将嵌套字典中的值和键映射到列

import pandas as pd

data = {
    'A': {'a': 1, 'b': 2, 'c': 3},
    'B': {'a': 4, 'b': 5, 'c': 6},
    'C': {'a': 7, 'b': 8, 'c': 9}
}

df = pd.DataFrame(data)

df = df.stack()

df = df.reset_index()

  level_0 level_1  0
0       A       a  1
1       A       b  2
2       A       c  3
3       B       a  4
4       B       b  5
5       B       c  6
6       C       a  7
7       C       b  8
8       C       c  9