BUG: Respect axis when doing DataFrame.expanding

Closes pandas-devgh-23372.

doc/source/whatsnew/v0.24.0.txt

@@ -1213,6 +1213,7 @@ Groupby/Resample/Rolling
 - Bug in :meth:`SeriesGroupBy.mean` when values were integral but could not fit inside of int64, overflowing instead. (:issue:`22487`)
 - :func:`RollingGroupby.agg` and :func:`ExpandingGroupby.agg` now support multiple aggregation functions as parameters (:issue:`15072`)
 - Bug in :meth:`DataFrame.resample` and :meth:`Series.resample` when resampling by a weekly offset (``'W'``) across a DST transition (:issue:`9119`, :issue:`21459`)
+- Bug in :meth:`DataFrame.expanding` in which the ``axis`` argument was not being respected during aggregations (:issue:`23372`)
 
 Reshaping
 ^^^^^^^^^

pandas/core/window.py

@@ -1867,6 +1867,15 @@ def _constructor(self):
 
     def _get_window(self, other=None):
         obj = self._selected_obj
+        axis = self.obj._get_axis_number(self.axis)
+
+        # If axis == 0, leave `obj` alone.
+        # If axis == 1, transpose.
+        #
+        # Other values of axis are invalid.
+        if axis == 1:
+            obj = obj.T
+
         if other is None:
             return (max(len(obj), self.min_periods) if self.min_periods
                     else len(obj))

pandas/tests/test_window.py

@@ -714,6 +714,16 @@ def test_iter_raises(self, klass):
         with pytest.raises(NotImplementedError):
             iter(obj.expanding(2))
 
+    def test_expanding_axis(self):
+        # see gh-23372.
+        df = DataFrame(np.ones((10, 20)))
+
+        exp_row = [np.nan] * 2 + [float(i) for i in range(3, 21)]
+        expected = DataFrame([exp_row] * 10)
+
+        result = df.expanding(3, axis=1).sum()
+        tm.assert_frame_equal(result, expected)
+
 
 class TestEWM(Base):