From a2217e0018141ac5724d1e1ea8dcfeadbc3ff30f Mon Sep 17 00:00:00 2001
From: Tom Augspurger <tom.w.augspurger@gmail.com>
Date: Thu, 5 Dec 2019 14:20:55 -0600
Subject: [PATCH 01/13] BUG: preserve EA dtype in transpose

---
 doc/source/whatsnew/v1.0.0.rst           |  1 +
 pandas/core/generic.py                   | 33 +++++++++++++++++++++---
 pandas/tests/extension/base/reshaping.py | 14 ++++++++++
 pandas/tests/extension/test_numpy.py     |  4 +++
 4 files changed, 48 insertions(+), 4 deletions(-)

diff --git a/doc/source/whatsnew/v1.0.0.rst b/doc/source/whatsnew/v1.0.0.rst
index f3b058c099c14..4143ad12ed17f 100644
--- a/doc/source/whatsnew/v1.0.0.rst
+++ b/doc/source/whatsnew/v1.0.0.rst
@@ -775,6 +775,7 @@ Reshaping
 - Bug where :meth:`DataFrame.equals` returned True incorrectly in some cases when two DataFrames had the same columns in different orders (:issue:`28839`)
 - Bug in :meth:`DataFrame.replace` that caused non-numeric replacer's dtype not respected (:issue:`26632`)
 - Bug in :func:`melt` where supplying mixed strings and numeric values for ``id_vars`` or ``value_vars`` would incorrectly raise a ``ValueError`` (:issue:`29718`)
+- Dtypes are now preserved when transposing a ``DataFrame`` where each column is the same extension dtyep (:issue:``)
 - Bug in :func:`merge_asof` merging on a tz-aware ``left_index`` and ``right_on`` a tz-aware column (:issue:`29864`)
 -
 
diff --git a/pandas/core/generic.py b/pandas/core/generic.py
index e19bf9c1c39ea..8072c5a984e86 100644
--- a/pandas/core/generic.py
+++ b/pandas/core/generic.py
@@ -721,12 +721,37 @@ def transpose(self, *args, **kwargs):
         new_axes = self._construct_axes_dict_from(
             self, [self._get_axis(x) for x in axes_names]
         )
-        new_values = self.values.transpose(axes_numbers)
-        if kwargs.pop("copy", None) or (len(args) and args[-1]):
-            new_values = new_values.copy()
+
+        if (
+            self._is_homogeneous_type
+            and len(self._data.blocks)
+            and is_extension_array_dtype(self._data.blocks[0].dtype)
+        ):
+            kwargs.pop("copy", None)  # by definition, we're copying
+            dtype = self._data.blocks[0].dtype
+            arr_type = dtype.construct_array_type()
+
+            # Slow, but unavoidable with 1D EAs.
+            new_values = []
+            for i in range(len(self)):
+                new_values.append(
+                    arr_type._from_sequence(
+                        [block.values[i] for block in self._data.blocks], dtype=dtype
+                    )
+                )
+            columns = new_axes.pop("columns")
+            new_values = dict(zip(columns, new_values))
+            result = self._constructor(new_values, **new_axes)
+
+        else:
+            new_values = self.values.transpose(axes_numbers)
+            if kwargs.pop("copy", None) or (len(args) and args[-1]):
+                new_values = new_values.copy()
+
+            result = self._constructor(new_values, **new_axes)
 
         nv.validate_transpose(tuple(), kwargs)
-        return self._constructor(new_values, **new_axes).__finalize__(self)
+        return result.__finalize__(self)
 
     def swapaxes(self, axis1, axis2, copy=True):
         """
diff --git a/pandas/tests/extension/base/reshaping.py b/pandas/tests/extension/base/reshaping.py
index 90e607343297d..bb9b21b66dbb5 100644
--- a/pandas/tests/extension/base/reshaping.py
+++ b/pandas/tests/extension/base/reshaping.py
@@ -295,3 +295,17 @@ def test_ravel(self, data):
         # Check that we have a view, not a copy
         result[0] = result[1]
         assert data[0] == data[1]
+
+    def test_transpose(self, data):
+        df = pd.DataFrame({"A": data[:4], "B": data[:4]}, index=["a", "b", "c", "d"])
+        result = df.T
+        expected = pd.DataFrame(
+            {
+                "a": type(data)._from_sequence([data[0]] * 2, dtype=data.dtype),
+                "b": type(data)._from_sequence([data[1]] * 2, dtype=data.dtype),
+                "c": type(data)._from_sequence([data[2]] * 2, dtype=data.dtype),
+                "d": type(data)._from_sequence([data[3]] * 2, dtype=data.dtype),
+            },
+            index=["A", "B"],
+        )
+        self.assert_frame_equal(result, expected)
diff --git a/pandas/tests/extension/test_numpy.py b/pandas/tests/extension/test_numpy.py
index 221cf0787d839..95ba2fc8f00f2 100644
--- a/pandas/tests/extension/test_numpy.py
+++ b/pandas/tests/extension/test_numpy.py
@@ -330,6 +330,10 @@ def test_merge_on_extension_array_duplicates(self, data):
         # Fails creating expected
         super().test_merge_on_extension_array_duplicates(data)
 
