BUG: retain extension dtypes in transpose

pandas/core/generic.py

@@ -725,7 +725,17 @@ def transpose(self, *args, **kwargs):
             new_values = new_values.copy()
 
         nv.validate_transpose(tuple(), kwargs)
-        return self._constructor(new_values, **new_axes).__finalize__(self)
+        result = self._constructor(new_values, **new_axes).__finalize__(self)
+
+        if self.ndim == 2 and self._is_homogeneous_type and len(self.columns):
+            if is_extension_array_dtype(self.dtypes.iloc[0]):
+                # Retain ExtensionArray dtypes through transpose;
+                # TODO: this can be made cleaner if/when (N, 1) EA are allowed
+                dtype = self.dtypes.iloc[0]
+                for col in result.columns:
+                    result[col] = result[col].astype(dtype)
+
+        return result
 
     def swapaxes(self, axis1, axis2, copy=True):
         """

pandas/tests/arithmetic/test_datetime64.py

@@ -153,12 +153,10 @@ def test_dt64arr_nat_comparison(self, tz_naive_fixture, box_with_array):
         ts = pd.Timestamp.now(tz)
         ser = pd.Series([ts, pd.NaT])
 
-        # FIXME: Can't transpose because that loses the tz dtype on
-        #  the NaT column
-        obj = tm.box_expected(ser, box, transpose=False)
+        obj = tm.box_expected(ser, box)
 
         expected = pd.Series([True, False], dtype=np.bool_)
-        expected = tm.box_expected(expected, xbox, transpose=False)
+        expected = tm.box_expected(expected, xbox)
 
         result = obj == ts
         tm.assert_equal(result, expected)
@@ -879,10 +877,8 @@ def test_dt64arr_add_sub_td64_nat(self, box_with_array, tz_naive_fixture):
         other = np.timedelta64("NaT")
         expected = pd.DatetimeIndex(["NaT"] * 9, tz=tz)
 
-        # FIXME: fails with transpose=True due to tz-aware DataFrame
-        #  transpose bug
-        obj = tm.box_expected(dti, box_with_array, transpose=False)
-        expected = tm.box_expected(expected, box_with_array, transpose=False)
+        obj = tm.box_expected(dti, box_with_array)
+        expected = tm.box_expected(expected, box_with_array)
 
         result = obj + other
         tm.assert_equal(result, expected)

pandas/tests/arithmetic/test_period.py

@@ -755,18 +755,16 @@ 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):
+    def test_pi_add_offset_n_gt1(self, box):
         # 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])
 
         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)
+        expected = tm.box_expected(expected, box)
 
         result = pi + per.freq
         tm.assert_equal(result, expected)
@@ -780,9 +778,8 @@ def test_pi_add_offset_n_gt1_not_divisible(self, box_with_array):
         pi = pd.PeriodIndex(["2016-01"], freq="2M")
         expected = pd.PeriodIndex(["2016-04"], freq="2M")
 
-        # FIXME: with transposing these tests fail
-        pi = tm.box_expected(pi, box_with_array, transpose=False)
-        expected = tm.box_expected(expected, box_with_array, transpose=False)
+        pi = tm.box_expected(pi, box_with_array)
+        expected = tm.box_expected(expected, box_with_array)
 
         result = pi + to_offset("3M")
         tm.assert_equal(result, expected)
@@ -984,16 +981,15 @@ 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):
+    def test_parr_add_sub_td64_nat(self, box):
         # 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")
 
-        obj = tm.box_expected(pi, box, transpose=transpose)
-        expected = tm.box_expected(expected, box, transpose=transpose)
+        obj = tm.box_expected(pi, box)
+        expected = tm.box_expected(expected, box)
 
         result = obj + other
         tm.assert_equal(result, expected)

pandas/tests/frame/test_operators.py

@@ -844,6 +844,22 @@ 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")