Split test and add docstring

Author: mroeschke

pandas/core/internals.py

@@ -2906,7 +2906,23 @@ def shift(self, periods, axis=0, mgr=None):
                                            placement=self.mgr_locs)]
 
     def diff(self, n, axis=0, mgr=None):
-        """1st discrete difference"""
+        """1st discrete difference
+
+        Parameters
+        ----------
+        n : int, number of periods to diff
+        axis : int, axis to diff upon. default 0
+        mgr : default None
+
+        Return
+        ------
+        A list with a new TimeDeltaBlock.
+
+        Note
+        ----
+        The arguments here are mimicking shift so they are called correctly
+        by apply.
+        """
         if axis == 0:
             # Cannot currently calculate diff across multiple blocks since this
             # function is invoked via apply

pandas/tests/frame/test_timeseries.py

@@ -57,28 +57,31 @@ def test_diff(self):
             1), 'z': pd.Series(1)}).astype('float64')
         assert_frame_equal(result, expected)
 
-    @pytest.mark.parametrize('axis', [0, 1])
     @pytest.mark.parametrize('tz', [None, 'UTC'])
-    def test_diff_datetime(self, axis, tz):
+    def test_diff_datetime_axis0(self, tz):
         # GH 18578
         df = DataFrame({0: date_range('2010', freq='D', periods=2, tz=tz),
                         1: date_range('2010', freq='D', periods=2, tz=tz)})
-        if axis == 1:
-            if tz is None:
-                result = df.diff(axis=axis)
-                expected = DataFrame({0: pd.TimedeltaIndex(['NaT', 'NaT']),
-                                      1: pd.TimedeltaIndex(['0 days',
-                                                            '0 days'])})
-                assert_frame_equal(result, expected)
-            else:
-                with pytest.raises(NotImplementedError):
-                    result = df.diff(axis=axis)
 
-        else:
-            result = df.diff(axis=axis)
-            expected = DataFrame({0: pd.TimedeltaIndex(['NaT', '1 days']),
-                                  1: pd.TimedeltaIndex(['NaT', '1 days'])})
+        result = df.diff(axis=0)
+        expected = DataFrame({0: pd.TimedeltaIndex(['NaT', '1 days']),
+                              1: pd.TimedeltaIndex(['NaT', '1 days'])})
+        assert_frame_equal(result, expected)
+
+    @pytest.mark.parametrize('tz', [None, 'UTC'])
+    def test_diff_datetime_axis1(self, tz):
+        # GH 18578
+        df = DataFrame({0: date_range('2010', freq='D', periods=2, tz=tz),
+                        1: date_range('2010', freq='D', periods=2, tz=tz)})
+        if tz is None:
+            result = df.diff(axis=1)
+            expected = DataFrame({0: pd.TimedeltaIndex(['NaT', 'NaT']),
+                                  1: pd.TimedeltaIndex(['0 days',
+                                                        '0 days'])})
             assert_frame_equal(result, expected)
+        else:
+            with pytest.raises(NotImplementedError):
+                result = df.diff(axis=1)
 
     def test_diff_timedelta(self):
         # GH 4533