Merge two columns when sign changes pandas

I would like to merge two columns into one but I am not sure how to do this efficiently. My df looks like this:

col1   col2
 0.4   -0.9
 0.2   -0.5
-0.1    0.2
-0.2    0.4
 0.8   -0.6

So if one column is positive, the other one is always negative. But I would like to have all negative numbers from column 1 replaced by all positive numbers from column 2. So it would look like this:

col1   
 0.4   
 0.2   
 0.2
 0.4
 0.8  

If you know an efficient solution to this I would really appreciate it!!

>Solution :

Find the rows where col1 is less than 0 and replace with col2:

df.loc[df['col1'] < 0, 'col1'] = df['col2']

result:

   col1  col2
0   0.4  -0.9
1   0.2  -0.5
2   0.2   0.2
3   0.4   0.4
4   0.8  -0.6