Get longest streak of consecutive weeks by group in pandas

Question

Currently I'm working with weekly data for different subjects, but it might have some long streaks without data, so, what I want to do, is to just keep the longest streak of consecutive weeks for every id. My data looks like this:

id    week
1      8
1      15
1      60
1      61
1      62
2      10
2      11
2      12
2      13
2      25
2      26

My expected output would be:

id    week
1      60
1      61
1      62
2      10
2      11
2      12
2      13

I got a bit close, trying to mark with a 1 when week==week.shift()+1. The problem is this approach doesn't mark the first occurrence in a streak, and also I can't filter the longest one:

df.loc[ (df['id'] == df['id'].shift())&(df['week'] == df['week'].shift()+1),'streak']=1

This, according to my example, would bring this:

id    week  streak
1      8     nan
1      15    nan
1      60    nan
1      61    1
1      62    1
2      10    nan
2      11    1
2      12    1
2      13    1
2      25    nan
2      26    1

Any ideas on how to achieve what I want?

Answer 1

Try this:

df['consec'] = df.groupby(['id',df['week'].diff(-1).ne(-1).shift().bfill().cumsum()]).transform('count')

df[df.groupby('id')['consec'].transform('max') == df.consec]

Output:

   id  week  consec
2   1    60       3
3   1    61       3
4   1    62       3
5   2    10       4
6   2    11       4
7   2    12       4
8   2    13       4