Python的熊貓數據框 - 創建更改列

Question

我有此列名稱的數據幀

timestamp,stockname,total volume traded 

有多種股票的名字在每一個時間框架

11:00,A,100 
11:00,B,500 
11:01,A,150 
11:01,B,600 
11:02,A,200 
11:02,B,650 

我想創建一個ChangeInVol塔，每個股票都有自己的差異，如

timestamp, stock,total volume, change in volume 
11:00,A,100,NaN 
11:00,B,500,NAN 
11:01,A,150,50 
11:01,B,600,100 
11:02,A,200,50 
11:03,B,650,50 

如果它是一個單一的股票，我可以hav大功告成

df['ChangeVol'] = df['TotalVol'] - df['TotalVol'].shift(1) 

但也有多個個股

Answer 1

極品sort_values + DataFrameGroupBy.diff：

#if columns not sorted 
df = df.sort_values(['timestamp','stockname']) 

df['change in volume'] = df.groupby('stockname')['total volume traded'].diff() 
print (df) 
    timestamp stockname total volume traded change in volume 
0  11:00   A     100    NaN 
1  11:00   B     500    NaN 
2  11:01   A     150    50.0 
3  11:01   B     600    100.0 
4  11:02   A     200    50.0 
5  11:02   B     650    50.0