Fill nan value when condition meet

Question

I have a dataframe as shown below:

df = pd.DataFrame({'A': [1, np.nan, np.nan, np.nan, 3, 3, 3, 3, np.nan, np.nan, np.nan, 5, 5, 5, 6, 6, 6, np.nan, np.nan, np.nan, 6, 7, 8, 9, 10,np.nan, np.nan, 10, 11]})

I wanna fill the nan value only when the previous non-nan value equals the flowing non-nan values.

Expected Output:

df = pd.DataFrame({'A': [1, np.nan, np.nan, np.nan, 3, 3, 3, 3, np.nan, np.nan, np.nan, 5, 5, 5, 6, 6, 6, np.nan, np.nan, np.nan, 6, 7, 8, 9, 10, np.nan, np.nan,10, 11],'fill_nan': [1, np.nan, np.nan, np.nan, 3, 3, 3, 3, np.nan, np.nan, np.nan, 5, 5, 5, 6, 6, 6, 6, 6, 6, 6, 7, 8, 9, 10, 10, 10,10, 11]})

Answer 1

Try comparing the bfill with ffill:

s = df.ffill()
out = s.where(s.eq(df.bfill()))

Output:

         A
0      1.0
1      NaN
2      NaN
3      NaN
4      3.0
5      3.0
6      3.0
7      3.0
8      NaN
9      NaN
10     NaN
11     5.0
12     5.0
13     5.0
14     6.0
15     6.0
16     6.0
17     6.0
18     6.0
19     6.0
20     6.0
21     7.0
22     8.0
23     9.0
24    10.0
25    10.0
26    10.0
27    10.0
28    11.0