+    @skip_nested
+    def test_transpose(self, data):
+        super().test_transpose(data)
+
 
 class TestSetitem(BaseNumPyTests, base.BaseSetitemTests):
     @skip_nested

From 4fb44c535ac25623760569f43a990d036952c610 Mon Sep 17 00:00:00 2001
From: Tom Augspurger <tom.w.augspurger@gmail.com>
Date: Thu, 5 Dec 2019 15:32:50 -0600
Subject: [PATCH 02/13] fix typo

---
 doc/source/whatsnew/v1.0.0.rst | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/doc/source/whatsnew/v1.0.0.rst b/doc/source/whatsnew/v1.0.0.rst
index 4143ad12ed17f..3dba0abaf7297 100644
--- a/doc/source/whatsnew/v1.0.0.rst
+++ b/doc/source/whatsnew/v1.0.0.rst
@@ -775,7 +775,7 @@ Reshaping
 - Bug where :meth:`DataFrame.equals` returned True incorrectly in some cases when two DataFrames had the same columns in different orders (:issue:`28839`)
 - Bug in :meth:`DataFrame.replace` that caused non-numeric replacer's dtype not respected (:issue:`26632`)
 - Bug in :func:`melt` where supplying mixed strings and numeric values for ``id_vars`` or ``value_vars`` would incorrectly raise a ``ValueError`` (:issue:`29718`)
-- Dtypes are now preserved when transposing a ``DataFrame`` where each column is the same extension dtyep (:issue:``)
+- Dtypes are now preserved when transposing a ``DataFrame`` where each column is the same extension dtype (:issue:`30091`)
 - Bug in :func:`merge_asof` merging on a tz-aware ``left_index`` and ``right_on`` a tz-aware column (:issue:`29864`)
 -
 

From e18a42607acc0f4040d87fdef1cf4741a9078508 Mon Sep 17 00:00:00 2001
From: Tom Augspurger <tom.w.augspurger@gmail.com>
Date: Thu, 5 Dec 2019 15:32:58 -0600
Subject: [PATCH 03/13] remove xpass

---
 pandas/tests/arithmetic/conftest.py    | 19 -------------------
 pandas/tests/arithmetic/test_period.py | 12 +++++-------
 2 files changed, 5 insertions(+), 26 deletions(-)

diff --git a/pandas/tests/arithmetic/conftest.py b/pandas/tests/arithmetic/conftest.py
index 1f8fdfd671856..64588af3e3053 100644
--- a/pandas/tests/arithmetic/conftest.py
+++ b/pandas/tests/arithmetic/conftest.py
@@ -235,25 +235,6 @@ def box_df_fail(request):
     return request.param
 
 
-@pytest.fixture(
-    params=[
-        (pd.Index, False),
-        (pd.Series, False),
-        (pd.DataFrame, False),
-        pytest.param((pd.DataFrame, True), marks=pytest.mark.xfail),
-        (tm.to_array, False),
-    ],
-    ids=id_func,
-)
-def box_transpose_fail(request):
-    """
-    Fixture similar to `box` but testing both transpose cases for DataFrame,
-    with the tranpose=True case xfailed.
-    """
-    # GH#23620
-    return request.param
-
-
 @pytest.fixture(params=[pd.Index, pd.Series, pd.DataFrame, tm.to_array], ids=id_func)
 def box_with_array(request):
     """
diff --git a/pandas/tests/arithmetic/test_period.py b/pandas/tests/arithmetic/test_period.py
index ed693d873efb8..ef95402c6215b 100644
--- a/pandas/tests/arithmetic/test_period.py
+++ b/pandas/tests/arithmetic/test_period.py
@@ -755,10 +755,10 @@ def test_pi_sub_isub_offset(self):
         rng -= pd.offsets.MonthEnd(5)
         tm.assert_index_equal(rng, expected)
 
