From 165fd722cb51e53dc7917d2cc62f3ec173983573 Mon Sep 17 00:00:00 2001
From: steveya <steveya@gmail.com>
Date: Mon, 7 Sep 2020 17:50:02 +0800
Subject: [PATCH 01/23] BUG: GH36113

---
 doc/source/whatsnew/v1.2.0.rst     |  2 +-
 pandas/core/reshape/reshape.py     |  2 +-
 pandas/tests/frame/test_reshape.py | 12 ++++++++++++
 3 files changed, 14 insertions(+), 2 deletions(-)

diff --git a/doc/source/whatsnew/v1.2.0.rst b/doc/source/whatsnew/v1.2.0.rst
index 9a778acba4764..d28bbdedec8e8 100644
--- a/doc/source/whatsnew/v1.2.0.rst
+++ b/doc/source/whatsnew/v1.2.0.rst
@@ -322,7 +322,7 @@ Reshaping
 - Bug in :meth:`DataFrame.pivot_table` with ``aggfunc='count'`` or ``aggfunc='sum'`` returning ``NaN`` for missing categories when pivoted on a ``Categorical``. Now returning ``0`` (:issue:`31422`)
 - Bug in :func:`union_indexes` where input index names are not preserved in some cases. Affects :func:`concat` and :class:`DataFrame` constructor (:issue:`13475`)
 - Bug in func :meth:`crosstab` when using multiple columns with ``margins=True`` and ``normalize=True`` (:issue:`35144`)
--
+- Bug in :meth:`DataFrame.stack` for empty DataFrame (:issue:`36113`)
 
 Sparse
 ^^^^^^
diff --git a/pandas/core/reshape/reshape.py b/pandas/core/reshape/reshape.py
index e81dd8f0c735c..ed5de5332c299 100644
--- a/pandas/core/reshape/reshape.py
+++ b/pandas/core/reshape/reshape.py
@@ -517,7 +517,7 @@ def factorize(index):
         # For homogeneous EAs, frame._values will coerce to object. So
         # we concatenate instead.
         dtypes = list(frame.dtypes._values)
-        dtype = dtypes[0]
+        dtype = dtypes[0] if len(dtypes) > 0 else object
 
         if is_extension_array_dtype(dtype):
             arr = dtype.construct_array_type()
diff --git a/pandas/tests/frame/test_reshape.py b/pandas/tests/frame/test_reshape.py
index d80ebaa09b6a8..c2eb6ba5b8eac 100644
--- a/pandas/tests/frame/test_reshape.py
+++ b/pandas/tests/frame/test_reshape.py
@@ -1273,6 +1273,18 @@ def test_stack_timezone_aware_values():
     tm.assert_series_equal(result, expected)
 
 
+def test_stack_empty_frame():
+    tm.assert_series_equal(
+        DataFrame().stack(), Series(index=MultiIndex([[], []], [[], []]))
+    )
+    tm.assert_series_equal(
+        DataFrame().stack(dropna=True), Series(index=MultiIndex([[], []], [[], []]))
+    )
+    tm.assert_frame_equal(
+        DataFrame().stack().unstack(), DataFrame()
+    )
+
+
 def test_unstacking_multi_index_df():
     # see gh-30740
     df = DataFrame(

From e0c1a8d0ee474e67e00338dd7ef262e098422436 Mon Sep 17 00:00:00 2001
From: steveya <steveya@gmail.com>
Date: Mon, 7 Sep 2020 23:30:50 +0800
Subject: [PATCH 02/23] modify tests to avoid deprrecated errors

---
 pandas/tests/frame/test_reshape.py | 8 +++-----
 1 file changed, 3 insertions(+), 5 deletions(-)

diff --git a/pandas/tests/frame/test_reshape.py b/pandas/tests/frame/test_reshape.py
index c2eb6ba5b8eac..4d0a9cd780b56 100644
--- a/pandas/tests/frame/test_reshape.py
+++ b/pandas/tests/frame/test_reshape.py
@@ -1275,14 +1275,12 @@ def test_stack_timezone_aware_values():
 
 def test_stack_empty_frame():
     tm.assert_series_equal(
-        DataFrame().stack(), Series(index=MultiIndex([[], []], [[], []]))
+        DataFrame().stack(), Series(index=MultiIndex([[], []], [[], []]), dtype=object)
     )
     tm.assert_series_equal(
-        DataFrame().stack(dropna=True), Series(index=MultiIndex([[], []], [[], []]))
-    )
-    tm.assert_frame_equal(
-        DataFrame().stack().unstack(), DataFrame()
+        DataFrame().stack(dropna=True), Series(index=MultiIndex([[], []], [[], []]), dtype=object)
     )
+    tm.assert_frame_equal(DataFrame().stack().unstack(), DataFrame())
 
 
 def test_unstacking_multi_index_df():

