Flatten a list in a pandas column into multiple rows?

Question

I am looking for a fast method that performs what is asked in the title, repeating values for the other columns. Example:

df=pd.DataFrame([[['foo','bar'],1],['baz',2]], columns=['A','B'])

df
       'A'         'B'   
0 ['foo','bar']     1    
1 'baz'             2    

Into:

df_2
     'A'         'B'   
0   'foo'         1
1   'bar'         1
2   'baz'         2