i want to put columns into multi index in dataframe

for example, dataframe is below

   A B
a  1 2 
b  3 4

I want to put column to index
the result i want is below

    value
a A   1
a B   2
b A   3
b B   4

how can i get the result?

>Solution :

You can do a dataframe melt, and then turn the variable column into an index column

pd.melt(df, value_vars=['A','B'], var_name='', ignore_index=False).set_index('',append=True)