From f765acf95947d896916dd4294540df3b4687ac23 Mon Sep 17 00:00:00 2001
From: steveya <steveya@gmail.com>
Date: Mon, 7 Sep 2020 23:32:26 +0800
Subject: [PATCH 03/23] PEP 8 compliant

---
 pandas/tests/frame/test_reshape.py | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/pandas/tests/frame/test_reshape.py b/pandas/tests/frame/test_reshape.py
index 4d0a9cd780b56..54f36fd350a3e 100644
--- a/pandas/tests/frame/test_reshape.py
+++ b/pandas/tests/frame/test_reshape.py
@@ -1275,10 +1275,12 @@ def test_stack_timezone_aware_values():
 
 def test_stack_empty_frame():
     tm.assert_series_equal(
-        DataFrame().stack(), Series(index=MultiIndex([[], []], [[], []]), dtype=object)
+        DataFrame().stack(), 
+        Series(index=MultiIndex([[], []], [[], []]), dtype=object)
     )
     tm.assert_series_equal(
-        DataFrame().stack(dropna=True), Series(index=MultiIndex([[], []], [[], []]), dtype=object)
+        DataFrame().stack(dropna=True), 
+        Series(index=MultiIndex([[], []], [[], []]), dtype=object)
     )
     tm.assert_frame_equal(DataFrame().stack().unstack(), DataFrame())
 

From 109d3120ec713c9f5c77704ba2c3a72d10d5671c Mon Sep 17 00:00:00 2001
From: steveya <steveya@gmail.com>
Date: Tue, 8 Sep 2020 00:05:28 +0800
Subject: [PATCH 04/23] remove trailing white space

---
 pandas/tests/frame/test_reshape.py | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/pandas/tests/frame/test_reshape.py b/pandas/tests/frame/test_reshape.py
index 54f36fd350a3e..fa631a8365703 100644
--- a/pandas/tests/frame/test_reshape.py
+++ b/pandas/tests/frame/test_reshape.py
@@ -1275,11 +1275,11 @@ def test_stack_timezone_aware_values():
 
 def test_stack_empty_frame():
     tm.assert_series_equal(
-        DataFrame().stack(), 
+        DataFrame().stack(),
         Series(index=MultiIndex([[], []], [[], []]), dtype=object)
     )
     tm.assert_series_equal(
-        DataFrame().stack(dropna=True), 
+        DataFrame().stack(dropna=True),
         Series(index=MultiIndex([[], []], [[], []]), dtype=object)
     )
     tm.assert_frame_equal(DataFrame().stack().unstack(), DataFrame())

From 519a140564923029c01b011d2a4198de107ec78e Mon Sep 17 00:00:00 2001
From: steveya <steveya@gmail.com>
Date: Tue, 8 Sep 2020 00:47:29 +0800
Subject: [PATCH 05/23] black format checked

---
 pandas/tests/frame/test_reshape.py | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/pandas/tests/frame/test_reshape.py b/pandas/tests/frame/test_reshape.py
index fa631a8365703..95ae3b887a5dc 100644
--- a/pandas/tests/frame/test_reshape.py
+++ b/pandas/tests/frame/test_reshape.py
@@ -1275,12 +1275,11 @@ def test_stack_timezone_aware_values():
 
 def test_stack_empty_frame():
     tm.assert_series_equal(
-        DataFrame().stack(),
-        Series(index=MultiIndex([[], []], [[], []]), dtype=object)
+        DataFrame().stack(), Series(index=MultiIndex([[], []], [[], []]), dtype=object)
     )
     tm.assert_series_equal(
         DataFrame().stack(dropna=True),
-        Series(index=MultiIndex([[], []], [[], []]), dtype=object)
+        Series(index=MultiIndex([[], []], [[], []]), dtype=object),
     )
     tm.assert_frame_equal(DataFrame().stack().unstack(), DataFrame())
 

From 9d20ff53320b0e22a4b1fa40c0d4809ca9cf104a Mon Sep 17 00:00:00 2001
From: steveya <steveya@gmail.com>
Date: Wed, 9 Sep 2020 14:15:19 +0800
Subject: [PATCH 06/23] DataFrame().stack should return an empty Series with
 dtype np.float64 instead of object

