Find cumcount and agg func based on past records of each group

Question

I have a dataframe like as shown below

df = pd.DataFrame(
    {'stud_name' : ['ABC', 'ABC','ABC','DEF', 
                  'DEF','DEF'],
     'qty' : [123,31,490,518,70,900],
     'trans_date' : ['13/11/2020','10/1/2018','11/11/2017','27/03/2016','13/05/2010','14/07/2008']})

I would like to do the below

a) for each stud_name, look at their past data (full past data) and compute the min, max and mean of qty column

Please note that the 1st record/row for every unique stud_name will be NA because there is no past data (history) to look at and compute the aggregate statistics

I tried something like below but the output is incorrect

df['trans_date'] = pd.to_datetime(df['trans_date'])
df.sort_values(by=['stud_name','trans_date'],inplace=True)
df['past_transactions'] = df.groupby('stud_name').cumcount()
df['past_max_qty'] = df.groupby('stud_name')['qty'].expanding().max().values
df['past_min_qty'] = df.groupby('stud_name')['qty'].expanding().min().values
df['past_avg_qty'] = df.groupby('stud_name')['qty'].expanding().mean().values

I expect my output to be like as shown below

Answer 1

We can use custom function to calculate the past statistics per student

def past_stats(q):
    return (
        q.expanding()
        .agg(['max', 'min', 'mean'])
        .shift().add_prefix('past_')
    )

df.join(df.groupby('stud_name')['qty'].apply(past_stats))

---

  stud_name  qty trans_date  past_max  past_min  past_mean
2       ABC  490 2017-11-11       NaN       NaN        NaN
1       ABC   31 2018-10-01     490.0     490.0      490.0
0       ABC  123 2020-11-13     490.0      31.0      260.5
5       DEF  900 2008-07-14       NaN       NaN        NaN
4       DEF   70 2010-05-13     900.0     900.0      900.0
3       DEF  518 2016-03-27     900.0      70.0      485.0