pandas多条件分组排序问题 row_number

df = pd.DataFrame({'key1' : ['a','a','a','b','b'],
    'key2' : ['c','d','c','c','d'],
    'data' : [1,10,2,3,30]})
  key1 key2  data
0    a    c     1
1    a    d    10
2    a    c     2
3    b    c     3
4    b    d    30
key1 key2  data  row_number
0    a    c     1     1
1    a    d    10     1
2    a    c     2     2
3    b    c     3     1
4    b    d    30     1
以key1、key2分组，按照data排序，取出序号应该怎么处理呢？搜索找到的以下方法没有成功
df['row_number'] = df['data'].groupby(df['key1','key2']).rank(ascending=True,method='first')