Pandas: How to fill null values with mean of a groupby?

Question

I have a dataset will some missing data that looks like this:

id    category     value
1     A            NaN
2     B            NaN
3     A            10.5
         


        

Answer 1

You can also use GroupBy + transform to fill NaN values with groupwise means. This method avoids inefficient apply + lambda. For example:

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