Python Pandas: Counting keys and summing up their values in a data frame

Question

Data frame df1 contains key, value pairs:

    key  val
0    1    7
1    2    5
2    2    5
3    3    4
4    3    4
5    3    4

How to get data frame df2 that for each key has a record with two fields: cnt equal to the number of times given key is found in df1 and sum equal to the sum of this key values ? Like this:

    cnt  key  sum
0    1    1    7
1    2    2   10
2    3    3   12

Answer 1

You can use agg with a list of summary functions:

df.groupby('key').val.agg(["count", "sum"]).reset_index()