---
 pandas/tests/frame/test_reshape.py | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/pandas/tests/frame/test_reshape.py b/pandas/tests/frame/test_reshape.py
index 95ae3b887a5dc..4e4bae5b82c0d 100644
--- a/pandas/tests/frame/test_reshape.py
+++ b/pandas/tests/frame/test_reshape.py
@@ -1275,11 +1275,11 @@ def test_stack_timezone_aware_values():
 
 def test_stack_empty_frame():
     tm.assert_series_equal(
-        DataFrame().stack(), Series(index=MultiIndex([[], []], [[], []]), dtype=object)
+        DataFrame().stack(), Series(index=MultiIndex([[], []], [[], []]), dtype=np.float64)
     )
     tm.assert_series_equal(
         DataFrame().stack(dropna=True),
-        Series(index=MultiIndex([[], []], [[], []]), dtype=object),
+        Series(index=MultiIndex([[], []], [[], []]), dtype=np.float64),
     )
     tm.assert_frame_equal(DataFrame().stack().unstack(), DataFrame())
 

From d460db6da6e3409714e568108e6952da5a7f03ef Mon Sep 17 00:00:00 2001
From: steveya <steveya@gmail.com>
Date: Wed, 9 Sep 2020 14:50:11 +0800
Subject: [PATCH 07/23] PEP8 again.

---
 pandas/tests/frame/test_reshape.py | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/pandas/tests/frame/test_reshape.py b/pandas/tests/frame/test_reshape.py
index 4e4bae5b82c0d..783e02427e0b4 100644
--- a/pandas/tests/frame/test_reshape.py
+++ b/pandas/tests/frame/test_reshape.py
@@ -1275,7 +1275,8 @@ def test_stack_timezone_aware_values():
 
 def test_stack_empty_frame():
     tm.assert_series_equal(
-        DataFrame().stack(), Series(index=MultiIndex([[], []], [[], []]), dtype=np.float64)
+        DataFrame().stack(), 
+        Series(index=MultiIndex([[], []], [[], []]), dtype=np.float64)
     )
     tm.assert_series_equal(
         DataFrame().stack(dropna=True),

From bae2bd8808c96a7d397ec09bc3245a17d25f2314 Mon Sep 17 00:00:00 2001
From: steveya <steveya@gmail.com>
Date: Wed, 9 Sep 2020 17:06:57 +0800
Subject: [PATCH 08/23] remove trailing space...\

---
 pandas/tests/frame/test_reshape.py | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/pandas/tests/frame/test_reshape.py b/pandas/tests/frame/test_reshape.py
index 783e02427e0b4..835180fd9e18e 100644
--- a/pandas/tests/frame/test_reshape.py
+++ b/pandas/tests/frame/test_reshape.py
@@ -1275,7 +1275,7 @@ def test_stack_timezone_aware_values():
 
 def test_stack_empty_frame():
     tm.assert_series_equal(
-        DataFrame().stack(), 
+        DataFrame().stack(),
         Series(index=MultiIndex([[], []], [[], []]), dtype=np.float64)
     )
     tm.assert_series_equal(

From 047ae40f50499d1054ac0370210b01baf3115cf6 Mon Sep 17 00:00:00 2001
From: steveya <steveya@gmail.com>
Date: Wed, 9 Sep 2020 17:44:39 +0800
Subject: [PATCH 09/23] add a comma to pass black lint

---
 pandas/tests/frame/test_reshape.py | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/pandas/tests/frame/test_reshape.py b/pandas/tests/frame/test_reshape.py
index 835180fd9e18e..6b25cbd063828 100644
--- a/pandas/tests/frame/test_reshape.py
+++ b/pandas/tests/frame/test_reshape.py
@@ -1276,7 +1276,7 @@ def test_stack_timezone_aware_values():
 def test_stack_empty_frame():
     tm.assert_series_equal(
         DataFrame().stack(),
-        Series(index=MultiIndex([[], []], [[], []]), dtype=np.float64)
+        Series(index=MultiIndex([[], []], [[], []]), dtype=np.float64),
     )
     tm.assert_series_equal(
         DataFrame().stack(dropna=True),

From c0fffe8c6bc01d192ee30399afe50793b5b5a038 Mon Sep 17 00:00:00 2001
From: steveya <steveya@gmail.com>
Date: Wed, 9 Sep 2020 21:23:36 +0800
Subject: [PATCH 10/23] simply fixes and parameterize tests

