slicing a datacolumn based on a condition in pandas

Question

I have a df,

    A    B
0   a    1.0
1   b,c  2.0
2
3   c,d  NaN

I am trying to slice df["A"] when df["B"] is NaN or empty space " " , I failed in slicing it by (,) Please help me to achieve it, my desired output is,

out_df,

    A    B
0   a    1.0
1   b,c  2.0
2
3   c    NaN

Answer 1

Use np.where i.e

df['new'] = np.where((pd.isnull(df['B']))|(df['B']==''), df['A'].str.split(',').str[0], df['A'])

or df.where i.e

df['A'].where((pd.notnull(df['B']))|(df['B']==''), df['A'].str.split(',').str[0])

    A    B  new
0    a    1    a
1  b,c    2  b,c
2               
3  c,d  NaN    c