defaultdict sample

 from collections import defaultdict

student_courses = [('alice','math'),('bob','math'),('alice','science')]

course_students = defaultdict(list)

student_courses_dd = defaultdict(list)

for student,course in student_courses:

  course_students[course].append(student)

  student_courses_dd[student].append(course)

for i in dict(course_students).items():

  print(*i)

print()

for i in dict(student_courses_dd).items():

  print(*i)

Output:

math ['alice', 'bob']
science ['alice']

alice ['math', 'science']
bob ['math']