From 1ec754f31d0a0654f3f8d6a3a9513590a187399f Mon Sep 17 00:00:00 2001
From: MBrouns <matthijs.brouns@gmail.com>
Date: Sat, 20 Jun 2020 12:50:32 +0200
Subject: [PATCH 1/4] 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
https://github.com/pandas-dev/pandas/pull/7934 for that same reason,
but was reintroduced for unclear reasons in a later refactoring
---
 doc/source/whatsnew/v1.1.0.rst                |  3 +++
 pandas/core/base.py                           | 18 +++++++++++------
 pandas/core/generic.py                        |  2 +-
 pandas/core/groupby/base.py                   |  9 +++++++--
 pandas/core/window/expanding.py               | 18 +++++++++++++++--
 .../test_moments_consistency_expanding.py     | 10 +++++-----
 pandas/tests/window/test_api.py               | 11 ++--------
 pandas/tests/window/test_expanding.py         | 20 ++++++++++++-------
 8 files changed, 59 insertions(+), 32 deletions(-)

diff --git a/doc/source/whatsnew/v1.1.0.rst b/doc/source/whatsnew/v1.1.0.rst
index 9bd4ddbb624d9..e7f417cbe52c3 100644
--- a/doc/source/whatsnew/v1.1.0.rst
+++ b/doc/source/whatsnew/v1.1.0.rst
@@ -820,6 +820,9 @@ 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`)
+
+
 
 .. ---------------------------------------------------------------------------
 
diff --git a/pandas/core/base.py b/pandas/core/base.py
index b62ef668df5e1..2297f8337ca0b 100644
--- a/pandas/core/base.py
+++ b/pandas/core/base.py
@@ -5,6 +5,7 @@
 import builtins
 import textwrap
 from typing import Any, Dict, FrozenSet, List, Optional, Union
+import warnings
 
 import numpy as np
 
@@ -581,12 +582,17 @@ def _shallow_copy(self, obj, **kwargs):
         """
         return a new object with the replacement attributes
         """
-        if isinstance(obj, self._constructor):
-            obj = obj.obj
-        for attr in self._attributes:
-            if attr not in kwargs:
-                kwargs[attr] = getattr(self, attr)
-        return self._constructor(obj, **kwargs)
+        with warnings.catch_warnings():
+            warnings.filterwarnings(
+                "ignore",
+                "The `center` argument on `expanding` will be removed in the future",
+            )
+            if isinstance(obj, self._constructor):
+                obj = obj.obj
+            for attr in self._attributes:
+                if attr not in kwargs:
+                    kwargs[attr] = getattr(self, attr)
+            return self._constructor(obj, **kwargs)
 
 
 class IndexOpsMixin:
diff --git a/pandas/core/generic.py b/pandas/core/generic.py
index d892e2487b31c..7e7a5ae92e2a2 100644
--- a/pandas/core/generic.py
+++ b/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)
 
diff --git a/pandas/core/groupby/base.py b/pandas/core/groupby/base.py
index e71b2f94c8014..7dded2ba258e3 100644
--- a/pandas/core/groupby/base.py
+++ b/pandas/core/groupby/base.py
@@ -4,6 +4,7 @@
 SeriesGroupBy and the DataFrameGroupBy objects.
 """
 import collections
+import warnings
 
 from pandas.core.dtypes.common import is_list_like, is_scalar
 
@@ -40,8 +41,12 @@ def _gotitem(self, key, ndim, subset=None):
             groupby = self._groupby[key]
         except IndexError:
             groupby = self._groupby
-
-        self = type(self)(subset, groupby=groupby, parent=self, **kwargs)
+        with warnings.catch_warnings():
+            warnings.filterwarnings(
+                "ignore",
+                "The `center` argument on `expanding` will be removed in the future",
+            )
+            self = type(self)(subset, groupby=groupby, parent=self, **kwargs)
         self._reset_cache()
         if subset.ndim == 2:
             if is_scalar(key) and key in subset or is_list_like(key):
diff --git a/pandas/core/window/expanding.py b/pandas/core/window/expanding.py
index bbc19fad8b799..6fcb3b5dcbc2b 100644
--- a/pandas/core/window/expanding.py
+++ b/pandas/core/window/expanding.py
@@ -1,5 +1,6 @@
 from textwrap import dedent
 from typing import Dict, Optional
+import warnings
 
 from pandas.compat.numpy import function as nv
 from pandas.util._decorators import Appender, Substitution, doc
@@ -57,7 +58,15 @@ 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",
+                FutureWarning,
+                stacklevel=3,
+            )
+        else:
+            center = False
         super().__init__(obj=obj, min_periods=min_periods, center=center, axis=axis)
 
