BUG: preserve frequency across Timestamp addition/subtraction (#4547)

Author: rosnfeld

doc/source/release.rst

@@ -218,6 +218,7 @@ Bug Fixes
 - Bug in ``DataFrame.to_stata`` that lead to data loss in certain cases, and could exported using the
   wrong data types and missing values (:issue:`6335`)
 - Inconsistent types in Timestamp addition/subtraction (:issue:`6543`)
+- Bug in preserving frequency across Timestamp addition/subtraction (:issue:`4547`)
 - Bug in indexing: empty list lookup caused ``IndexError`` exceptions (:issue:`6536`, :issue:`6551`)
 
 

pandas/tseries/tests/test_tslib.py

@@ -330,6 +330,20 @@ def test_addition_subtraction_types(self):
             self.assertEqual(type(timestamp_instance + timedelta64_instance), Timestamp)
             self.assertEqual(type(timestamp_instance - timedelta64_instance), Timestamp)
 
+    def test_addition_subtraction_preserve_frequency(self):
+        timestamp_instance = date_range('2014-03-05', periods=1, freq='D')[0]
+        timedelta_instance = datetime.timedelta(days=1)
+        original_freq = timestamp_instance.freq
+        self.assertEqual((timestamp_instance + 1).freq, original_freq)
+        self.assertEqual((timestamp_instance - 1).freq, original_freq)
+        self.assertEqual((timestamp_instance + timedelta_instance).freq, original_freq)
+        self.assertEqual((timestamp_instance - timedelta_instance).freq, original_freq)
+
+        if not _np_version_under1p7:
+            timedelta64_instance = np.timedelta64(1, 'D')
+            self.assertEqual((timestamp_instance + timedelta64_instance).freq, original_freq)
+            self.assertEqual((timestamp_instance - timedelta64_instance).freq, original_freq)
+
 if __name__ == '__main__':
     nose.runmodule(argv=[__file__, '-vvs', '-x', '--pdb', '--pdb-failure'],
                    exit=False)

pandas/tslib.pyx

@@ -681,17 +681,17 @@ cdef class _Timestamp(datetime):
 
         if is_timedelta64_object(other):
             other_int = other.astype('timedelta64[ns]').astype(int)
-            return Timestamp(self.value + other_int, tz=self.tzinfo)
+            return Timestamp(self.value + other_int, tz=self.tzinfo, offset=self.offset)
 
         if is_integer_object(other):
             if self.offset is None:
                 raise ValueError("Cannot add integral value to Timestamp "
                                  "without offset.")
-            return Timestamp((self.offset * other).apply(self))
+            return Timestamp((self.offset * other).apply(self), offset=self.offset)
 
         if isinstance(other, timedelta) or hasattr(other, 'delta'):
             nanos = _delta_to_nanoseconds(other)
-            return Timestamp(self.value + nanos, tz=self.tzinfo)
+            return Timestamp(self.value + nanos, tz=self.tzinfo, offset=self.offset)
 
         result = datetime.__add__(self, other)
         if isinstance(result, datetime):