可能的解决方案：

out = df1.set_index('ID')
out.update(df2.set_index('ID'))
out = out.reset_index()

输出：

ID  PRICE  WEIGHT  STOCK
0   1     20   100.0   30.0
1   2     30    90.0   40.0
2   3     40   400.0   70.0

import pandas as pd

df1 = pd.DataFrame({'ID': [1, 2, 3],
                   'PRICE': [20, 30, 40],
                   'WEIGHT': [100, 200, 300],
                   'STOCK': [30, 10, 20],
                   })

df2 = pd.DataFrame({'ID': [2, 3],
                   'WEIGHT': [90, 400],
                   'STOCK': [40, 70],
                   })

df2 = df2.set_index('ID')
df1 = df1.set_index('ID')
df1.loc[df2.index,['WEIGHT', 'STOCK']] = df2[['WEIGHT', 'STOCK']]

print(df1.reset_index())

ID  PRICE  WEIGHT  STOCK
0   1     20     100     30
1   2     30      90     40
2   3     40     400     70