Deprecate center on df.expanding (pandas-dev#34887)

doc/source/whatsnew/v1.1.0.rst

@@ -825,6 +825,9 @@ Deprecations
   precision through the ``rtol``, and ``atol`` parameters, thus deprecating the
   ``check_less_precise`` parameter. (:issue:`13357`).
 - :func:`DataFrame.melt` accepting a value_name that already exists is deprecated, and will be removed in a future version (:issue:`34731`)
+- 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

@@ -10500,8 +10500,18 @@ 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)
+            if center is not None:
+                warnings.warn(
+                    "The `center` argument on `expanding` "
+                    "will be removed in the future",
+                    FutureWarning,
+                    stacklevel=2,
+                )
+            else:
+                center = False
+
             return Expanding(self, min_periods=min_periods, center=center, axis=axis)
 
         cls.expanding = expanding

pandas/core/window/expanding.py

@@ -57,7 +57,7 @@ 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):
         super().__init__(obj=obj, min_periods=min_periods, center=center, axis=axis)
 
     @property

pandas/tests/window/moments/test_moments_consistency_expanding.py

@@ -119,8 +119,8 @@ def test_expanding_corr_pairwise(frame):
     ids=["sum", "mean", "max", "min"],
 )
 def test_expanding_func(func, static_comp, has_min_periods, series, frame, nan_locs):
-    def expanding_func(x, min_periods=1, center=False, axis=0):
-        exp = x.expanding(min_periods=min_periods, center=center, axis=axis)
+    def expanding_func(x, min_periods=1, axis=0):
+        exp = x.expanding(min_periods=min_periods, axis=axis)
         return getattr(exp, func)()
 
     _check_expanding(
@@ -166,7 +166,7 @@ def test_expanding_apply_consistency(
 
     with warnings.catch_warnings():
         warnings.filterwarnings(
-            "ignore", message=".*(empty slice|0 for slice).*", category=RuntimeWarning,
+            "ignore", message=".*(empty slice|0 for slice).*", category=RuntimeWarning
         )
         # test consistency between expanding_xyz() and either (a)
         # expanding_apply of Series.xyz(), or (b) expanding_apply of
@@ -267,7 +267,7 @@ def test_expanding_consistency(consistency_data, min_periods):
     # with empty/0-length Series/DataFrames
     with warnings.catch_warnings():
         warnings.filterwarnings(
-            "ignore", message=".*(empty slice|0 for slice).*", category=RuntimeWarning,
+            "ignore", message=".*(empty slice|0 for slice).*", category=RuntimeWarning
         )
 
         # test consistency between different expanding_* moments
@@ -454,7 +454,7 @@ def test_expanding_cov_pairwise_diff_length():
 def test_expanding_corr_pairwise_diff_length():
     # GH 7512
     df1 = DataFrame(
-        [[1, 2], [3, 2], [3, 4]], columns=["A", "B"], index=Index(range(3), name="bar"),
+        [[1, 2], [3, 2], [3, 4]], columns=["A", "B"], index=Index(range(3), name="bar")
     )
     df1a = DataFrame(
         [[1, 2], [3, 4]], index=Index([0, 2], name="bar"), columns=["A", "B"]

pandas/tests/window/test_api.py

@@ -107,10 +107,7 @@ def test_agg():
 
     with pytest.raises(SpecificationError, match=msg):
         r.aggregate(
-            {
-                "A": {"mean": "mean", "sum": "sum"},
-                "B": {"mean2": "mean", "sum2": "sum"},
-            }
+            {"A": {"mean": "mean", "sum": "sum"}, "B": {"mean2": "mean", "sum2": "sum"}}
         )
 
     result = r.aggregate({"A": ["mean", "std"], "B": ["mean", "std"]})
@@ -191,11 +188,7 @@ def test_count_nonnumeric_types():
         "dt_nat",
         "periods_nat",
     ]
-    dt_nat_col = [
-        Timestamp("20170101"),
-        Timestamp("20170203"),
-        Timestamp(None),
-    ]
+    dt_nat_col = [Timestamp("20170101"), Timestamp("20170203"), Timestamp(None)]
 
     df = DataFrame(
         {

pandas/tests/window/test_expanding.py

@@ -16,6 +16,9 @@ def test_doc_string():
     df.expanding(2).sum()
 
 
+@pytest.mark.filterwarnings(
+    "ignore:The `center` argument on `expanding` will be removed in the future"
+)
 def test_constructor(which):
     # GH 12669
 
@@ -213,3 +216,16 @@ 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()