在尝试使用sort_values（）方法对熊猫数据框进行排序时，出现意外的KeyError。我对此感到困惑。这是我的玩具数据和代码：
toy.to_json()
'{"count":{"P1550":46,"P1943":311,"P1974":353,"P2123":1,"P2305":451,"P2368":2,"P2378":468,"P2396":412,"P2403":1,"P2413":367},"mean":{"P1550":3.0,"P1943":24.0,"P1974":404.0,"P2123":27.0,"P2305":24.0,"P2368":3.0,"P2378":404.0,"P2396":13.0,"P2403":2.0,"P2413":2.0}}'
toy
count mean
promo_id
P1550 46 3.0
P1943 311 24.0
P1974 353 404.0
P2123 1 27.0
P2305 451 24.0
P2368 2 3.0
P2378 468 404.0
P2396 412 13.0
P2403 1 2.0
P2413 367 2.0
toy.sort_values(by = ['mean'], axis = 1)
KeyError: 'mean'