根据其他列用该列中的其他值填充 NaN 值

Question

我有一个像这样的熊猫数据框：

INPUT df:

Student_ID   Subject     Weekly_Feedback
100101         NaN         Excellent: 5
100101         NaN         very good:4
100101       Physics       Good: 3
100101         NaN         Excellent: 5
100102         NaN         Excellent: 5
100102       Chemistry     Good: 3
100102         NaN         Good: 3
100102         NaN         very Good: 3
100102         NaN         Needs improvement: 2

我想使用 Subject 填充 NaN 列中的行，该列具有 Student_ID 值的相应主题。

Expected OUTPUT df:

Student_ID   Subject     Weekly_Feedback
100101       Physics      Excellent: 5
100101       Physics      very good:4
100101       Physics      Good: 3
100101       Physics      Excellent: 5
100102       Chemistry    Excellent: 5
100102       Chemistry    Good: 3
100102       Chemistry    Good: 3
100102       Chemistry    very Good: 3
100102       Chemistry    Needs improvement: 2