-    def test_pi_add_offset_n_gt1(self, box_transpose_fail):
+    @pytest.mark.parametrize("transpose", [True, False])
+    def test_pi_add_offset_n_gt1(self, box, transpose):
         # GH#23215
         # add offset to PeriodIndex with freq.n > 1
-        box, transpose = box_transpose_fail
 
         per = pd.Period("2016-01", freq="2M")
         pi = pd.PeriodIndex([per])
@@ -984,10 +984,9 @@ def test_pi_add_sub_timedeltalike_freq_mismatch_monthly(self, mismatched_freq):
         with pytest.raises(IncompatibleFrequency, match=msg):
             rng -= other
 
-    def test_parr_add_sub_td64_nat(self, box_transpose_fail):
+    @pytest.mark.parametrize("transpose", [True, False])
+    def test_parr_add_sub_td64_nat(self, box, transpose):
         # GH#23320 special handling for timedelta64("NaT")
-        box, transpose = box_transpose_fail
-
         pi = pd.period_range("1994-04-01", periods=9, freq="19D")
         other = np.timedelta64("NaT")
         expected = pd.PeriodIndex(["NaT"] * 9, freq="19D")
@@ -1011,10 +1010,9 @@ def test_parr_add_sub_td64_nat(self, box_transpose_fail):
             TimedeltaArray._from_sequence(["NaT"] * 9),
         ],
     )
-    def test_parr_add_sub_tdt64_nat_array(self, box_df_fail, other):
+    def test_parr_add_sub_tdt64_nat_array(self, box, other):
         # FIXME: DataFrame fails because when when operating column-wise
         #  timedelta64 entries become NaT and are treated like datetimes
-        box = box_df_fail
 
         pi = pd.period_range("1994-04-01", periods=9, freq="19D")
         expected = pd.PeriodIndex(["NaT"] * 9, freq="19D")

From 9be80b7436479eef98931811deaf22ea1d3d93a8 Mon Sep 17 00:00:00 2001
From: Tom Augspurger <tom.w.augspurger@gmail.com>
Date: Fri, 6 Dec 2019 11:44:39 -0600
Subject: [PATCH 04/13] simplify

---
 pandas/core/frame.py           | 31 +++++++++++++--
 pandas/core/generic.py         | 69 ----------------------------------
 pandas/core/reshape/reshape.py |  4 ++
 3 files changed, 32 insertions(+), 72 deletions(-)

diff --git a/pandas/core/frame.py b/pandas/core/frame.py
index fde3d1657b4f2..0278f900d0c8c 100644
--- a/pandas/core/frame.py
+++ b/pandas/core/frame.py
@@ -2501,7 +2501,7 @@ def memory_usage(self, index=True, deep=False):
             )
         return result
 
-    def transpose(self, *args, **kwargs):
+    def transpose(self, *args, copy=False, **kwargs):
         """
         Transpose index and columns.
 
@@ -2593,8 +2593,33 @@ def transpose(self, *args, **kwargs):
         1    object
         dtype: object
         """
-        nv.validate_transpose(args, dict())
-        return super().transpose(1, 0, **kwargs)
+        if args == () or args == (None,):
+            args = (1, 0)
+        else:
+            raise ValueError(
+                "the 'axes' parameter is not supported in pandas.DataFrame.transpose"
+            )
+        # construct the args
+
+        if self._is_homogeneous_type and is_extension_array_dtype(self.iloc[:, 0]):
+            dtype = self.dtypes.iloc[0]
+            arr_type = dtype.construct_array_type()
+            values = self.values
+
+            new_values = [arr_type._from_sequence(row, dtype=dtype) for row in values]
+            result = self._constructor(
+                dict(zip(self.index, new_values)), index=self.columns
+            )
+
+        else:
+            new_values = self.values.transpose(args)
+            if copy:
+                new_values = new_values.copy()
+            result = self._constructor(
+                new_values, index=self.columns, columns=self.index
+            )
+
+        return result.__finalize__(self)
 
     T = property(transpose)
 
diff --git a/pandas/core/generic.py b/pandas/core/generic.py
index 8072c5a984e86..808eff45e88a4 100644
--- a/pandas/core/generic.py
+++ b/pandas/core/generic.py
@@ -684,75 +684,6 @@ def _set_axis(self, axis, labels):
         self._data.set_axis(axis, labels)
         self._clear_item_cache()
 
