how to update existing data frame in pandas?

Question

Given these two data frames:

>>> df1 = pd.DataFrame({\'c1\':[\'a\',\'a\',\'b\',\'b\'], \'c2\':[\'x\',\'y\',\'x\',\'y\'], \'val\':0})
>>> df         


        

Answer 1

Yes, take a look at combine_first or update. For example:

>>> df1['val'] = df2['val'].combine_first(df1['val'])
>>> df1
Out[26]:
    c1  c2  val
0    a   x   12
1    a   y   31
2    b   x   14
3    b   y   0

EDIT: to combine according to c1 and c2 ignoring the current index:

>>> df1['val'] = df2.set_index(['c1','c2'])['val'].combine_first(df1.set_index(['c1','c2'])['val']).values
>> df1
Out[25]:
    c1  c2  val
0    a   x   12
1    a   y   31
2    b   x   0
3    b   y   14