如何使用 groupby() 函数计算 DataFrame 中按特定列分组数据的平均值？

df = pd.dataframe(    {        "major": ["math", "physics", "math", "math", "physics", "math", "math", "physics", "demography"],        "grade": ["3", "2", "2", "1", "1", "3", "3", "2", "2"],        "age": [20, 18, 19, 21, 20, 19, 19, 18, 20],    })

df.groupby(["major"]).mean("grade")

Grade                1           2           3MajorDemography<br>Math<br>Pyhsics