Invert axis parameter of DataFrame.diff to match the convention of np.diff

Author: evanpw

pandas/core/frame.py

@@ -3583,21 +3583,22 @@ def unstack(self, level=-1):
     #----------------------------------------------------------------------
     # Time series-related
 
-    def diff(self, periods=1, axis=1):
+    def diff(self, periods=1, axis=0):
         """
         1st discrete difference of object
 
         Parameters
         ----------
         periods : int, default 1
             Periods to shift for forming difference
-        axis : {0 or 'index', 1 or 'columns'}
+        axis : {0 or 'index', 1 or 'columns'}, default 0
 
         Returns
         -------
         diffed : DataFrame
         """
-        axis = self._get_axis_number(axis)
+        # Invert the axis to match the convention of np.diff
+        axis = 1 - self._get_axis_number(axis)
 
         new_data = self._data.diff(n=periods, axis=axis)
         return self._constructor(new_data)

pandas/tests/test_frame.py

@@ -9957,8 +9957,8 @@ def test_diff_float_n(self):
 
     def test_diff_axis(self):
         df = DataFrame([[1., 2.], [3., 4.]])
-        assert_frame_equal(df.diff(axis=0), DataFrame([[np.nan, 1.], [np.nan, 1.]]))
-        assert_frame_equal(df.diff(axis=1), DataFrame([[np.nan, np.nan], [2., 2.]]))
+        assert_frame_equal(df.diff(axis=1), DataFrame([[np.nan, 1.], [np.nan, 1.]]))
+        assert_frame_equal(df.diff(axis=0), DataFrame([[np.nan, np.nan], [2., 2.]]))
 
     def test_pct_change(self):
         rs = self.tsframe.pct_change(fill_method=None)