Numerical Computing with Python

How it works...

Throughout this recipe, the query method is used to filter data instead of boolean indexing. Refer to the Improving readability of Boolean indexing with the query method recipe from Chapter 11, Boolean Indexing, for more information.

Our goal is to find the percentage weight loss for each month for each person. One way to accomplish this task is to calculate each week's weight loss relative to the start of each month. This specific task is perfectly suited to the transform groupby method. The transform method accepts a function as its one required parameter. This function gets implicitly passed each non-grouping column (or only the columns specified in the indexing operator as was done in this recipe with Weight). It must ...