allStudentsAAA = ['Bob', 'John', 'Carl', 'Joe', 'Hao', 'Ali']

# Create a set of allStudents
allStudentsSet = set(allStudentsAAA)

# Create an empty set
gradedStudents = set() # Empty set


# Fill in the gradedStudents set with people that have grades
def whichHaveGrades():
    global allStudentsSet, gradedStudents
    for student in allStudentsSet:
        answer = raw_input('Does %s have a grade?' %(student))
        if answer == 'y':
           gradedStudents.add(student)

def main():
    global allStudentsSet, gradedStudents
    whichHaveGrades()

    # Display the ones without grades
    # How?
    ungraded = allStudentsSet.difference(gradedStudents)
    print ungraded


main()