-    def transpose(self, *args, **kwargs):
-        """
-        Permute the dimensions of the %(klass)s
-
-        Parameters
-        ----------
-        args : %(args_transpose)s
-        copy : bool, default False
-            Make a copy of the underlying data. Mixed-dtype data will
-            always result in a copy
-        **kwargs
-            Additional keyword arguments will be passed to the function.
-
-        Returns
-        -------
-        y : same as input
-
-        Examples
-        --------
-        >>> p.transpose(2, 0, 1)
-        >>> p.transpose(2, 0, 1, copy=True)
-        """
-
-        # construct the args
-        axes, kwargs = self._construct_axes_from_arguments(
-            args, kwargs, require_all=True
-        )
-        axes_names = tuple(self._get_axis_name(axes[a]) for a in self._AXIS_ORDERS)
-        axes_numbers = tuple(self._get_axis_number(axes[a]) for a in self._AXIS_ORDERS)
-
-        # we must have unique axes
-        if len(axes) != len(set(axes)):
-            raise ValueError(f"Must specify {self._AXIS_LEN} unique axes")
-
-        new_axes = self._construct_axes_dict_from(
-            self, [self._get_axis(x) for x in axes_names]
-        )
-
-        if (
-            self._is_homogeneous_type
-            and len(self._data.blocks)
-            and is_extension_array_dtype(self._data.blocks[0].dtype)
-        ):
-            kwargs.pop("copy", None)  # by definition, we're copying
-            dtype = self._data.blocks[0].dtype
-            arr_type = dtype.construct_array_type()
-
-            # Slow, but unavoidable with 1D EAs.
-            new_values = []
-            for i in range(len(self)):
-                new_values.append(
-                    arr_type._from_sequence(
-                        [block.values[i] for block in self._data.blocks], dtype=dtype
-                    )
-                )
-            columns = new_axes.pop("columns")
-            new_values = dict(zip(columns, new_values))
-            result = self._constructor(new_values, **new_axes)
-
-        else:
-            new_values = self.values.transpose(axes_numbers)
-            if kwargs.pop("copy", None) or (len(args) and args[-1]):
-                new_values = new_values.copy()
-
-            result = self._constructor(new_values, **new_axes)
-
-        nv.validate_transpose(tuple(), kwargs)
-        return result.__finalize__(self)
-
     def swapaxes(self, axis1, axis2, copy=True):
         """
         Interchange axes and swap values axes appropriately.
diff --git a/pandas/core/reshape/reshape.py b/pandas/core/reshape/reshape.py
index a8dcc995e48da..85d4cc33d1a1a 100644
--- a/pandas/core/reshape/reshape.py
+++ b/pandas/core/reshape/reshape.py
@@ -491,6 +491,10 @@ def _unstack_extension_series(series, level, fill_value):
     return concat(out, axis="columns", copy=False, keys=result.columns)
 
 
+def _transpose_extension_arrays(df):
+    pass
+
+
 def stack(frame, level=-1, dropna=True):
     """
     Convert DataFrame to Series with multi-level Index. Columns become the

From bfdfccf2304255c95c7d8169ba18ae2d026c444b Mon Sep 17 00:00:00 2001
From: Tom Augspurger <tom.w.augspurger@gmail.com>
Date: Fri, 6 Dec 2019 12:04:38 -0600
Subject: [PATCH 05/13] simplify

---
 pandas/core/frame.py                     | 1 +
 pandas/tests/extension/base/reshaping.py | 2 ++
 pandas/tests/extension/json/test_json.py | 4 ++++
 3 files changed, 7 insertions(+)

diff --git a/pandas/core/frame.py b/pandas/core/frame.py
index 10aa32335ddcc..66faab9df421a 100644
--- a/pandas/core/frame.py
+++ b/pandas/core/frame.py
@@ -2595,6 +2595,7 @@ def transpose(self, *args, copy=False, **kwargs):
         1    object
         dtype: object
         """
+        nv.validate_transpose(args, dict())
         if args == () or args == (None,):
             args = (1, 0)
         else:
diff --git a/pandas/tests/extension/base/reshaping.py b/pandas/tests/extension/base/reshaping.py
index bb9b21b66dbb5..c38ed020afb95 100644
--- a/pandas/tests/extension/base/reshaping.py
+++ b/pandas/tests/extension/base/reshaping.py
@@ -309,3 +309,5 @@ def test_transpose(self, data):
             index=["A", "B"],
         )
         self.assert_frame_equal(result, expected)
+
+        self.assert_frame_equal(np.transpose(np.transpose(expected)), expected)
diff --git a/pandas/tests/extension/json/test_json.py b/pandas/tests/extension/json/test_json.py
index 7e027a65eec3a..51f913b741a81 100644
--- a/pandas/tests/extension/json/test_json.py
+++ b/pandas/tests/extension/json/test_json.py
@@ -162,6 +162,10 @@ def test_unstack(self, data, index):
         # this matches otherwise
         return super().test_unstack(data, index)
 
