explain

This code defines a function average_subject_grades that calculates the average grade for each subject from a list of subject_grades.

The function starts by creating an empty dictionary subject_average_grades, which will be used to store the accumulated total grade and number of grades for each subject.

In the first for loop, the code iterates over each subject_grade in the input list subject_grades. For each iteration, it extracts the subject and the grade from the subject_grade object, and adds it to the dictionary subject_average_grades. If the subject is not already a key in the dictionary, the code initializes a new entry with the subject as the key and a list [grade, 1] as the value, where grade is the first grade for the subject and 1 is the count of grades for the subject. If the subject already exists as a key in the dictionary, the code adds the grade to the accumulated total grade and increments the count by 1.

In the second for loop, the code iterates over the items in the dictionary subject_average_grades, which are pairs of subject and the list (total_grade, count). The code then calculates the average grade for each subject by dividing the total grade by the count and updates the value for the subject key in the dictionary subject_average_grades.

Finally, the function returns the dictionary subject_average_grades, which contains the average grade for each subject.

The code then creates a list of subject grades using a list comprehension and passes it to the average_subject_grades function. The resulting dictionary of average subject grades is then iterated over and printed, with a message indicating the average grade for each subject.