擅长:python、mysql、java
<p><code>groupby + transform</code>然后填写:</p>
<pre><code>df['Weight'].fillna(df.groupby("Class")['Weight'].transform("mean"))
</code></pre>
<hr/>
<pre><code>0 5.600000
1 8.600000
2 8.366667
3 5.900000
4 5.650000
5 5.716667
6 8.100000
7 8.400000
Name: Weight, dtype: float64
</code></pre>