+    @pytest.mark.skip(reason="Inconsistent sizes.")
+    def test_transpose(self, data):
+        super().test_transpose(data)
+
 
 class TestGetitem(BaseJSON, base.BaseGetitemTests):
     pass

From 10d81bd018d2008b5bdf774b89696b09a01cec71 Mon Sep 17 00:00:00 2001
From: Tom Augspurger <tom.w.augspurger@gmail.com>
Date: Fri, 6 Dec 2019 12:06:09 -0600
Subject: [PATCH 06/13] steal tests

---
 pandas/tests/frame/test_operators.py | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)

diff --git a/pandas/tests/frame/test_operators.py b/pandas/tests/frame/test_operators.py
index f3e61dffb500d..4ca1ee50e97b3 100644
--- a/pandas/tests/frame/test_operators.py
+++ b/pandas/tests/frame/test_operators.py
@@ -891,6 +891,22 @@ def test_no_warning(self, all_arithmetic_operators):
 
 
 class TestTranspose:
+    @pytest.mark.parametrize(
+        "ser",
+        [
+            pd.date_range("2016-04-05 04:30", periods=3, tz="UTC"),
+            pd.period_range("1994", freq="A", periods=3),
+            pd.period_range("1969", freq="9s", periods=1),
+            pd.date_range("2016-04-05 04:30", periods=3).astype("category"),
+            pd.date_range("2016-04-05 04:30", periods=3, tz="UTC").astype("category"),
+        ],
+    )
+    def test_transpose_retains_extension_dtype(self, ser):
+        # case with more than 1 column, must have same dtype
+        df = pd.DataFrame({"a": ser, "b": ser})
+        result = df.T
+        assert (result.dtypes == ser.dtype).all()
+
     def test_transpose_tzaware_1col_single_tz(self):
         # GH#26825
         dti = pd.date_range("2016-04-05 04:30", periods=3, tz="UTC")

From 132472d46748e8bf52a955e471267942515283fa Mon Sep 17 00:00:00 2001
From: Tom Augspurger <tom.w.augspurger@gmail.com>
Date: Fri, 6 Dec 2019 12:11:28 -0600
Subject: [PATCH 07/13] update docs

---
 pandas/core/frame.py | 21 ++++++++++-----------
 1 file changed, 10 insertions(+), 11 deletions(-)

diff --git a/pandas/core/frame.py b/pandas/core/frame.py
index 66faab9df421a..5c373ec2b5fab 100644
--- a/pandas/core/frame.py
+++ b/pandas/core/frame.py
@@ -2503,7 +2503,7 @@ def memory_usage(self, index=True, deep=False):
             )
         return result
 
-    def transpose(self, *args, copy=False, **kwargs):
+    def transpose(self, *args, copy=False):
         """
         Transpose index and columns.
 
@@ -2513,9 +2513,14 @@ def transpose(self, *args, copy=False, **kwargs):
 
         Parameters
         ----------
-        *args, **kwargs
-            Additional arguments and keywords have no effect but might be
-            accepted for compatibility with numpy.
+        *args : tuple, optional
+            Accepted for compatibility with NumPy.
+        copy : bool, default False
+            Whether to copy the data after transposing, even for DataFrames
+            with a single dtype.
+
+            Note that a copy is always required for mixed dtype DataFrames,
+            or for DataFrames with any extension types.
 
         Returns
         -------
@@ -2596,12 +2601,6 @@ def transpose(self, *args, copy=False, **kwargs):
         dtype: object
         """
         nv.validate_transpose(args, dict())
-        if args == () or args == (None,):
-            args = (1, 0)
-        else:
-            raise ValueError(
-                "the 'axes' parameter is not supported in pandas.DataFrame.transpose"
-            )
         # construct the args
 
         if self._is_homogeneous_type and is_extension_array_dtype(self.iloc[:, 0]):
@@ -2615,7 +2614,7 @@ def transpose(self, *args, copy=False, **kwargs):
             )
 
         else:
-            new_values = self.values.transpose(args)
+            new_values = self.values.T
             if copy:
                 new_values = new_values.copy()
             result = self._constructor(

From 6aae8d69fa26ed1aa657127daae61ad75fd2ada1 Mon Sep 17 00:00:00 2001
From: Tom Augspurger <tom.w.augspurger@gmail.com>
Date: Fri, 6 Dec 2019 13:17:59 -0600
Subject: [PATCH 08/13] fixup

