test_df = pd.DataFrame({'a':[np.nan,np.nan,np.nan,4,np.nan,np.nan,6]})
test_df
    a
0   NaN
1   NaN
2   NaN
3   4.0
4   NaN
5   NaN
6   6.0

I'm trying to backfill with the real value divided by the number of na values itself. The following is what I'm trying to get

    a
0   1.0
1   1.0
2   1.0
3   1.0
4   2.0
5   2.0
6   2.0

CodePudding user response：

Try:

# identify the blocks by cumsum on the reversed non-nan series
groups = test_df['a'].notna()[::-1].cumsum()

# groupby and transform 
test_df['a'] = test_df['a'].fillna(0).groupby(groups).transform('mean')

Output:

     a
0  1.0
1  1.0
2  1.0
3  1.0
4  2.0
5  2.0
6  2.0

CodePudding user response：

IIUC use:

# get reverse group
group = test_df.loc[::-1,'a'].notna().cumsum()

# get size and divide
test_df['a'] = (test_df['a']
                .bfill()
                .div(test_df.groupby(group)['a'].transform('size'))
               )

Or with rdiv:

test_df['a'] = (test_df
                .groupby(group)['a']
                .transform('size')
                .rdiv(test_df['a'].bfill())
                 )

Output (as new column for clarity):

     a   a2
0  NaN  1.0
1  NaN  1.0
2  NaN  1.0
3  4.0  1.0
4  NaN  2.0
5  NaN  2.0
6  6.0  2.0