Python · OOP: classes and objects

A class is a blueprint, and an object (instance) is one thing built from it.

• __init__ runs when an object is created and sets up the instance attributes.
• self is the instance itself. Python passes it automatically, so obj.method(x) becomes Class.method(obj, x).
• Class attributes are defined in the class body and shared by every instance. Instance attributes are set on self and belong to one object.
• @property lets a method be read like an attribute. That’s useful for computed values or for validating input.

class Student:
    university = "PDEU"                  # class attribute (shared)

    def __init__(self, name, marks):
        self.name = name                 # instance attributes
        self._marks = marks

    @property
    def marks(self):
        return self._marks

    @marks.setter
    def marks(self, value):
        if not 0 <= value <= 100:
            raise ValueError("marks must be 0..100")
        self._marks = value

    def passed(self, cutoff=40):
        return self._marks >= cutoff

a = Student("Sahaj", 88)
b = Student("Ravi", 35)
print(a.name, a.marks, a.passed(), b.passed())
print(a.university, b.university, Student.university)
a.marks = 91
print(a.marks)
try:
    a.marks = 150
except ValueError as e:
    print("ValueError:", e)
print(Student.passed(b, cutoff=30))   # what a.method() really does

Output:

Sahaj 88 True False
PDEU PDEU PDEU
91
ValueError: marks must be 0..100
True