Pandas Cookbook

Filtering for states with a minority majority

In Chapter 4, Selecting Subsets of Data, we marked every row as True or False before filtering out the False rows. In a similar fashion, it is possible to mark entire groups of data as either True or False before filtering out the False groups. To do this, we first form groups with the groupby method and then apply the filter method. The filter method accepts a function that must return either True or False to indicate whether a group is kept or not.

This filter method applied after a call to the groupby method is completely different than the DataFrame filter method covered in the Selecting columns with methods recipe from Chapter 2, Essential DataFrame Operations.