---
 pandas/core/reshape/reshape.py     |  4 ++--
 pandas/tests/frame/test_reshape.py | 25 +++++++++++++++----------
 2 files changed, 17 insertions(+), 12 deletions(-)

diff --git a/pandas/core/reshape/reshape.py b/pandas/core/reshape/reshape.py
index ed5de5332c299..3f482064cdea2 100644
--- a/pandas/core/reshape/reshape.py
+++ b/pandas/core/reshape/reshape.py
@@ -513,11 +513,11 @@ def factorize(index):
             verify_integrity=False,
         )
 
-    if frame._is_homogeneous_type:
+    if not frame.empty and frame._is_homogeneous_type:
         # For homogeneous EAs, frame._values will coerce to object. So
         # we concatenate instead.
         dtypes = list(frame.dtypes._values)
-        dtype = dtypes[0] if len(dtypes) > 0 else object
+        dtype = dtypes[0]
 
         if is_extension_array_dtype(dtype):
             arr = dtype.construct_array_type()
diff --git a/pandas/tests/frame/test_reshape.py b/pandas/tests/frame/test_reshape.py
index 6b25cbd063828..e86731e5aa599 100644
--- a/pandas/tests/frame/test_reshape.py
+++ b/pandas/tests/frame/test_reshape.py
@@ -1273,16 +1273,21 @@ def test_stack_timezone_aware_values():
     tm.assert_series_equal(result, expected)
 
 
-def test_stack_empty_frame():
-    tm.assert_series_equal(
-        DataFrame().stack(),
-        Series(index=MultiIndex([[], []], [[], []]), dtype=np.float64),
-    )
-    tm.assert_series_equal(
-        DataFrame().stack(dropna=True),
-        Series(index=MultiIndex([[], []], [[], []]), dtype=np.float64),
-    )
-    tm.assert_frame_equal(DataFrame().stack().unstack(), DataFrame())
+@pytest.mark.parametrize("dropna", [True, False])
+def test_stack_empty_frame(dropna):
+    # GH 36113
+    expected = Series(index=MultiIndex([[], []], [[], []]), dtype=np.float64)
+    result = DataFrame().stack(dropna=dropna)
+    tm.assert_series_equal(result, expected)
+
+
+@pytest.mark.parametrize("dropna", [True, False])
+@pytest.mark.parametrize("fill_value", [None, 0])
+def test_stack_unstack_empty_frame(dropna, fill_value):
+    # GH 36113
+    expected = DataFrame()
+    result = DataFrame().stack(dropna=dropna).unstack(fill_value=fill_value)
+    tm.assert_frame_equal(result, expected)
 
 
 def test_unstacking_multi_index_df():

From 6b2b9bdedafe0fc5ac196fc5d0af2e202e267731 Mon Sep 17 00:00:00 2001
From: steveya <steveya@gmail.com>
Date: Sat, 12 Sep 2020 22:52:14 +0800
Subject: [PATCH 11/23] add error messages when unstack frame and series with
 single level index

---
 pandas/core/reshape/reshape.py | 13 ++++++++++++-
 1 file changed, 12 insertions(+), 1 deletion(-)

diff --git a/pandas/core/reshape/reshape.py b/pandas/core/reshape/reshape.py
index 3f482064cdea2..c71afc28f0325 100644
--- a/pandas/core/reshape/reshape.py
+++ b/pandas/core/reshape/reshape.py
@@ -399,6 +399,12 @@ def _unstack_multiple(data, clocs, fill_value=None):
 
 
 def unstack(obj, level, fill_value=None):
+    # GH 36113
+    # Give nicer error messages when unstack a Index that is not
+    # a MultiIndex.
+    if not isinstance(obj.index, MultiIndex):
+        raise ValueError("index must be a MultiIndex to unstack")
+
     if isinstance(level, (tuple, list)):
         if len(level) != 1:
             # _unstack_multiple only handles MultiIndexes,
@@ -414,8 +420,13 @@ def unstack(obj, level, fill_value=None):
     if isinstance(obj, DataFrame):
         if isinstance(obj.index, MultiIndex):
             return _unstack_frame(obj, level, fill_value=fill_value)
-        else:
+        elif isinstance(obj.columns, MultiIndex):
             return obj.T.stack(dropna=False)
+        else:
+            raise ValueError(
+                "either index or column of a DataFrame need to "
+                "be a MultiIndex to unstack."
+            )
     else:
         if is_extension_array_dtype(obj.dtype):
             return _unstack_extension_series(obj, level, fill_value)

