This is my dataframe:
date ids
0 2011-04-23 [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13,...
1 2011-04-24 [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13,...
2 2011-04-25 [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13,...
3 2011-04-26 NaN
4 2011-04-27 [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13,...
5 2011-04-28 [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13,...
I want to replace Nan with [[],[],[],[]]. How to do that?
df['ids'].fillna("").apply(list)
Is working well for a 1 element list, but how can we use it with 4 element list?
You can't use fillna with lists, but you can create a Series containing your list repeated for the length of the dataframe, and assign that to the b where b is NaN:
df.loc[df['b'].isna(), 'b'] = pd.Series([ [[]]*4 ] * len(df))