Pandas: filling missing values by mean in each group

One way would be to use transform:

>>> df
  name  value
0    A      1
1    A    NaN
2    B    NaN
3    B      2
4    B      3
5    B      1
6    C      3
7    C    NaN
8    C      3
>>> df["value"] = df.groupby("name").transform(lambda x: x.fillna(x.mean()))
>>> df
  name  value
0    A      1
1    A      1
2    B      2
3    B      2
4    B      3
5    B      1
6    C      3
7    C      3
8    C      3