python - backward filling with condition -


i apply backward filling on particular column of dataframe following condition: have "colum_a" can assume 4 values, called a, b, c, d and, backward filling should work in following: 

if first not nan a, backward_filling a;  if first not nan b, backward_filling b;  if first not nan c, backward_filling b;  if first not nan d, backward_filling c;  if column_a contains nan, backward_filling d

for example:

input df:

colum_a  nan  nan  b  b  c  c

output df:

colum_a  b  b  c  c  d  d

please, appreciated. best regards, carlo

i think need map bfill condition:

#get mask filling nans m = df['colum_a'].isnull() d = {'a':'a','b':'b','c':'b','d':'c'} #d if values nan df['colum_b'] = 'd' if m.all() else np.where(m, df['colum_a'].map(d).bfill(),df['colum_a']) #alternative #df['colum_b'] = 'd' if m.all() else df['colum_a'].mask(m, df['colum_a'].map(d).bfill()) print (df)    colum_a colum_b 0      nan       b 1      nan       b 2        b       b 3              4      nan       b 5        c       c 6        c       c 7      nan       c 8      nan       c 9      nan       c 10       d       d 11       d       d 12             13       c       c 14     nan       15             16     nan     nan

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