如何根据大熊猫另一栏中的条件计算记录的频率？

I have a table like this:

我有这样一张桌子：

In [2]: df = pd.DataFrame({
   ...:     'donorID':[101,101,101,102,103,101,101,102,103],
   ...:     'recipientID':[11,11,21,21,31,11,21,31,31],
   ...:     'amount':[100,200,500,200,200,300,200,200,100],
   ...:     'year':[2014,2014,2014,2014,2014,2015,2015,2015,2015]
   ...: })

In [3]: df
Out[3]:
   amount  donorID  recipientID  year
0     100      101           11  2014
1     200      101           11  2014
2     500      101           21  2014
3     200      102           21  2014
4     200      103           31  2014
5     300      101           11  2015
6     200      101           21  2015
7     200      102           31  2015
8     100      103           31  2015
In [2]: