> This has a great SQL-ish API. Python is similar but starts to be a little clunky since it requires you to think about indices:

groupby has an as_index parameter for this very purpose

> Deducting the discount

You focus on doing the subtraction during the group by. Is there any good reason for this? You could either do it as a step before, or after summing up both columns. Putting too many things into one command is not good practice yet you benchmark the language based on how easy it is to do said bad practice