Pandas: filling missing values by mean in each group

Question

This should be straightforward, but the closest thing I\'ve found is this post: pandas: Filling missing values within a group, and I still can\'t solve my problem....

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Answer 1

fillna + groupby + transform + mean

This seems intuitive:

df['value'] = df['value'].fillna(df.groupby('name')['value'].transform('mean'))

The groupby + transform syntax maps the groupwise mean to the index of the original dataframe. This is roughly equivalent to @DSM's solution, but avoids the need to define an anonymous lambda function.