How to link two classes in python for eg a student class and a subject class such that a student class contains a number objects of subject class - python-3.x

Question

class Subject:
def __init__(self, name, midTerm, attend, classtest, endterm):
self.name = name
self.midTerm = midTerm
self.attend = attend
self.classtest = classtest
self.endterm = endterm
self.total = 0
self.grade = 'F'
newAttend = attendanceMarks(self.attend)
if newAttend != -1:
self.total = newAttend + midTermMarks(midTerm) + classTestMarks(classtest) + endTermMarks(endterm)
self.grade = gradeCal(self.total)
else:
self.grade = 'F'
class Student:
sub = []
def __init__(self, name, roll, s1, s2, s3, s4):
self.name = name
self.roll = roll
self.sub.append(s1)
self.sub.append(s2)
self.sub.append(s3)
self.sub.append(s4)
self.tTotal = self.sub[0].total + self.sub[1].total + self.sub[2].total + self.sub[3].total
self.tGrade = gradeCal(self.tTotal / 4)
st1 = Student("Akash", 17, Subject("Compiler Design", 50, 76, 50, 100), Subject("Software Engineering", 50, 76, 50, 100), Subject("Machine Learning", 50, 76, 50, 100), Subject("Natural Language Processing", 50, 76, 50, 100))
st2 = Student("Juan", 23, Subject("Compiler Design", 30, 76, 25, 50), Subject("Software Engineering", 25, 76, 34, 60), Subject("Machine Learning", 39, 76, 45, 66), Subject("Natural Language Processing", 32, 76, 44, 77))
st1.tTotal
st2.tTotal
Here both st1.tTotal and st2.tTotal gives the same output. Some of the methods used above are already defined and the code executes successfully. The total and grade inside subject class are calculated in terms of 100 marks and grades A-F respectively.

Answer 1

The reason why st1.tTotal and st2.tTotal give the same output is that you're defining Student.sub as a class variable so it is shared across all Student instances. You should instead make Student.sub an instance attribute by defining it in the __init__ method and bind it to self so that each Student object has its own sub list.
Change:
class Student:
sub = []
def __init__(self, name, roll, s1, s2, s3, s4):
self.name = name
self.roll = roll
self.sub.append(s1)
self.sub.append(s2)
self.sub.append(s3)
self.sub.append(s4)
to:
class Student:
def __init__(self, name, roll, s1, s2, s3, s4):
self.name = name
self.roll = roll
self.sub = []
self.sub.append(s1)
self.sub.append(s2)
self.sub.append(s3)
self.sub.append(s4)
or to allow a variable number of subjects for each Student, use a variable argument with an asterisk:
class Student:
def __init__(self, name, roll, *sub):
self.name = name
self.roll = roll
self.sub = sub