From efc060325b50ec31ee0e1b16a784dcbeb4b7016e Mon Sep 17 00:00:00 2001
From: steveya <steveya@gmail.com>
Date: Sat, 12 Sep 2020 23:25:41 +0800
Subject: [PATCH 12/23] apply ValueError location

---
 pandas/core/reshape/reshape.py | 14 +++++++++-----
 1 file changed, 9 insertions(+), 5 deletions(-)

diff --git a/pandas/core/reshape/reshape.py b/pandas/core/reshape/reshape.py
index c71afc28f0325..80d1633c3b4e2 100644
--- a/pandas/core/reshape/reshape.py
+++ b/pandas/core/reshape/reshape.py
@@ -399,11 +399,6 @@ def _unstack_multiple(data, clocs, fill_value=None):
 
 
 def unstack(obj, level, fill_value=None):
-    # GH 36113
-    # Give nicer error messages when unstack a Index that is not
-    # a MultiIndex.
-    if not isinstance(obj.index, MultiIndex):
-        raise ValueError("index must be a MultiIndex to unstack")
 
     if isinstance(level, (tuple, list)):
         if len(level) != 1:
@@ -423,11 +418,20 @@ def unstack(obj, level, fill_value=None):
         elif isinstance(obj.columns, MultiIndex):
             return obj.T.stack(dropna=False)
         else:
+            # GH 36113
+            # Give nicer error messages when unstack a Index that is not
+            # a MultiIndex.
             raise ValueError(
                 "either index or column of a DataFrame need to "
                 "be a MultiIndex to unstack."
             )
     else:
+        # GH 36113
+        # Give nicer error messages when unstack a Index that is not
+        # a MultiIndex.
+        if not isinstance(obj.index, MultiIndex):
+            raise ValueError("index must be a MultiIndex to unstack")
+
         if is_extension_array_dtype(obj.dtype):
             return _unstack_extension_series(obj, level, fill_value)
         unstacker = _Unstacker(

From dac5f3238c7614307b4454aecad6c107fc790ee4 Mon Sep 17 00:00:00 2001
From: steveya <steveya@gmail.com>
Date: Sun, 13 Sep 2020 18:33:55 +0800
Subject: [PATCH 13/23] change the place where error is raised

---
 pandas/core/reshape/reshape.py | 21 ++++++---------------
 1 file changed, 6 insertions(+), 15 deletions(-)

diff --git a/pandas/core/reshape/reshape.py b/pandas/core/reshape/reshape.py
index 80d1633c3b4e2..80889cccf71d7 100644
--- a/pandas/core/reshape/reshape.py
+++ b/pandas/core/reshape/reshape.py
@@ -415,23 +415,14 @@ def unstack(obj, level, fill_value=None):
     if isinstance(obj, DataFrame):
         if isinstance(obj.index, MultiIndex):
             return _unstack_frame(obj, level, fill_value=fill_value)
-        elif isinstance(obj.columns, MultiIndex):
-            return obj.T.stack(dropna=False)
         else:
-            # GH 36113
-            # Give nicer error messages when unstack a Index that is not
-            # a MultiIndex.
-            raise ValueError(
-                "either index or column of a DataFrame need to "
-                "be a MultiIndex to unstack."
-            )
-    else:
+            return obj.T.stack(dropna=False)
+    elif not isinstance(obj.index, MultiIndex):
         # GH 36113
-        # Give nicer error messages when unstack a Index that is not
-        # a MultiIndex.
-        if not isinstance(obj.index, MultiIndex):
-            raise ValueError("index must be a MultiIndex to unstack")
-
+        # Give nicer error messages when unstack a  Series whose
+        # Index is not a MultiIndex.
+        raise ValueError("index must be a MultiIndex to unstack")
+    else:
         if is_extension_array_dtype(obj.dtype):
             return _unstack_extension_series(obj, level, fill_value)
         unstacker = _Unstacker(

From 6524a6c0286d9ebd3f1edaeb9fc0eaa021c71c65 Mon Sep 17 00:00:00 2001
From: steveya <steveya@gmail.com>
Date: Wed, 16 Sep 2020 17:07:15 +0800
Subject: [PATCH 14/23] add a test for unstack series with one level of index.
 elaborate changes in v1.2.0.rst

