Pandas merge two dataframes summing values [duplicate]

Question

Suppose I have two dataframes with partly repeated entries:

source1=pandas.DataFrame({'key':['a','b'],'value':[1,2]})
#  key  value
#0   a      1
#1   b      2
source2=pandas.DataFrame({'key':['b','c'],'value':[3,0]})
#  key  value
#0   b      3
#1   c      0

What do I need to do with source1 and source2 in order to get resulting frame with following entries:

#  key  value
#0   a      1
#1   b      5
#2   c      0

Answer 1

Just add

source1.set_index('key').add(source2.set_index('key'), fill_value=0)

If key is already the index, just use

source1.add(source2, fill_value=0)

You man want to .reset_index() at the end if you don't want key as index