---
 pandas/core/frame.py                     | 8 +++++---
 pandas/tests/extension/base/reshaping.py | 4 ++--
 2 files changed, 7 insertions(+), 5 deletions(-)

diff --git a/pandas/core/frame.py b/pandas/core/frame.py
index 5c373ec2b5fab..ea68f41ca348f 100644
--- a/pandas/core/frame.py
+++ b/pandas/core/frame.py
@@ -2503,7 +2503,7 @@ def memory_usage(self, index=True, deep=False):
             )
         return result
 
-    def transpose(self, *args, copy=False):
+    def transpose(self, *args, copy: bool = False):
         """
         Transpose index and columns.
 
@@ -2603,8 +2603,10 @@ def transpose(self, *args, copy=False):
         nv.validate_transpose(args, dict())
         # construct the args
 
-        if self._is_homogeneous_type and is_extension_array_dtype(self.iloc[:, 0]):
-            dtype = self.dtypes.iloc[0]
+        dtypes = list(self.dtypes)
+        if self._is_homogeneous_type and is_extension_array_dtype(dtypes[0]):
+            # is_homogeneous_type implies that we have at least one column.
+            dtype = dtypes[0]
             arr_type = dtype.construct_array_type()
             values = self.values
 
diff --git a/pandas/tests/extension/base/reshaping.py b/pandas/tests/extension/base/reshaping.py
index c38ed020afb95..89c9ed3674a66 100644
--- a/pandas/tests/extension/base/reshaping.py
+++ b/pandas/tests/extension/base/reshaping.py
@@ -309,5 +309,5 @@ def test_transpose(self, data):
             index=["A", "B"],
         )
         self.assert_frame_equal(result, expected)
-
-        self.assert_frame_equal(np.transpose(np.transpose(expected)), expected)
+        self.assert_frame_equal(np.transpose(np.transpose(df)), df)
+        self.assert_frame_equal(np.transpose(np.transpose(df[["A"]])), df[["A"]])

From 9d703a29bfe483db8ba6583568a256ee44e56eed Mon Sep 17 00:00:00 2001
From: Tom Augspurger <tom.w.augspurger@gmail.com>
Date: Fri, 6 Dec 2019 15:33:48 -0600
Subject: [PATCH 09/13] empty

---
 pandas/core/frame.py | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/pandas/core/frame.py b/pandas/core/frame.py
index 648711bc95f7c..44ab039e7853b 100644
--- a/pandas/core/frame.py
+++ b/pandas/core/frame.py
@@ -2595,8 +2595,7 @@ def transpose(self, *args, copy: bool = False):
         # construct the args
 
         dtypes = list(self.dtypes)
-        if self._is_homogeneous_type and is_extension_array_dtype(dtypes[0]):
-            # is_homogeneous_type implies that we have at least one column.
+        if self._is_homogeneous_type and dtypes and is_extension_array_dtype(dtypes[0]):
             dtype = dtypes[0]
             arr_type = dtype.construct_array_type()
             values = self.values

From feecee8dc60d661cd26e19c3b4e46e85b40fb774 Mon Sep 17 00:00:00 2001
From: Tom Augspurger <tom.w.augspurger@gmail.com>
Date: Thu, 12 Dec 2019 08:02:54 -0600
Subject: [PATCH 10/13] box

---
 pandas/core/frame.py                   |  1 +
 pandas/core/reshape/reshape.py         |  4 ----
 pandas/tests/arithmetic/test_period.py | 18 +++++++++---------
 3 files changed, 10 insertions(+), 13 deletions(-)

diff --git a/pandas/core/frame.py b/pandas/core/frame.py
index 2dfbbac3f4492..889cfd43ea582 100644
--- a/pandas/core/frame.py
+++ b/pandas/core/frame.py
@@ -2595,6 +2595,7 @@ def transpose(self, *args, copy: bool = False):
 
         dtypes = list(self.dtypes)
         if self._is_homogeneous_type and dtypes and is_extension_array_dtype(dtypes[0]):
+            # We have EAs with the same dtype. We can preserve that dtype in transpose.
             dtype = dtypes[0]
             arr_type = dtype.construct_array_type()
             values = self.values
diff --git a/pandas/core/reshape/reshape.py b/pandas/core/reshape/reshape.py
index 85d4cc33d1a1a..a8dcc995e48da 100644
--- a/pandas/core/reshape/reshape.py
+++ b/pandas/core/reshape/reshape.py
@@ -491,10 +491,6 @@ def _unstack_extension_series(series, level, fill_value):
     return concat(out, axis="columns", copy=False, keys=result.columns)
 
