Using Pandas join to fill in columns

Question

I have two DataFrames that roughly look like

(ID) (Category) (Value1)  (Value2)

111   1          5          7
112   1          3          8
113   2          6          9
114   3          2          6

and

(Category)  (Value1 Average for Category) (Value2 Average for Category)

1              4                              5 
2              6                              7
3              9                              2

Ultimately, I'd like to join the two DataFrames so that each ID can have the average value for its category in the row with it. I'm having trouble finding the right way to join/merge/etc. that will fill in columns by checking the category from the other DateFrame. Does anyone have any idea where to start?

Answer 1

只需在第一个数据帧上执行df1.groupby(['ID', 'Category']).transform(func='mean')即可获得所需的数据帧。

Answer 2

You are simply looking for a join , in pandas we use pd.merge for that like the following:

df3 = pd.merge(df1, df2, on='Category')

    ID  Category    Value1  Value2  Value 1 Average Value 2 Average
0   111 1           5       7       4               5
1   112 1           3       8       4               5
2   113 2           6       9       6               7
3   114 3           2       6       9               2

Official documentation of pandas on merging:
https://pandas.pydata.org/pandas-docs/stable/reference/api/pandas.DataFrame.merge.html

Here is a good explanation on joins: Pandas Merging 101