 0, 1, 1],
+        [0, None, None, None],
+        pytest.param([None, None, None, None], marks=pytest.mark.xfail),
+    ],
+)
+def test_size_axis_1(df, axis_1, by, sort, dropna):
+    # GH#45715
+    counts = {key: sum(value == key for value in by) for key in dict.fromkeys(by)}
+    if dropna:
+        counts = {key: value for key, value in counts.items() if key is not None}
+    expected = DataFrame(counts, index=df.index)
+    if sort:
+        expected = expected.sort_index(axis=1)
+    grouped = df.groupby(by=by, axis=axis_1, sort=sort, dropna=dropna)
+    result = grouped.size()
+    tm.assert_frame_equal(result, expected)
+
+
 @pytest.mark.parametrize("by", ["A", "B", ["A", "B"]])
 @pytest.mark.parametrize("sort", [True, False])
 def test_size_sort(sort, by):

pandas/tests/groupby/transform/test_transform.py

@@ -191,6 +191,29 @@ def test_transform_axis_1(request, transformation_func):
     tm.assert_equal(result, expected)
 
 
+def test_transform_axis_1_reducer(request, reduction_func):
+    # GH#45715
+    if reduction_func in (
+        "corrwith",
+        "first",
+        "idxmax",
+        "idxmin",
+        "last",
+        "ngroup",
+        "nth",
+    ):
+        marker = pytest.mark.xfail(reason="transform incorrectly fails - GH#45986")
+        request.node.add_marker(marker)
+    df = DataFrame({"a": [1, 2], "b": [3, 4], "c": [5, 6]}, index=["x", "y"])
+    result = df.groupby([0, 0, 1], axis=1).transform(reduction_func)
+    if reduction_func == "size":
+        # size doesn't behave in the same manner; hardcode expected result
+        expected = DataFrame(2 * [[2, 2, 1]], index=df.index, columns=df.columns)
+    else:
+        expected = df.T.groupby([0, 0, 1]).transform(reduction_func).T
+    tm.assert_equal(result, expected)
+
+
 def test_transform_axis_ts(tsframe):
 
     # make sure that we are setting the axes