TST: Added DF test cases for first last valid index (#20499)

Author: asdf8601

pandas/tests/frame/test_timeseries.py

@@ -530,6 +530,16 @@ def test_first_last_valid(self):
         assert frame.last_valid_index() is None
         assert frame.first_valid_index() is None
 
+        # GH20499: its preserves freq with holes
+        frame.index = date_range("20110101", periods=N, freq="B")
+        frame.iloc[1] = 1
+        frame.iloc[-2] = 1
+        assert frame.first_valid_index() == frame.index[1]
+        assert frame.last_valid_index() == frame.index[-2]
+        assert frame.first_valid_index().freq == frame.index.freq
+        assert frame.last_valid_index().freq == frame.index.freq
+
+
     def test_at_time_frame(self):
         rng = date_range('1/1/2000', '1/5/2000', freq='5min')
         ts = DataFrame(np.random.randn(len(rng), 2), index=rng)