---
 doc/source/whatsnew/v1.2.0.rst     | 4 ++--
 pandas/tests/frame/test_reshape.py | 9 ++++++++-
 2 files changed, 10 insertions(+), 3 deletions(-)

diff --git a/doc/source/whatsnew/v1.2.0.rst b/doc/source/whatsnew/v1.2.0.rst
index d28bbdedec8e8..e39b739f1b7ab 100644
--- a/doc/source/whatsnew/v1.2.0.rst
+++ b/doc/source/whatsnew/v1.2.0.rst
@@ -322,8 +322,8 @@ Reshaping
 - Bug in :meth:`DataFrame.pivot_table` with ``aggfunc='count'`` or ``aggfunc='sum'`` returning ``NaN`` for missing categories when pivoted on a ``Categorical``. Now returning ``0`` (:issue:`31422`)
 - Bug in :func:`union_indexes` where input index names are not preserved in some cases. Affects :func:`concat` and :class:`DataFrame` constructor (:issue:`13475`)
 - Bug in func :meth:`crosstab` when using multiple columns with ``margins=True`` and ``normalize=True`` (:issue:`35144`)
-- Bug in :meth:`DataFrame.stack` for empty DataFrame (:issue:`36113`)
-
+- Bug in :meth:`DataFrame.stack` where an empty DataFrame.stack would raise an error (:issue:`36113`). Now returning an empty Series with empty MultiIndex.
+- Bug in :meth:`Series.unstack`. Now a Series with single level of Index trying to unstack would raise a ValueError. (:issue:`36113`)
 Sparse
 ^^^^^^
 
diff --git a/pandas/tests/frame/test_reshape.py b/pandas/tests/frame/test_reshape.py
index e86731e5aa599..9f6a00fbd45d7 100644
--- a/pandas/tests/frame/test_reshape.py
+++ b/pandas/tests/frame/test_reshape.py
@@ -1285,11 +1285,18 @@ def test_stack_empty_frame(dropna):
 @pytest.mark.parametrize("fill_value", [None, 0])
 def test_stack_unstack_empty_frame(dropna, fill_value):
     # GH 36113
-    expected = DataFrame()
     result = DataFrame().stack(dropna=dropna).unstack(fill_value=fill_value)
+    expected = DataFrame()
     tm.assert_frame_equal(result, expected)
 
 
+def test_unstack_single_index_series():
+    # GH 36113
+    msg = "index must be a MultiIndex to unstack"
+    with pytest.raises(ValueError, match=msg):
+        Series().unstack()
+
+
 def test_unstacking_multi_index_df():
     # see gh-30740
     df = DataFrame(

From 6c7110124b1f2cd00bc08dd20ccdb314e5a280c2 Mon Sep 17 00:00:00 2001
From: steveya <steveya@gmail.com>
Date: Tue, 22 Sep 2020 20:30:10 +0800
Subject: [PATCH 15/23] adding type information to exception message.

---
 pandas/core/reshape/reshape.py | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/pandas/core/reshape/reshape.py b/pandas/core/reshape/reshape.py
index 80889cccf71d7..1449db10ecd42 100644
--- a/pandas/core/reshape/reshape.py
+++ b/pandas/core/reshape/reshape.py
@@ -421,7 +421,9 @@ def unstack(obj, level, fill_value=None):
         # GH 36113
         # Give nicer error messages when unstack a  Series whose
         # Index is not a MultiIndex.
-        raise ValueError("index must be a MultiIndex to unstack")
+        raise ValueError(
+            "index must be a MultiIndex to unstack, " f"{type(obj.index)} was passed"
+        )
     else:
         if is_extension_array_dtype(obj.dtype):
             return _unstack_extension_series(obj, level, fill_value)

From f2f29bcd8cfc9eb22e7be4ab0dbaf9721424ddfb Mon Sep 17 00:00:00 2001
From: steveya <steveya@gmail.com>
Date: Sat, 24 Oct 2020 18:27:19 +0800
Subject: [PATCH 16/23] fix black format problem

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

diff --git a/pandas/core/reshape/reshape.py b/pandas/core/reshape/reshape.py
index 260e2f8d7d156..2995eedc66cce 100644
--- a/pandas/core/reshape/reshape.py
+++ b/pandas/core/reshape/reshape.py
@@ -422,7 +422,7 @@ def unstack(obj, level, fill_value=None):
         # Give nicer error messages when unstack a  Series whose
         # Index is not a MultiIndex.
         raise ValueError(
-            "index must be a MultiIndex to unstack, " f"{type(obj.index)} was passed"
+            f"index must be a MultiIndex to unstack, {type(obj.index)} was passed"
         )
     else:
         if is_extension_array_dtype(obj.dtype):