 
-def _transpose_extension_arrays(df):
-    pass
-
-
 def stack(frame, level=-1, dropna=True):
     """
     Convert DataFrame to Series with multi-level Index. Columns become the
diff --git a/pandas/tests/arithmetic/test_period.py b/pandas/tests/arithmetic/test_period.py
index ef95402c6215b..f9fc766c29f8f 100644
--- a/pandas/tests/arithmetic/test_period.py
+++ b/pandas/tests/arithmetic/test_period.py
@@ -756,7 +756,7 @@ def test_pi_sub_isub_offset(self):
         tm.assert_index_equal(rng, expected)
 
     @pytest.mark.parametrize("transpose", [True, False])
-    def test_pi_add_offset_n_gt1(self, box, transpose):
+    def test_pi_add_offset_n_gt1(self, box_with_array, transpose):
         # GH#23215
         # add offset to PeriodIndex with freq.n > 1
 
@@ -765,8 +765,8 @@ def test_pi_add_offset_n_gt1(self, box, transpose):
 
         expected = pd.PeriodIndex(["2016-03"], freq="2M")
 
-        pi = tm.box_expected(pi, box, transpose=transpose)
-        expected = tm.box_expected(expected, box, transpose=transpose)
+        pi = tm.box_expected(pi, box_with_array, transpose=transpose)
+        expected = tm.box_expected(expected, box_with_array, transpose=transpose)
 
         result = pi + per.freq
         tm.assert_equal(result, expected)
@@ -985,14 +985,14 @@ def test_pi_add_sub_timedeltalike_freq_mismatch_monthly(self, mismatched_freq):
             rng -= other
 
     @pytest.mark.parametrize("transpose", [True, False])
-    def test_parr_add_sub_td64_nat(self, box, transpose):
+    def test_parr_add_sub_td64_nat(self, box_with_array, transpose):
         # GH#23320 special handling for timedelta64("NaT")
         pi = pd.period_range("1994-04-01", periods=9, freq="19D")
         other = np.timedelta64("NaT")
         expected = pd.PeriodIndex(["NaT"] * 9, freq="19D")
 
-        obj = tm.box_expected(pi, box, transpose=transpose)
-        expected = tm.box_expected(expected, box, transpose=transpose)
+        obj = tm.box_expected(pi, box_with_array, transpose=transpose)
+        expected = tm.box_expected(expected, box_with_array, transpose=transpose)
 
         result = obj + other
         tm.assert_equal(result, expected)
@@ -1010,15 +1010,15 @@ def test_parr_add_sub_td64_nat(self, box, transpose):
             TimedeltaArray._from_sequence(["NaT"] * 9),
         ],
     )
-    def test_parr_add_sub_tdt64_nat_array(self, box, other):
+    def test_parr_add_sub_tdt64_nat_array(self, box_with_array, other):
         # FIXME: DataFrame fails because when when operating column-wise
         #  timedelta64 entries become NaT and are treated like datetimes
 
         pi = pd.period_range("1994-04-01", periods=9, freq="19D")
         expected = pd.PeriodIndex(["NaT"] * 9, freq="19D")
 
-        obj = tm.box_expected(pi, box)
-        expected = tm.box_expected(expected, box)
+        obj = tm.box_expected(pi, box_with_array)
+        expected = tm.box_expected(expected, box_with_array)
 
         result = obj + other
         tm.assert_equal(result, expected)

From 9abd6c279b5002595370cc51f892309f718f41aa Mon Sep 17 00:00:00 2001
From: Tom Augspurger <tom.w.augspurger@gmail.com>
Date: Fri, 20 Dec 2019 06:49:43 -0600
Subject: [PATCH 11/13] update test.

---
 pandas/tests/arithmetic/test_period.py   | 3 ---
 pandas/tests/extension/json/test_json.py | 2 +-
 pandas/tests/frame/test_operators.py     | 9 ++++++---
 3 files changed, 7 insertions(+), 7 deletions(-)

diff --git a/pandas/tests/arithmetic/test_period.py b/pandas/tests/arithmetic/test_period.py
index f9fc766c29f8f..621c759cb550f 100644
--- a/pandas/tests/arithmetic/test_period.py
+++ b/pandas/tests/arithmetic/test_period.py
@@ -1011,9 +1011,6 @@ def test_parr_add_sub_td64_nat(self, box_with_array, transpose):
         ],
     )
     def test_parr_add_sub_tdt64_nat_array(self, box_with_array, other):
-        # FIXME: DataFrame fails because when when operating column-wise
-        #  timedelta64 entries become NaT and are treated like datetimes
-
         pi = pd.period_range("1994-04-01", periods=9, freq="19D")
         expected = pd.PeriodIndex(["NaT"] * 9, freq="19D")
 
