This is a continuation of my Sales Report in Pandas post and it begins with the final table from it. Here it is for one random customer.

report[report['CustomerID'] == 12347.0]

It represents an activity of this particular customer for the whole available date range aggregated by month. You can see that this activity is not regular.

First, I want to add a flag whether or not customer was active in the previous month.

report['active_prev'] = (report.sort_values(by=['month'], ascending=True)
.groupby(['CustomerID'])['active'].shift(1))

This hard stuff is something like a window function in sql. Because I have many customers I have to somehow say to pandas that they should be treated separately, so that’s what groupby for. And also I want months to be in order, that’s what sort_values for. Finally, shift gives the value from the previous row (month in this case). …