Include issue number in tests and whatsnew

Author: evanpw

doc/source/whatsnew/v0.16.1.txt

@@ -16,7 +16,7 @@ We recommend that all users upgrade to this version.
 
 Enhancements
 ~~~~~~~~~~~~
-- ``DataFrame.diff`` now takes an ``axis`` parameter
+- ``DataFrame.diff`` now takes an ``axis`` parameter (:issue:`9727`)
 
 
 

pandas/tests/test_frame.py

@@ -9956,6 +9956,7 @@ def test_diff_float_n(self):
         assert_frame_equal(rs, xp)
 
     def test_diff_axis(self):
+        # GH 9727
         df = DataFrame([[1., 2.], [3., 4.]])
         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.]]))