BUG: Fix+test dataframe tranpose with datetimeTZ

pandas/core/frame.py

@@ -568,7 +568,9 @@ def _get_axes(N, K, index=index, columns=columns):
         # if we don't have a dtype specified, then try to convert objects
         # on the entire block; this is to convert if we have datetimelike's
         # embedded in an object type
-        if dtype is None and is_object_dtype(values):
+        if dtype is None and is_object_dtype(values) and values.shape[0] == 1:
+            # only do this inference for single-column DataFrame, otherwise
+            #  create_block_manager_from_blocks will raise a ValueError
             values = maybe_infer_to_datetimelike(values)
 
         return create_block_manager_from_blocks([values], [columns, index])

pandas/core/generic.py

@@ -22,19 +22,22 @@
     is_bool_dtype,
     is_numeric_dtype,
     is_datetime64_any_dtype,
-    is_timedelta64_dtype,
+    is_datetime64_dtype,
     is_datetime64tz_dtype,
+    is_timedelta64_dtype,
     is_list_like,
     is_dict_like,
     is_re_compilable,
     is_period_arraylike,
     is_object_dtype,
     is_extension_array_dtype,
     pandas_dtype)
-from pandas.core.dtypes.cast import maybe_promote, maybe_upcast_putmask
+from pandas.core.dtypes.cast import (
+    maybe_promote, maybe_upcast_putmask, maybe_infer_to_datetimelike)
 from pandas.core.dtypes.inference import is_hashable
 from pandas.core.dtypes.missing import isna, notna
-from pandas.core.dtypes.generic import ABCSeries, ABCPanel, ABCDataFrame
+from pandas.core.dtypes.generic import (
+    ABCSeries, ABCPanel, ABCDataFrame, ABCDatetimeIndex)
 
 from pandas.core.base import PandasObject, SelectionMixin
 from pandas.core.index import (Index, MultiIndex, ensure_index,
@@ -683,12 +686,20 @@ 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)
+        values = self.values
+        if isinstance(values, ABCDatetimeIndex):
+            # we must case to numpy array otherwise transpose raises ValueError
+            values = np.array(values.astype(np.object)).reshape(self.shape)
+
+        new_values = values.transpose(axes_numbers)
         if kwargs.pop('copy', None) or (len(args) and args[-1]):
             new_values = new_values.copy()
 
         nv.validate_transpose_for_generic(self, kwargs)
-        return self._constructor(new_values, **new_axes).__finalize__(self)
+        result = self._constructor(new_values, **new_axes)
+
+        result = maybe_restore_dtypes(result, self)
+        return result.__finalize__(self)
 
     def swapaxes(self, axis1, axis2, copy=True):
         """
@@ -10753,6 +10764,35 @@ def logical_func(self, axis=0, bool_only=None, skipna=True, level=None,
     return set_function_name(logical_func, name, cls)
 
 
+def maybe_restore_dtypes(result, orig):
+    # GH#23730
+    if orig.ndim != 2:
+        return result
+
+    if orig.size == 0:
+        # ensure both orig.dtypes and result.dtypes have length >= 1
+        return result
+
+    if ((result.dtypes == np.object_).all() and
+            not (orig.dtypes == np.object_).any()):
+        # the transpose was lossy
+        if (orig.dtypes == orig.dtypes[0]).all():
+            if is_datetime64tz_dtype(orig.dtypes[0]):
+                tz = orig.dtypes[0].tz
+                for col in result.columns:
+                    result[col] = maybe_infer_to_datetimelike(result[col])
+                    if (is_datetime64_dtype(result[col]) and
+                            isna(result[col]).all()):
+                        # all-NaT gets inferred as tz-naive
+                        result[col] = pd.DatetimeIndex(result[col], tz=tz)
+
+            else:
+                # TODO: consider doing something useful in this case?
+                pass
+
+    return result
+
+
 # install the indexes
 for _name, _indexer in indexing.get_indexers_list():
     NDFrame._create_indexer(_name, _indexer)

pandas/core/internals/managers.py

@@ -15,8 +15,9 @@
     find_common_type, infer_dtype_from_scalar, maybe_convert_objects,
     maybe_promote)
 from pandas.core.dtypes.common import (
-    _NS_DTYPE, is_datetimelike_v_numeric, is_extension_array_dtype,
-    is_extension_type, is_numeric_v_string_like, is_scalar)
+    _NS_DTYPE, is_datetime64tz_dtype, is_datetimelike_v_numeric,
+    is_extension_array_dtype, is_extension_type, is_numeric_v_string_like,
+    is_scalar)
 import pandas.core.dtypes.concat as _concat
 from pandas.core.dtypes.generic import ABCExtensionArray, ABCSeries
 from pandas.core.dtypes.missing import isna
@@ -773,6 +774,9 @@ def _interleave(self):
             dtype = dtype.subtype
         elif is_extension_array_dtype(dtype):
             dtype = 'object'
+        elif is_datetime64tz_dtype(dtype):
+            # TODO: avoid this conversion by allowing 2D DatetimeArray
+            dtype = 'object'
 
         result = np.empty(self.shape, dtype=dtype)
 

pandas/tests/arithmetic/conftest.py

@@ -163,6 +163,19 @@ def box_df_fail(request):
     return request.param
 
 
+@pytest.fixture(params=[(pd.Index, False),
+                        (pd.Series, False),
+                        (pd.DataFrame, False),
+                        (pd.DataFrame, True)],
+                ids=lambda x: x[0].__name__ + '-' + str(x[1]))
+def box_with_transpose(request):
+    """
+    Fixture similar to `box` but testing both transpose cases for DataFrame
+    """
+    # GH#23620
+    return request.param
+
+
 @pytest.fixture(params=[(pd.Index, False),
                         (pd.Series, False),
                         (pd.DataFrame, False),
@@ -178,6 +191,20 @@ def box_transpose_fail(request):
     return request.param
 
 
+@pytest.fixture(params=[(pd.Index, False),
+                        (pd.Series, False),
+                        (pd.DataFrame, False),
+                        (pd.DataFrame, True),
+                        (tm.to_array, False)],
+                ids=id_func)
+def box_T_with_array(request):
+    """
+    Like `box`, but specific to datetime64 for also testing pandas Array,
+    and both transpose cases for DataFrame
+    """
+    return request.param
+
+
 @pytest.fixture(params=[pd.Index, pd.Series, pd.DataFrame, tm.to_array],
                 ids=id_func)
 def box_with_array(request):