Matching the behaviour to pandas>=1.0.0 at most.

databricks/koalas/tests/test_expanding.py

@@ -14,6 +14,8 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 #
+from distutils.version import LooseVersion
+
 import numpy as np
 import pandas as pd
 
@@ -60,7 +62,10 @@ def test_expanding_repr(self):
         self.assertEqual(repr(ks.range(10).expanding(5)), "Expanding [min_periods=5]")
 
     def test_expanding_count(self):
-        self._test_expanding_func("count")
+        # The behaviour of Expanding.count are different between pandas>=1.0.0 and lower,
+        # and we're following the behaviour of latest version of pandas.
+        if LooseVersion(pd.__version__) >= LooseVersion('1.0.0'):
+            self._test_expanding_func("count")
 
     def test_expanding_min(self):
         self._test_expanding_func("min")
@@ -115,7 +120,10 @@ def _test_groupby_expanding_func(self, f):
             repr(getattr(pdf.groupby([("a", "x"), ("a", "y")]).expanding(2), f)().sort_index()))
 
     def test_groupby_expanding_count(self):
-        self._test_groupby_expanding_func("count")
+        # The behaviour of ExpandingGroupby.count are different between pandas>=1.0.0 and lower,
+        # and we're following the behaviour of latest version of pandas.
+        if LooseVersion(pd.__version__) >= LooseVersion('1.0.0'):
+            self._test_groupby_expanding_func("count")
 
     def test_groupby_expanding_min(self):
         self._test_groupby_expanding_func("min")

databricks/koalas/window.py

@@ -55,15 +55,6 @@ def count(self):
         def count(scol):
             return F.count(scol).over(self._window)
 
-        if LooseVersion(pd.__version__) >= LooseVersion('1.0.0'):
-            if isinstance(self, (Expanding, ExpandingGroupby)):
-                def count_expanding(scol):
-                    return F.when(
-                        F.row_number().over(self._unbounded_window) >= self._min_periods,
-                        F.count(scol).over(self._window)
-                    ).otherwise(F.lit(None))
-                return self._apply_as_series_or_frame(count_expanding).astype('float64')
-
         return self._apply_as_series_or_frame(count).astype('float64')
 
     def sum(self):
@@ -1104,7 +1095,12 @@ def count(self):
         2  2.0
         3  3.0
         """
-        return super(Expanding, self).count()
+        def count(scol):
+            return F.when(
+                F.row_number().over(self._unbounded_window) >= self._min_periods,
+                F.count(scol).over(self._window)
+            ).otherwise(F.lit(None))
+        return self._apply_as_series_or_frame(count).astype('float64')
 
     def sum(self):
         """