From 148b77d6f44a67d49ec63052e36caf7eb38d2821 Mon Sep 17 00:00:00 2001
From: steveya <steveya@gmail.com>
Date: Thu, 5 Nov 2020 14:24:14 +0800
Subject: [PATCH 17/23] fix unittest assert error message

---
 pandas/tests/frame/test_stack_unstack.py | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/pandas/tests/frame/test_stack_unstack.py b/pandas/tests/frame/test_stack_unstack.py
index d9610a4233717..bc3ea19665ac3 100644
--- a/pandas/tests/frame/test_stack_unstack.py
+++ b/pandas/tests/frame/test_stack_unstack.py
@@ -1194,7 +1194,7 @@ def test_stack_unstack_empty_frame(dropna, fill_value):
 
 def test_unstack_single_index_series():
     # GH 36113
-    msg = "index must be a MultiIndex to unstack"
+    msg = r"index must be a MultiIndex to unstack.*"
     with pytest.raises(ValueError, match=msg):
         Series().unstack()
 

From 99f828009fba37d86fcdbeb80ae8e694672ca2cb Mon Sep 17 00:00:00 2001
From: steveya <steveya@gmail.com>
Date: Thu, 5 Nov 2020 20:34:38 +0800
Subject: [PATCH 18/23] change dtype of empty series and dataframe in test

---
 pandas/tests/frame/test_stack_unstack.py | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/pandas/tests/frame/test_stack_unstack.py b/pandas/tests/frame/test_stack_unstack.py
index bc3ea19665ac3..0da0f4bb2b5c3 100644
--- a/pandas/tests/frame/test_stack_unstack.py
+++ b/pandas/tests/frame/test_stack_unstack.py
@@ -1179,7 +1179,7 @@ def test_stack_timezone_aware_values():
 def test_stack_empty_frame(dropna):
     # GH 36113
     expected = Series(index=MultiIndex([[], []], [[], []]), dtype=np.float64)
-    result = DataFrame().stack(dropna=dropna)
+    result = DataFrame(dtype=np.float64).stack(dropna=dropna)
     tm.assert_series_equal(result, expected)
 
 
@@ -1187,8 +1187,8 @@ def test_stack_empty_frame(dropna):
 @pytest.mark.parametrize("fill_value", [None, 0])
 def test_stack_unstack_empty_frame(dropna, fill_value):
     # GH 36113
-    result = DataFrame().stack(dropna=dropna).unstack(fill_value=fill_value)
-    expected = DataFrame()
+    result = DataFrame(dtype=np.intp).stack(dropna=dropna).unstack(fill_value=fill_value)
+    expected = DataFrame(dtype=np.intp)
     tm.assert_frame_equal(result, expected)
 
 
@@ -1196,7 +1196,7 @@ def test_unstack_single_index_series():
     # GH 36113
     msg = r"index must be a MultiIndex to unstack.*"
     with pytest.raises(ValueError, match=msg):
-        Series().unstack()
+        Series(dtype=np.intp).unstack()
 
 
 def test_unstacking_multi_index_df():

From c4e244a715b01c60eeb337ecadafd559ea38a675 Mon Sep 17 00:00:00 2001
From: steveya <steveya@gmail.com>
Date: Thu, 5 Nov 2020 22:33:21 +0800
Subject: [PATCH 19/23] formatting

---
 pandas/tests/frame/test_stack_unstack.py | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/pandas/tests/frame/test_stack_unstack.py b/pandas/tests/frame/test_stack_unstack.py
index 0da0f4bb2b5c3..22eeb9493154b 100644
--- a/pandas/tests/frame/test_stack_unstack.py
+++ b/pandas/tests/frame/test_stack_unstack.py
@@ -1187,7 +1187,9 @@ def test_stack_empty_frame(dropna):
 @pytest.mark.parametrize("fill_value", [None, 0])
 def test_stack_unstack_empty_frame(dropna, fill_value):
     # GH 36113
-    result = DataFrame(dtype=np.intp).stack(dropna=dropna).unstack(fill_value=fill_value)
+    result = (
+        DataFrame(dtype=np.intp).stack(dropna=dropna).unstack(fill_value=fill_value)
+    )
     expected = DataFrame(dtype=np.intp)
     tm.assert_frame_equal(result, expected)
 

