我有以下Python pandas数据帧:
id| country | 2016 | 2017 | 2018
--+----------+------+------+------
0 | saudi | A | null | B
1 | china | C | A | B
2 | pakistan | null | null | C
我想要:
id| country | year | value
--+----------+------+------
0 | saudi | 2016 | A
1 | saudi | 2018 | B
2 | china | 2016 | C
3 | china | 2017 | A
4 | china | 2018 | …