Deprecate center on df.expanding

`df.expanding(center=True)` currently returns non-sensical results and it
is unclear what results would be expected. It was previously removed in
#7934 for that same reason,
but was reintroduced for unclear reasons in a later refactoring

Author: MBrouns

doc/source/whatsnew/v1.1.0.rst

@@ -781,6 +781,7 @@ Deprecations
 - The ``squeeze`` keyword in the ``groupby`` function is deprecated and will be removed in a future version (:issue:`32380`)
 - The ``tz`` keyword in :meth:`Period.to_timestamp` is deprecated and will be removed in a future version; use `per.to_timestamp(...).tz_localize(tz)`` instead (:issue:`34522`)
 - :meth:`DatetimeIndex.to_perioddelta` is deprecated and will be removed in a future version.  Use ``index - index.to_period(freq).to_timestamp()`` instead (:issue:`34853`)
+- the ``center`` keyword in the :meth:`DataFrame.expanding`  function is deprecated and will be removed in a future version (:issue:`20647`)
 
 .. ---------------------------------------------------------------------------
 

pandas/core/generic.py

@@ -10501,7 +10501,7 @@ def rolling(
         cls.rolling = rolling
 
         @doc(Expanding)
-        def expanding(self, min_periods=1, center=False, axis=0):
+        def expanding(self, min_periods=1, center=None, axis=0):
             axis = self._get_axis_number(axis)
             return Expanding(self, min_periods=min_periods, center=center, axis=axis)
 

pandas/core/window/expanding.py

@@ -1,3 +1,4 @@
+import warnings
 from textwrap import dedent
 from typing import Dict, Optional
 
@@ -57,7 +58,19 @@ class Expanding(_Rolling_and_Expanding):
 
     _attributes = ["min_periods", "center", "axis"]
 
-    def __init__(self, obj, min_periods=1, center=False, axis=0, **kwargs):
+    def __init__(self, obj, min_periods=1, center=None, axis=0, **kwargs):
+        if center is not None:
+            warnings.warn(
+                """
+                the `center` argument on `expanding` will be removed in the future.
+                Please be aware that there are issues with its current implementation,
+                and use it with caution
+                (https://github.com/pandas-dev/pandas/issues/20647)""",
+                FutureWarning,
+                stacklevel=3,
+            )
+        else:
+            center = False
         super().__init__(obj=obj, min_periods=min_periods, center=center, axis=axis)
 
     @property

pandas/tests/window/test_expanding.py

@@ -210,3 +210,16 @@ def test_iter_expanding_series(ser, expected, min_periods):
 
     for (expected, actual) in zip(expected, ser.expanding(min_periods)):
         tm.assert_series_equal(actual, expected)
+
+
+def test_center_deprecate_warning():
+    # GH 20647
+    df = pd.DataFrame()
+    with tm.assert_produces_warning(FutureWarning):
+        df.expanding(center=True)
+
+    with tm.assert_produces_warning(FutureWarning):
+        df.expanding(center=False)
+
+    with tm.assert_produces_warning(None):
+        df.expanding()