diff --git a/pandas/tests/extension/json/test_json.py b/pandas/tests/extension/json/test_json.py
index 51f913b741a81..476dc4f1e6fa1 100644
--- a/pandas/tests/extension/json/test_json.py
+++ b/pandas/tests/extension/json/test_json.py
@@ -162,7 +162,7 @@ def test_unstack(self, data, index):
         # this matches otherwise
         return super().test_unstack(data, index)
 
-    @pytest.mark.skip(reason="Inconsistent sizes.")
+    @pytest.mark.xfail(reason="Inconsistent sizes.")
     def test_transpose(self, data):
         super().test_transpose(data)
 
diff --git a/pandas/tests/frame/test_operators.py b/pandas/tests/frame/test_operators.py
index 4ca1ee50e97b3..137a284931725 100644
--- a/pandas/tests/frame/test_operators.py
+++ b/pandas/tests/frame/test_operators.py
@@ -897,15 +897,18 @@ class TestTranspose:
             pd.date_range("2016-04-05 04:30", periods=3, tz="UTC"),
             pd.period_range("1994", freq="A", periods=3),
             pd.period_range("1969", freq="9s", periods=1),
-            pd.date_range("2016-04-05 04:30", periods=3).astype("category"),
+            pytest.param(
+                pd.date_range("2016-04-05 04:30", periods=3).astype("category"),
+                marks=pytest.mark.xfail(reason="buggy"),
+            ),
             pd.date_range("2016-04-05 04:30", periods=3, tz="UTC").astype("category"),
         ],
     )
     def test_transpose_retains_extension_dtype(self, ser):
         # case with more than 1 column, must have same dtype
         df = pd.DataFrame({"a": ser, "b": ser})
-        result = df.T
-        assert (result.dtypes == ser.dtype).all()
+        result = df.T.T
+        tm.assert_frame_equal(result, df)
 
     def test_transpose_tzaware_1col_single_tz(self):
         # GH#26825

From f6b3c37ed9f54cd217972d4720164dce79d6ef4f Mon Sep 17 00:00:00 2001
From: Tom Augspurger <tom.w.augspurger@gmail.com>
Date: Fri, 20 Dec 2019 08:13:06 -0600
Subject: [PATCH 12/13] filter

---
 pandas/tests/frame/test_operators.py | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/pandas/tests/frame/test_operators.py b/pandas/tests/frame/test_operators.py
index 137a284931725..5416a33c2a222 100644
--- a/pandas/tests/frame/test_operators.py
+++ b/pandas/tests/frame/test_operators.py
@@ -1,5 +1,6 @@
 from decimal import Decimal
 import operator
+import warnings
 
 import numpy as np
 import pytest
@@ -908,7 +909,14 @@ def test_transpose_retains_extension_dtype(self, ser):
         # case with more than 1 column, must have same dtype
         df = pd.DataFrame({"a": ser, "b": ser})
         result = df.T.T
-        tm.assert_frame_equal(result, df)
+        # For Categorical[datetime64[ns, tz]], we have a warning that the
+        # dtype will change to preserve tz in the future. We don't care,
+        # since happeneing to both sides here.
+        with warnings.catch_warnings():
+            warnings.filterwarnings(
+                "ignore", "Converting timezone-aware", FutureWarning
+            )
+            tm.assert_frame_equal(result, df)
 
     def test_transpose_tzaware_1col_single_tz(self):
         # GH#26825

From 6d9daa8b676b6556dfa5c57058b156220afb312a Mon Sep 17 00:00:00 2001
From: jbrockmendel <jbrockmendel@gmail.com>
Date: Thu, 26 Dec 2019 17:11:04 -0800
Subject: [PATCH 13/13] fixup unused import

---
 pandas/tests/frame/test_operators.py | 1 -
 1 file changed, 1 deletion(-)

diff --git a/pandas/tests/frame/test_operators.py b/pandas/tests/frame/test_operators.py
index aa0c8e4ad640b..a4f1c0688b144 100644
--- a/pandas/tests/frame/test_operators.py
+++ b/pandas/tests/frame/test_operators.py
@@ -1,6 +1,5 @@
 from decimal import Decimal
 import operator
-import warnings
 
 import numpy as np
 import pytest