From 668189fa2cd478ceb5f5280ca7236884fc7c2cae Mon Sep 17 00:00:00 2001
From: steveya <steveya@gmail.com>
Date: Fri, 20 Nov 2020 14:19:10 +0800
Subject: [PATCH 20/23] change intp to int64 in testing of stack unstack empty
 frame

---
 pandas/tests/frame/test_stack_unstack.py | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/pandas/tests/frame/test_stack_unstack.py b/pandas/tests/frame/test_stack_unstack.py
index 22eeb9493154b..9544bb44d19df 100644
--- a/pandas/tests/frame/test_stack_unstack.py
+++ b/pandas/tests/frame/test_stack_unstack.py
@@ -1188,9 +1188,9 @@ def test_stack_empty_frame(dropna):
 def test_stack_unstack_empty_frame(dropna, fill_value):
     # GH 36113
     result = (
-        DataFrame(dtype=np.intp).stack(dropna=dropna).unstack(fill_value=fill_value)
+        DataFrame(dtype=np.int64).stack(dropna=dropna).unstack(fill_value=fill_value)
     )
-    expected = DataFrame(dtype=np.intp)
+    expected = DataFrame(dtype=np.int64)
     tm.assert_frame_equal(result, expected)
 
 
@@ -1198,7 +1198,7 @@ def test_unstack_single_index_series():
     # GH 36113
     msg = r"index must be a MultiIndex to unstack.*"
     with pytest.raises(ValueError, match=msg):
-        Series(dtype=np.intp).unstack()
+        Series(dtype=np.int64).unstack()
 
 
 def test_unstacking_multi_index_df():

From 4f95523670304ab7a74068e19c6fa65ba161b5ac Mon Sep 17 00:00:00 2001
From: steveya <steveya@gmail.com>
Date: Sun, 22 Nov 2020 19:34:10 +0800
Subject: [PATCH 21/23] ensure indexer is of type int64

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

diff --git a/pandas/core/sorting.py b/pandas/core/sorting.py
index 2a0da8b0fb35c..729f517c789a7 100644
--- a/pandas/core/sorting.py
+++ b/pandas/core/sorting.py
@@ -610,7 +610,7 @@ def compress_group_index(group_index, sort: bool = True):
     if sort and len(obs_group_ids) > 0:
         obs_group_ids, comp_ids = _reorder_by_uniques(obs_group_ids, comp_ids)
 
-    return comp_ids, obs_group_ids
+    return ensure_int64(comp_ids), ensure_int64(obs_group_ids)
 
 
 def _reorder_by_uniques(uniques, labels):

From 475f158b898f5fc1759375a155b532429f4738bc Mon Sep 17 00:00:00 2001
From: steveya <steveya@gmail.com>
Date: Thu, 26 Nov 2020 17:31:21 +0800
Subject: [PATCH 22/23] remove xfail

---
 pandas/tests/reshape/test_pivot.py | 1 -
 1 file changed, 1 deletion(-)

diff --git a/pandas/tests/reshape/test_pivot.py b/pandas/tests/reshape/test_pivot.py
index 5a28cd5c418f0..800adc41123ed 100644
--- a/pandas/tests/reshape/test_pivot.py
+++ b/pandas/tests/reshape/test_pivot.py
@@ -2102,7 +2102,6 @@ def test_pivot_duplicates(self):
         with pytest.raises(ValueError, match="duplicate entries"):
             data.pivot("a", "b", "c")
 
-    @pytest.mark.xfail(not IS64, reason="GH 36579: fail on 32-bit system")
     def test_pivot_empty(self):
         df = DataFrame(columns=["a", "b", "c"])
         result = df.pivot("a", "b", "c")

From bdf49d3851e75cacb0306e04edf91c2f281c56e9 Mon Sep 17 00:00:00 2001
From: steveya <steveya@gmail.com>
Date: Thu, 26 Nov 2020 18:48:15 +0800
Subject: [PATCH 23/23] remove unsed import

---
 pandas/tests/reshape/test_pivot.py | 2 --
 1 file changed, 2 deletions(-)

diff --git a/pandas/tests/reshape/test_pivot.py b/pandas/tests/reshape/test_pivot.py
index 800adc41123ed..f9b2a02920841 100644
--- a/pandas/tests/reshape/test_pivot.py
+++ b/pandas/tests/reshape/test_pivot.py
@@ -4,8 +4,6 @@
 import numpy as np
 import pytest
 
-from pandas.compat import IS64
-
 import pandas as pd
 from pandas import (
     Categorical,