Q:

pandas adding data from index table to dataframe

I have a pandas dataframe monthlyTempDiff with contents as:
monthlyAdder.ix[1:3]
         City1     City2    City3
monthID                              
1        0.01      0.1      0.02
2        0.04      0.2      0.03
8        0.17      0.3      0.05

Column monthID is also the index.
I have another table fwdTempTable
fwdTempTable.ix[1:3]
             City1     City2    City3      City4        monthID
DateTime                                                   
2017-1-01    22        24       26         16           1
2017-8-01    23        25       27         17           8
2017-2-01    13        15       17         27           2

I want to add to fwdTempTable, data from monthlyAdder based on combination of monthID and City1, City2, City3 to obtain the following:
fwdTempTable.ix[1:3]
             City1     City2    City3      City4        monthID
DateTime                                                   
2017-1-01    22.01     24.1     26.02      16           1
2017-8-01    23.17     25.3     27.05      17           8
2017-2-01    13.04     15.2     17.03      27           2

i.e. for each row of dataframe fwdTempTable, I extract the City1, City2 or City3 category and map it along with monthID to dataframe monthlyAdder. After mapping to monthlyAdder, I will get the right adder for a city and a month. I adjust the city data in fwdTempTable with this adder for that month.
These dataframes are large and I can't think beyond for loop on each column of fwdTempTable. 

A:

Let's try set_index and add:
fwdTempTable.set_index('monthID', append=True).add(monthlyAdder, fill_value=0).reset_index('monthID')

Output:
           monthID  City1  City2  City3  City4
DateTime                                      
2017-1-01        1  22.01   24.1  26.02   16.0
2017-8-01        8  23.17   25.3  27.05   17.0
2017-2-01        2  13.04   15.2  17.03   27.0

