Class and instance variables

Definition

A class variable belongs to the class and is shared by all its objects. An instance variable belongs to one object and is set through self. Reading looks on the object first and then on the class.

In simple words

Every student in a school shares the same school name, a class variable. Each has their own roll number, an instance variable.

Why it matters

Mixing them up causes a famous bug: a list stored as a class variable gets shared, so adding to one object's list changes everyone's.

How it works

In code

class_vars.py

class Student:
    school = "Pathshala"        # class variable: shared
    shared_marks = []           # the trap

    def __init__(self, name):
        self.name = name        # instance variable: one per object
        self.marks = []

a = Student("Riya")
c = Student("Aman")
print(a.school, c.school)

a.marks.append(90)
print(a.marks, c.marks)

a.shared_marks.append(90)
print(a.shared_marks, c.shared_marks)   # both changed

Student.school = "New School"
print(a.school)
a.school = "Mine only"                  # now hides the class variable, on a only
print(a.school, c.school)

Output

Pathshala Pathshala
[90] []
[90] [90]
New School
Mine only New School

Common mistakes

Putting items = [] at class level and expecting one list per object.
Assigning self.count += 1 to update a shared counter. It creates an instance variable. Write Student.count += 1.

Check yourself

1. Where should per-object data be created?

2. Two objects share a list at class level. One appends. What does the other see?

Summary

Class variables are shared, and instance variables are per object.
Create mutable data in __init__.

Finished reading?

Sources: The Python Tutorial, python.org