FIX DataFrame diff with timedelta pandas-dev#4533

Author: hayd

pandas/core/common.py

@@ -813,7 +813,8 @@ def diff(arr, n, axis=0):
     na_indexer[axis] = slice(None, n) if n >= 0 else slice(n, None)
     out_arr[tuple(na_indexer)] = na
 
-    if arr.ndim == 2 and arr.dtype.name in _diff_special:
+    if (arr.ndim == 2 and arr.dtype.name in _diff_special
+                      and dtype != 'timedelta64[ns]'):
         f = _diff_special[arr.dtype.name]
         f(arr, out_arr, n, axis)
     else:

pandas/tests/test_frame.py

@@ -9285,6 +9285,16 @@ def test_diff_float_n(self):
         xp = self.tsframe.diff(1)
         assert_frame_equal(rs, xp)
 
+    def test_diff_timedelta(self):
+        df = DataFrame(dict(time=[Timestamp('20130101 9:01'),
+                                  Timestamp('20130101 9:02')],
+                            value=[1.0,2.0]))
+        res = df.diff()
+        exp = DataFrame([[pd.NaT, np.nan],
+                         [np.timedelta64(int(6e10), 'ns'), 1]],
+                         columns=['time', 'value'])
+        assert_frame_equal(res, exp)
+
     def test_pct_change(self):
         rs = self.tsframe.pct_change(fill_method=None)
         assert_frame_equal(rs, self.tsframe / self.tsframe.shift(1) - 1)