Pandas fillna with string values from 2 other columns

I have a df with 3 columns, City, State, and MSA. Some of the MSA values are NaN. I would like to fill the MSA NaN values with a concatenation of City and State. I can fill MSA with City using df.MSA_CBSA.fillna(df.City, inplace=True), but some cities in different states have the same name.

>Solution :

Keep using the vectorized operation that you suggested. Notice that the argument can receive a combination from the other instances:

df.MSA.fillna(df.City + "," + df.State, inplace=True)