How to convert index of a pandas dataframe into a column

How to convert an index of a dataframe into a column?

For example:

        gi       ptt_loc
 0  384444683      593  
 1  384444684      594 
 2  384444686      596  

to

    index1    gi       ptt_loc
 0  0     384444683      593  
 1  1     384444684      594 
 2  2     384444686      596