     @property
@@ -129,7 +138,12 @@ def aggregate(self, func, *args, **kwargs):
     @Substitution(name="expanding")
     @Appender(_shared_docs["count"])
     def count(self, **kwargs):
-        return super().count(**kwargs)
+        with warnings.catch_warnings():
+            warnings.filterwarnings(
+                "ignore",
+                "The `center` argument on `expanding` will be removed in the future",
+            )
+            return super().count(**kwargs)
 
     @Substitution(name="expanding")
     @Appender(_shared_docs["apply"])
diff --git a/pandas/tests/window/moments/test_moments_consistency_expanding.py b/pandas/tests/window/moments/test_moments_consistency_expanding.py
index ee3579d76d1db..3ec91dcb60610 100644
--- a/pandas/tests/window/moments/test_moments_consistency_expanding.py
+++ b/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"]
diff --git a/pandas/tests/window/test_api.py b/pandas/tests/window/test_api.py
index 33fb79d98a324..2c3d8b4608806 100644
--- a/pandas/tests/window/test_api.py
+++ 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(
         {
diff --git a/pandas/tests/window/test_expanding.py b/pandas/tests/window/test_expanding.py
index 30d65ebe84a1f..1c53d72ec719f 100644
--- a/pandas/tests/window/test_expanding.py
+++ b/pandas/tests/window/test_expanding.py
@@ -18,13 +18,10 @@ def test_doc_string():
 
 def test_constructor(which):
     # GH 12669
-
     c = which.expanding
 
     # valid
     c(min_periods=1)
-    c(min_periods=1, center=True)
-    c(min_periods=1, center=False)
 
     # not valid
     for w in [2.0, "foo", np.array([2])]:
@@ -32,10 +29,6 @@ def test_constructor(which):
         with pytest.raises(ValueError, match=msg):
             c(min_periods=w)
 
-        msg = "center must be a boolean"
-        with pytest.raises(ValueError, match=msg):
-            c(min_periods=1, center=w)
-
 
 @pytest.mark.parametrize("method", ["std", "mean", "sum", "max", "min", "var"])
 def test_numpy_compat(method):
@@ -213,3 +206,16 @@ 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()

From c1da8160e7ecd204db5ed5b14139ee78a73d8b5e Mon Sep 17 00:00:00 2001
From: MBrouns <matthijs.brouns@gmail.com>
Date: Tue, 7 Jul 2020 09:31:12 +0200
Subject: [PATCH 2/4] remove filterwarnings from core copying behaviour

---
 pandas/core/base.py             | 18 ++++++------------
 pandas/core/generic.py          | 10 ++++++++++
 pandas/core/groupby/base.py     |  9 ++-------
 pandas/core/window/expanding.py |  8 --------
 4 files changed, 18 insertions(+), 27 deletions(-)

diff --git a/pandas/core/base.py b/pandas/core/base.py
index 2297f8337ca0b..b62ef668df5e1 100644
--- a/pandas/core/base.py
+++ b/pandas/core/base.py
@@ -5,7 +5,6 @@
 import builtins
 import textwrap
 from typing import Any, Dict, FrozenSet, List, Optional, Union
-import warnings
 
 import numpy as np
 
@@ -582,17 +581,12 @@ def _shallow_copy(self, obj, **kwargs):
         """
         return a new object with the replacement attributes
         """
-        with warnings.catch_warnings():
-            warnings.filterwarnings(
-                "ignore",
-                "The `center` argument on `expanding` will be removed in the future",
-            )
-            if isinstance(obj, self._constructor):
-                obj = obj.obj
-            for attr in self._attributes:
-                if attr not in kwargs:
-                    kwargs[attr] = getattr(self, attr)
-            return self._constructor(obj, **kwargs)
+        if isinstance(obj, self._constructor):
+            obj = obj.obj
+        for attr in self._attributes:
+            if attr not in kwargs:
+                kwargs[attr] = getattr(self, attr)
+        return self._constructor(obj, **kwargs)
 
 
 class IndexOpsMixin:
diff --git a/pandas/core/generic.py b/pandas/core/generic.py
index 7e7a5ae92e2a2..ff4632efaeebb 100644
--- a/pandas/core/generic.py
+++ b/pandas/core/generic.py
@@ -10503,6 +10503,16 @@ def rolling(
         @doc(Expanding)
         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=3,
+                )
+            else:
+                center = False
+
             return Expanding(self, min_periods=min_periods, center=center, axis=axis)
 
         cls.expanding = expanding
diff --git a/pandas/core/groupby/base.py b/pandas/core/groupby/base.py
index 7dded2ba258e3..e71b2f94c8014 100644
--- a/pandas/core/groupby/base.py
+++ b/pandas/core/groupby/base.py
@@ -4,7 +4,6 @@
 SeriesGroupBy and the DataFrameGroupBy objects.
 """
 import collections
-import warnings
 
