BUG: Fix inserting TImedelta into Series, closes pandas-dev#22717

Author: jbrockmendel

pandas/core/indexing.py

@@ -19,6 +19,7 @@
     is_scalar,
     is_sequence,
     is_sparse,
+    is_timedelta64_dtype,
 )
 from pandas.core.dtypes.generic import ABCDataFrame, ABCSeries
 from pandas.core.dtypes.missing import _infer_fill_value, isna
@@ -429,11 +430,23 @@ def _setitem_with_indexer(self, indexer, value):
                     # this preserves dtype of the value
                     new_values = Series([value])._values
                     if len(self.obj._values):
-                        try:
-                            new_values = np.concatenate([self.obj._values, new_values])
-                        except TypeError:
+                        if is_timedelta64_dtype(
+                            new_values
+                        ) and not is_timedelta64_dtype(self.obj):
+                            # GH#22717 np.concatenate incorrect casts
+                            #  timedelta64 to integer
                             as_obj = self.obj.astype(object)
-                            new_values = np.concatenate([as_obj, new_values])
+                            new_values = np.concatenate(
+                                [as_obj, np.array([value], dtype=object)]
+                            )
+                        else:
+                            try:
+                                new_values = np.concatenate(
+                                    [self.obj._values, new_values]
+                                )
+                            except TypeError:
+                                as_obj = self.obj.astype(object)
+                                new_values = np.concatenate([as_obj, new_values])
                     self.obj._data = self.obj._constructor(
                         new_values, index=new_index, name=self.obj.name
                     )._data

pandas/tests/series/indexing/test_indexing.py

@@ -653,6 +653,15 @@ def test_timedelta_assignment():
     expected.loc[[1, 2, 3]] = pd.Timedelta(np.timedelta64(20, "m"))
     tm.assert_series_equal(s, expected)
 
+    # GH#22717 inserting a Timedelta should _not_ cast to int64
+    ser = pd.Series(["x"])
+    ser["td"] = pd.Timedelta("9 days")
+    assert isinstance(ser["td"], pd.Timedelta)
+
+    ser = pd.Series(["x"])
+    ser.loc["td"] = pd.Timedelta("9 days")
+    assert isinstance(ser["td"], pd.Timedelta)
+
 
 def test_underlying_data_conversion():
     # GH 4080