core: coerce result to dtype in Block.eval if dtype explicitly given

doc/source/whatsnew/v0.24.0.txt

@@ -436,6 +436,7 @@ Datetimelike
 
 - Fixed bug where two :class:`DateOffset` objects with different ``normalize`` attributes could evaluate as equal (:issue:`21404`)
 - Fixed bug where :meth:`Timestamp.resolution` incorrectly returned 1-microsecond ``timedelta`` instead of 1-nanosecond :class:`Timedelta` (:issue:`21336`,:issue:`21365`)
+- Fixed bug where :class:`DataFrame` with ``dtype='datetime64[ns]'`` operating with :class:`DateOffset` could cast to ``dtype='object'`` (:issue:`21610`)
 
 Timedelta
 ^^^^^^^^^

pandas/core/internals.py

@@ -35,7 +35,7 @@
     is_float_dtype, is_numeric_dtype,
     is_numeric_v_string_like, is_extension_type,
     is_extension_array_dtype,
-    is_list_like,
+    is_list_like, is_offsetlike,
     is_re,
     is_re_compilable,
     is_scalar,
@@ -710,6 +710,10 @@ def _try_cast_result(self, result, dtype=None):
         """ try to cast the result to our original type, we may have
         roundtripped thru object in the mean-time
         """
+        # coerce to `dtype` if explicitly given
+        if isinstance(dtype, np.dtype):
+            return result.astype(dtype)
+
         if dtype is None:
             dtype = self.dtype
 
@@ -1315,7 +1319,8 @@ def shift(self, periods, axis=0, mgr=None):
 
         return [self.make_block(new_values)]
 
-    def eval(self, func, other, errors='raise', try_cast=False, mgr=None):
+    def eval(self, func, other, errors='raise', try_cast=False, mgr=None,
+             dtype=None):
         """
         evaluate the block; return result block from the result
 
@@ -1327,7 +1332,8 @@ def eval(self, func, other, errors='raise', try_cast=False, mgr=None):
             - ``raise`` : allow exceptions to be raised
             - ``ignore`` : suppress exceptions. On error return original object
 
-        try_cast : try casting the results to the input type
+        try_cast : try casting the results to ``dtype`` or the input type
+        dtype: if not ``None`` and ``try_cast=Ture``, coerce the result to it
 
         Returns
         -------
@@ -1362,10 +1368,15 @@ def eval(self, func, other, errors='raise', try_cast=False, mgr=None):
             values, values_mask, other, other_mask = self._try_coerce_args(
                 transf(values), other)
         except TypeError:
+            # `Timestamp` operate with `DateOffset` must cast to `object`,
+            #  then cast back to `datetime64[ns]` after operation.
+            if (dtype is None and is_datetime64_dtype(self)
+                    and is_offsetlike(other)):
+                dtype = self.dtype
+
             block = self.coerce_to_target_dtype(orig_other)
-            return block.eval(func, orig_other,
-                              errors=errors,
-                              try_cast=try_cast, mgr=mgr)
+            return block.eval(func, orig_other, errors=errors,
+                              try_cast=try_cast, mgr=mgr, dtype=dtype)
 
         # get the result, may need to transpose the other
         def get_result(other):
@@ -1449,7 +1460,7 @@ def handle_error():
 
         # try to cast if requested
         if try_cast:
-            result = self._try_cast_result(result)
+            result = self._try_cast_result(result, dtype=dtype)
 
         result = _block_shape(result, ndim=self.ndim)
         return [self.make_block(result)]

pandas/tests/frame/test_arithmetic.py

@@ -211,6 +211,11 @@ def test_df_sub_datetime64_not_ns(self):
                                  pd.Timedelta(days=2)])
         tm.assert_frame_equal(res, expected)
 
+    def test_timestamp_df_add_dateoffset(self):
+        expected = pd.DataFrame([pd.Timestamp('2019')])
+        result = pd.DataFrame([pd.Timestamp('2018')]) + pd.DateOffset(years=1)
+        tm.assert_frame_equal(result, expected)
+
     @pytest.mark.parametrize('data', [
         [1, 2, 3],
         [1.1, 2.2, 3.3],