ERR: Clarify exceptions for invalid datetimelike operations

pandas/core/indexes/datetimelike.py

@@ -621,7 +621,9 @@ def _convert_scalar_indexer(self, key, kind=None):
                 ._convert_scalar_indexer(key, kind=kind))
 
     def _add_datelike(self, other):
-        raise AbstractMethodError(self)
+        raise TypeError("cannot add {0} and {1}"
+                        .format(type(self).__name__,
+                                type(other).__name__))
 
     def _sub_datelike(self, other):
         raise AbstractMethodError(self)
@@ -647,16 +649,13 @@ def __add__(self, other):
                     return other._add_delta(self)
                 raise TypeError("cannot add TimedeltaIndex and {typ}"
                                 .format(typ=type(other)))
-            elif isinstance(other, Index):
-                raise TypeError("cannot add {typ1} and {typ2}"
-                                .format(typ1=type(self).__name__,
-                                        typ2=type(other).__name__))
             elif isinstance(other, (DateOffset, timedelta, np.timedelta64,
                                     Timedelta)):
                 return self._add_delta(other)
             elif is_integer(other):
                 return self.shift(other)
-            elif isinstance(other, (Timestamp, datetime)):
+            elif isinstance(other, (Index, Timestamp, datetime,
+                                    np.datetime64)):
                 return self._add_datelike(other)
             else:  # pragma: no cover
                 return NotImplemented

pandas/core/indexes/datetimes.py

@@ -747,7 +747,9 @@ def _add_datelike(self, other):
         # adding a timedeltaindex to a datetimelike
         if other is libts.NaT:
             return self._nat_new(box=True)
-        raise TypeError("cannot add a datelike to a DatetimeIndex")
+        raise TypeError("cannot add {0} and {1}"
+                        .format(type(self).__name__,
+                                type(other).__name__))
 
     def _sub_datelike(self, other):
         # subtract a datetime from myself, yielding a TimedeltaIndex

pandas/tests/indexes/datetimes/test_ops.py

@@ -434,30 +434,53 @@ def test_add_iadd(self):
             tm.assert_index_equal(rng, expected)
 
         idx = DatetimeIndex(['2011-01-01', '2011-01-02'])
-        msg = "cannot add a datelike to a DatetimeIndex"
+        msg = "cannot add DatetimeIndex and Timestamp"
         with tm.assert_raises_regex(TypeError, msg):
             idx + Timestamp('2011-01-01')
 
         with tm.assert_raises_regex(TypeError, msg):
             Timestamp('2011-01-01') + idx
 
+    def test_add_dti_ts(self):
+        dti = DatetimeIndex(['2011-01-01', '2011-01-02'])
+        ts = Timestamp('2011-01-01')
+        msg = 'cannot add DatetimeIndex and Timestamp'
+
+        with tm.assert_raises_regex(TypeError, msg):
+            dti + ts
+
+        with tm.assert_raises_regex(TypeError, msg):
+            ts + dti
+
+    def test_add_dti_dt64(self):
+        dti = DatetimeIndex(['2011-01-01', '2011-01-02'])
+        dt64 = np.datetime64('2005-02-25')
+        msg = 'cannot add DatetimeIndex and datetime64'
+
+        with tm.assert_raises_regex(TypeError, msg):
+            dti + dt64
+
+        with tm.assert_raises_regex(TypeError, msg):
+            dt64 + dti
+
     def test_add_dti_dti(self):
         # previously performed setop (deprecated in 0.16.0), now raises
         # TypeError (GH14164)
 
         dti = date_range('20130101', periods=3)
         dti_tz = date_range('20130101', periods=3).tz_localize('US/Eastern')
+        msg = 'cannot add DatetimeIndex and DatetimeIndex'
 
-        with pytest.raises(TypeError):
+        with tm.assert_raises_regex(TypeError, msg):
             dti + dti
 
-        with pytest.raises(TypeError):
+        with tm.assert_raises_regex(TypeError, msg):
             dti_tz + dti_tz
 
-        with pytest.raises(TypeError):
+        with tm.assert_raises_regex(TypeError, msg):
             dti_tz + dti
 
-        with pytest.raises(TypeError):
+        with tm.assert_raises_regex(TypeError, msg):
             dti + dti_tz
 
     def test_difference(self):