tuples = list(zip(*[['bar', 'bar', 'baz', 'baz', 'foo', 'foo', 'qux', 'qux'],
                    ['one', 'two', 'one', 'two', 'one', 'two', 'one', 'two']]))
index = pd.MultiIndex.from_tuples(tuples, names=['first', 'second'])
df = pd.DataFrame(np.random.randn(8, 2), index=index, columns=['A', 'B'])
df2 = df[:4]

df2:
                     A         B
first second                    
bar   one     0.029399 -0.542108
      two     0.282696 -0.087302
baz   one    -1.575170  1.771208
      two     0.816482  1.100230
      
stacked = df2.stack()  # The stack() method “compresses” a level in the DataFrame’s columns.
stacked:
first second
bar  one     A   0.029399
             B   -0.542108
     two     A   0.282696
             B   -0.087302
baz  one     A   -1.575170
             B    1.771208
     two     A    0.816482
             B    1.100230      

stacked.unstack()   # which by default unstacks the last level
stacked.unstack(1)  # 
stacked.unstack(0)  # 即把0列展开在最后  

first          bar       baz
second
one      A  0.029399 -1.575170
         B -0.542108  1.771208
two      A  0.282696  0.816482
         B -0.087302  1.100230