Calculating weighted mean SAT scores per state with apply

The groupby object has four methods that accept a function (or functions) to perform a calculation on each group. These four methods are .agg, .filter, .transform, and .apply. Each of the first three of these methods has a very specific output that the function must return. .agg must return a scalar value, .filter must return a Boolean, and .transform must return a Series or DataFrame with the same length as the passed group. The .apply method, however, may return a scalar value, a Series, or even a DataFrame of any shape, therefore making it very flexible. It is also called only once per group (on a DataFrame), while the .transform and .agg methods get called once for each aggregating column (on a Series). The .apply method's ability to return a single object when operating on multiple columns at the same time makes the calculation in this recipe possible.

In this recipe, we calculate the weighted average of both...