pandas.DataFrame.unstack抄书笔记

pandas.DataFrame.unstack¶

DataFrame.unstack(level=- 1, fill_value=None)[source]

Pivot a level of the (necessarily hierarchical) index labels.

Returns a DataFrame having a new level of column labels whose inner-most level consists of the pivoted index labels.

If the index is not a MultiIndex, the output will be a Series (the analogue of stack when the columns are not a MultiIndex).

Parameters

levelint, str, or list of these, default -1 (last level)

Level(s) of index to unstack, can pass level name.

fill_valueint, str or dict

Replace NaN with this value if the unstack produces missing values.

Returns

Series or DataFrame

这是一个有意思的方法，实在很有意思。unstack，当dataframe对象是有一个多层index的时候，会从index中抽出去，填补到columns中去，

如果一个多层index的Series，就会变成了DataFrame对象。

如果是一个单层的index的DataFrame对象，[如果列的索引是多层索引],则会返回一个Series对象，列索引变成行索引的多层索引的第一层。

Example

index = pd.MultiIndex.from_tuples([('one', 'a'), ('one', 'b'),
                                   ('two', 'a'), ('two', 'b')])
s = pd.Series(np.arange(1.0, 5.0), index=index)
s

one  a   1.0
     b   2.0
two  a   3.0
     b   4.0
dtype: float64

　　

In [12]: s                                                                                                                  
Out[12]: 
one  a    1.0
     b    2.0
two  a    3.0
     b    4.0
dtype: float64

In [13]: s.unstack()                                                                                                        
Out[13]: 
       a    b
one  1.0  2.0
two  3.0  4.0

In [14]: s.unstack(-1)                                                                                                      
Out[14]: 
       a    b
one  1.0  2.0
two  3.0  4.0

In [15]: s.unstack(0)                                                                                                       
Out[15]: 
   one  two
a  1.0  3.0
b  2.0  4.0