Fill nan value when condition meet

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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]})

fillna pandas dataframe nan python
1 Answers

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
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