BUG: transpose casts mixed dtypes to object

doc/source/whatsnew/v1.4.0.rst

@@ -488,6 +488,7 @@ Reshaping
 - Improved error message when creating a :class:`DataFrame` column from a multi-dimensional :class:`numpy.ndarray` (:issue:`42463`)
 - :func:`concat` creating :class:`MultiIndex` with duplicate level entries when concatenating a :class:`DataFrame` with duplicates in :class:`Index` and multiple keys (:issue:`42651`)
 - Bug in :meth:`pandas.cut` on :class:`Series` with duplicate indices (:issue:`42185`) and non-exact :meth:`pandas.CategoricalIndex` (:issue:`42425`)
+- Bug in :meth:`DataFrame.transpose`, where mixed dtypes were cast to ``object`` (:issue:`43337`)
 - Bug in :meth:`DataFrame.append` failing to retain dtypes when appended columns do not match (:issue:`43392`)
 - Bug in :func:`concat` of ``bool`` and ``boolean`` dtypes resulting in ``object`` dtype instead of ``boolean`` dtype (:issue:`42800`)
 - Bug in :func:`crosstab` when inputs are are categorical Series, there are categories that are not present in one or both of the Series, and ``margins=True``. Previously the margin value for missing categories was ``NaN``. It is now correctly reported as 0 (:issue:`43505`)

pandas/core/frame.py

@@ -3400,9 +3400,13 @@ def transpose(self, *args, copy: bool = False) -> DataFrame:
 
         else:
             new_arr = self.values.T
-            if copy:
-                new_arr = new_arr.copy()
-            result = self._constructor(new_arr, index=self.columns, columns=self.index)
+            common_dtype = find_common_type(dtypes) if len(dtypes) > 0 else None
+            result = self._constructor(
+                new_arr,
+                index=self.columns,
+                columns=self.index,
+                dtype=common_dtype,
+            )
 
         return result.__finalize__(self, method="transpose")
 

pandas/tests/frame/methods/test_transpose.py

@@ -37,6 +37,7 @@ def test_transpose_tzaware_2col_mixed_tz(self):
         df4 = DataFrame({"A": dti, "B": dti2})
         assert (df4.dtypes == [dti.dtype, dti2.dtype]).all()
         assert (df4.T.dtypes == object).all()
+        print(df4._can_fast_transpose, df4.T._can_fast_transpose)
         tm.assert_frame_equal(df4.T.T, df4)
 
     @pytest.mark.parametrize("tz", [None, "America/New_York"])
@@ -57,6 +58,7 @@ def test_transpose_object_to_tzaware_mixed_tz(self):
         df2 = DataFrame([dti, dti2])
         assert (df2.dtypes == object).all()
         res2 = df2.T
+        print("\n", res2.dtypes, [dti.dtype, dti2.dtype])
         assert (res2.dtypes == [dti.dtype, dti2.dtype]).all()
 
     def test_transpose_uint64(self, uint64_frame):
@@ -103,3 +105,10 @@ def test_transpose_get_view_dt64tzget_view(self):
 
         rtrip = result._mgr.blocks[0].values
         assert np.shares_memory(arr._data, rtrip._data)
+
+    def test_transpose_mixed_dtypes(self):
+        # GH#43337
+        df = DataFrame({"a": [1], "b": [2]}).astype({"b": "Int64"})
+        result = df.T
+        expected = DataFrame([1, 2], index=["a", "b"], dtype="Int64")
+        tm.assert_frame_equal(result, expected)