python-如何将多索引更改为平面列名称

我有这个数据框：

import pandas as pd
df = pd.DataFrame(data={'Status' : ['green','green','red','blue','red','yellow','black'],
 'Group' : ['A','A','B','C','A','B','C'],
 'City' : ['Toronto','Montreal','Vancouver','Toronto','Edmonton','Winnipeg','Windsor'],
 'Sales' : [13,6,16,8,4,3,1]})
df.drop('Status',axis=1,inplace=True)
ndf = pd.pivot_table(df,values=['Sales'],index=['City'],columns=['Group'],fill_value=0,margins=False)

看起来像这样：

In [321]: ndf
Out[321]:
          Sales
Group         A   B  C
City
Edmonton      4   0  0
Montreal      6   0  0
Toronto      13   0  8
Vancouver     0  16  0
Windsor       0   0  1
Winnipeg      0   3  0

我如何展平它,使其成为简单的数据框架：

    City          A   B  C
    Edmonton      4   0  0
    Montreal      6   0  0
    Toronto      13   0  8
    Vancouver     0  16  0
    Windsor       0   0  1
    Winnipeg      0   3  0

解决方法:

ndf = ndf.Sales.reset_index()
ndf

Out[4]: 
Group       City   A   B  C
0       Edmonton   4   0  0
1       Montreal   6   0  0
2        Toronto  13   0  8
3      Vancouver   0  16  0
4        Windsor   0   0  1
5       Winnipeg   0   3  0