pandas: combine two columns in a DataFrame

Question

I have a pandas DataFrame that has multiple columns in it:

Index: 239897 entries, 2012-05-11 15:20:00 to 2012-06-02 23:44:51
Data columns:
foo           


        

Answer 1

More modern pandas versions (since at least 0.12) have the combine_first() and update() methods for DataFrame and Series objects. For example if your DataFrame were called df, you would do:

df.bar.combine_first(df.foo)

which would only alter Nan values of the bar column to match the foo column, and would do so inplace. To overwrite non-Nan values in bar with those in foo, you would use the update() method.