merging two csv using python

Question

I wanted to merge two csv files into new output file in this manner:

filea.csv:

A  B  C
1  a  y 
2  b   
3  f  z

fileb.csv

A B C 
1 Z c  
4 x z
6 j l

output.csv

A B C
1 a y
2 b
3 f z
4 x z
6 j l

hence , I retain contents of filea.csv and added '4','6' row to output.csv. I tried pandas lib with 'isinfunction' but unable to do it .