Task $27$

Let's make a smaller dataframe to use while we are working out the larger problem.

Create a dataframe called 'schools$\_$subset' from 'schools' that includes only Grade $3$ for the year $2012.$ We are not going to need all the columns, so only add district, grade, year, and mean scale score to the new dataframe.

Display the schools$\_$subset dataframe.

Task $28$

Look up the DataFrame rank method. We will use this method to set the ranks. Since we want the highest rank to be $1,$ we need to set the 'ascending' parameter to the rank call to False.

Make a function called add$\_$default$\_$rank that takes a dataframe the first parameter and a 'column' parameter with the default 'Mean Scale Score' as the second paramter.

This function should create a new column called 'Default Rank' in the passed dataframe. The value of the new column should be the rank for the passed 'column' parameter.

Demonstrate your add$\_$default$\_$rank function works by calling the function on the 'students$\_$subset' dataframe. Print the 'schools$\_$subset' dataframe before and after the invocation of the function to confirm that $(1)$ the 'Default Rank' column was added and $(2)$ that the rank assigned to each row is correct based upon the 'Mean Scaled Score' value.

Task $29$

Now that you have the add$\_$default$\_$rank function that works on a dataframe, you can use the apply method to apply that method to each group from a groupby.

Group the full 'schools' dataframe by grade and year, then apply add$\_$default$\_$rank to the groups. Store the results of this into a new 'schools$2\text{'}$ dataframe.

Display the resulting 'schools$2\text{'}$ dataframe.

Task $30$

The 'schools$2\text{'}$ now has a rank for each district for each grade and each year.

To fulfill the Mayor's ranking request, we can now produce the ordered list of districts with the top performers at the top of the list.

To do this, sum the 'Default Rank' column grouping by District and sorting the result using the sort$\_$values method with "ascending" set to True and show the results.