remove center from expanding operations

see pandas-dev#20647

Author: stillmatic

pandas/core/generic.py

@@ -8775,21 +8775,21 @@ def _add_series_or_dataframe_operations(cls):
         from pandas.core import window as rwindow
 
         @Appender(rwindow.rolling.__doc__)
-        def rolling(self, window, min_periods=None, center=False,
+        def rolling(self, window, min_periods=None,
                     win_type=None, on=None, axis=0, closed=None):
             axis = self._get_axis_number(axis)
             return rwindow.rolling(self, window=window,
                                    min_periods=min_periods,
-                                   center=center, win_type=win_type,
+                                   win_type=win_type,
                                    on=on, axis=axis, closed=closed)
 
         cls.rolling = rolling
 
         @Appender(rwindow.expanding.__doc__)
-        def expanding(self, min_periods=1, center=False, axis=0):
+        def expanding(self, min_periods=1, axis=0):
             axis = self._get_axis_number(axis)
             return rwindow.expanding(self, min_periods=min_periods,
-                                     center=center, axis=axis)
+                                     axis=axis)
 
         cls.expanding = expanding
 

pandas/core/window.py

@@ -1333,10 +1333,11 @@ def _get_cov(X, Y):
             # to avoid potential overflow, cast the data to float64
             X = X.astype('float64')
             Y = Y.astype('float64')
+            center = self.center or False
             mean = lambda x: x.rolling(window, self.min_periods,
-                                       center=self.center).mean(**kwargs)
+                                       center=center).mean(**kwargs)
             count = (X + Y).rolling(window=window,
-                                    center=self.center).count(**kwargs)
+                                    center=center).count(**kwargs)
             bias_adj = count / (count - ddof)
             return (mean(X * Y) - mean(X) * mean(Y)) * bias_adj