 from pandas.core.dtypes.common import is_list_like, is_scalar
 
@@ -41,12 +40,8 @@ def _gotitem(self, key, ndim, subset=None):
             groupby = self._groupby[key]
         except IndexError:
             groupby = self._groupby
-        with warnings.catch_warnings():
-            warnings.filterwarnings(
-                "ignore",
-                "The `center` argument on `expanding` will be removed in the future",
-            )
-            self = type(self)(subset, groupby=groupby, parent=self, **kwargs)
+
+        self = type(self)(subset, groupby=groupby, parent=self, **kwargs)
         self._reset_cache()
         if subset.ndim == 2:
             if is_scalar(key) and key in subset or is_list_like(key):
diff --git a/pandas/core/window/expanding.py b/pandas/core/window/expanding.py
index 6fcb3b5dcbc2b..af04d24e0fb44 100644
--- a/pandas/core/window/expanding.py
+++ b/pandas/core/window/expanding.py
@@ -59,14 +59,6 @@ class Expanding(_Rolling_and_Expanding):
     _attributes = ["min_periods", "center", "axis"]
 
     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",
-                FutureWarning,
-                stacklevel=3,
-            )
-        else:
-            center = False
         super().__init__(obj=obj, min_periods=min_periods, center=center, axis=axis)
 
     @property

From 31356f2bc30c6263bcce5e09079c648ef1a29f4f Mon Sep 17 00:00:00 2001
From: MBrouns <matthijs.brouns@gmail.com>
Date: Tue, 7 Jul 2020 14:30:29 +0200
Subject: [PATCH 3/4] correct stacklevel on deprecationwarning

---
 pandas/core/generic.py | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/pandas/core/generic.py b/pandas/core/generic.py
index ff4632efaeebb..fa15385fed930 100644
--- a/pandas/core/generic.py
+++ b/pandas/core/generic.py
@@ -10508,7 +10508,7 @@ def expanding(self, min_periods=1, center=None, axis=0):
                     "The `center` argument on `expanding` "
                     "will be removed in the future",
                     FutureWarning,
-                    stacklevel=3,
+                    stacklevel=2,
                 )
             else:
                 center = False

From 33f3fb7a4e292e1044f4a02a513125fe9cdf0c5d Mon Sep 17 00:00:00 2001
From: MBrouns <matthijs.brouns@gmail.com>
Date: Tue, 7 Jul 2020 15:42:42 +0200
Subject: [PATCH 4/4] revert changes to constructor test + remove filterwarning
 in count as that shouldn't throw warnings anymore

---
 pandas/core/window/expanding.py       |  8 +-------
 pandas/tests/window/test_expanding.py | 10 ++++++++++
 2 files changed, 11 insertions(+), 7 deletions(-)

diff --git a/pandas/core/window/expanding.py b/pandas/core/window/expanding.py
index af04d24e0fb44..8267cd4f0971e 100644
--- a/pandas/core/window/expanding.py
+++ b/pandas/core/window/expanding.py
@@ -1,6 +1,5 @@
 from textwrap import dedent
 from typing import Dict, Optional
-import warnings
 
 from pandas.compat.numpy import function as nv
 from pandas.util._decorators import Appender, Substitution, doc
@@ -130,12 +129,7 @@ def aggregate(self, func, *args, **kwargs):
     @Substitution(name="expanding")
     @Appender(_shared_docs["count"])
     def count(self, **kwargs):
-        with warnings.catch_warnings():
-            warnings.filterwarnings(
-                "ignore",
-                "The `center` argument on `expanding` will be removed in the future",
-            )
-            return super().count(**kwargs)
+        return super().count(**kwargs)
 
     @Substitution(name="expanding")
     @Appender(_shared_docs["apply"])
diff --git a/pandas/tests/window/test_expanding.py b/pandas/tests/window/test_expanding.py
index 1c53d72ec719f..146eca07c523e 100644
--- a/pandas/tests/window/test_expanding.py
+++ b/pandas/tests/window/test_expanding.py
@@ -16,12 +16,18 @@ 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
+
     c = which.expanding
 
     # valid
     c(min_periods=1)
+    c(min_periods=1, center=True)
+    c(min_periods=1, center=False)
 
     # not valid
     for w in [2.0, "foo", np.array([2])]:
@@ -29,6 +35,10 @@ def test_constructor(which):
         with pytest.raises(ValueError, match=msg):
             c(min_periods=w)
 
+        msg = "center must be a boolean"
+        with pytest.raises(ValueError, match=msg):
+            c(min_periods=1, center=w)
+
 
 @pytest.mark.parametrize("method", ["std", "mean", "sum", "max", "min", "var"])
 def test_numpy_compat(method):