Class attributes in Python

Introduction

Class attributes store information shared by all objects of a class. They help keep common data in one place.

When you want all objects of a class to share the same value, like a company name for all employees.

To keep track of data that belongs to the class itself, such as counting how many objects have been created.

When you want to set default values that apply to every object unless changed.

To save memory by not repeating the same value inside every object.

Syntax

Python

class ClassName:
    class_attribute = value

Class attributes are defined directly inside the class, but outside any methods.

All instances share the same class attribute unless overridden in the instance.

Examples

Here, species is a class attribute shared by all Dog objects.

Python

class Dog:
    species = "Canis familiaris"

Both car1 and car2 share the same wheels class attribute.

Python

class Car:
    wheels = 4

car1 = Car()
car2 = Car()
print(car1.wheels)  # prints 4
print(car2.wheels)  # prints 4

This class attribute total_books counts how many Book objects are created.

Python

class Book:
    total_books = 0

    def __init__(self):
        Book.total_books += 1

Sample Program

This program shows how the class attribute school_name is shared by all Student objects. Changing it on the class changes it for all students.

Python

class Student:
    school_name = "Greenwood High"

    def __init__(self, name):
        self.name = name

# Create two students
student1 = Student("Alice")
student2 = Student("Bob")

# Print their school name (class attribute)
print(student1.name + " goes to " + student1.school_name)
print(student2.name + " goes to " + student2.school_name)

# Change class attribute
Student.school_name = "Sunrise Academy"

print(student1.name + " now goes to " + student1.school_name)
print(student2.name + " now goes to " + student2.school_name)

Important Notes

If you assign a value to a class attribute using an instance (like student1.school_name = 'New School'), it creates a new instance attribute and does not change the class attribute.

Use class attributes for data shared by all instances, and instance attributes for data unique to each object.

Summary

Class attributes hold data shared by all objects of a class.

They are defined inside the class but outside methods.

Changing a class attribute affects all instances unless they have their own attribute with the same name.

Practice

1. What is a class attribute in Python?
class Car:
wheels = 4
Here, what does wheels represent?

easy

A. A value shared by all Car objects

B. A value unique to each Car object

C. A method inside the Car class

D. A variable defined inside a method

Solution

1. Step 1: Identify the attribute location

   The attribute wheels is defined inside the class but outside any method.

2. Step 2: Understand class attribute behavior

   Attributes defined this way are shared by all instances of the class.

3. Final Answer:

   A value shared by all Car objects -> Option A

4. Quick Check:

   Class attribute = shared value [OK]

Hint: Class attributes are outside methods, shared by all instances [OK]

Common Mistakes:

• Thinking class attributes are unique per object
• Confusing class attributes with instance attributes
• Assuming class attributes are methods

2. Which of the following is the correct way to define a class attribute color with value "red" inside a class Fruit?

easy

A. class Fruit:
def __init__(self):
color = "red"

B. class Fruit:
def color(self):
return "red"

C. class Fruit:
def __init__(self, color):
self.color = color

D. class Fruit:
color = "red"

Solution

1. Step 1: Identify class attribute syntax

   A class attribute is defined directly inside the class, outside any method.

2. Step 2: Check each option

   class Fruit: color = "red" correctly defines color as a class attribute. class Fruit: def __init__(self): color = "red" defines a local variable inside __init__. class Fruit: def color(self): return "red" defines a method, not an attribute. class Fruit: def __init__(self, color): self.color = color defines an instance attribute.

3. Final Answer:

   class Fruit: color = "red" -> Option D

4. Quick Check:

   Class attribute = direct assignment inside class [OK]

Hint: Class attributes are assigned directly inside class, outside methods [OK]

Common Mistakes:

• Defining attribute inside __init__ without self
• Confusing methods with attributes
• Using self for class attributes

3. What will be the output of this code?

class Dog:
    species = "Canine"

dog1 = Dog()
dog2 = Dog()
Dog.species = "Wolf"
print(dog1.species)
print(dog2.species)

medium

A. Canine\nCanine

B. Wolf\nCanine

C. Wolf\nWolf

D. Canine\nWolf

Solution

1. Step 1: Understand class attribute change

   The attribute species is a class attribute shared by all instances.

2. Step 2: Effect of changing class attribute

   Changing Dog.species to "Wolf" updates the value for all instances that don't have their own species attribute.

3. Final Answer:

   Wolf\nWolf -> Option C

4. Quick Check:

   Changing class attribute affects all instances [OK]

Hint: Changing class attribute changes it for all instances without override [OK]

Common Mistakes:

• Thinking instances keep old class attribute values
• Confusing instance and class attributes
• Expecting different outputs for dog1 and dog2

4. Find the error in this code snippet:

class Book:
    pages = 100

    def __init__(self, pages):
        pages = pages

b = Book(200)
print(b.pages)

medium

A. The instance attribute pages is not set properly

B. The class attribute pages is overwritten incorrectly

C. Syntax error in __init__ method

D. Cannot print pages attribute directly

Solution

1. Step 1: Analyze __init__ method

   The line pages = pages assigns the parameter to itself, not to the instance.

2. Step 2: Understand instance attribute setting

   To set an instance attribute, it should be self.pages = pages. Without self., the instance attribute is not created.

3. Final Answer:

   The instance attribute pages is not set properly -> Option A

4. Quick Check:

   Use self.pages to set instance attribute [OK]

Hint: Use self.attribute to set instance attributes inside __init__ [OK]

Common Mistakes:

• Forgetting self. when assigning instance attributes
• Assuming parameter assignment sets instance attribute
• Confusing class and instance attributes

5. You want to keep track of how many objects of class Student are created using a class attribute count. Which code correctly updates count each time a new Student is made?

hard

A. class Student:
count = 0
def __init__(self):
self.count += 1

B. class Student:
count = 0
def __init__(self):
Student.count += 1

C. class Student:
def __init__(self):
count = 0
count += 1

D. class Student:
count = 0
def __init__(self):
count += 1

Solution

1. Step 1: Identify class attribute usage

   count is a class attribute, so it should be accessed via the class name inside methods.

2. Step 2: Check each option for correct increment

   class Student: count = 0 def __init__(self): Student.count += 1 uses Student.count += 1, correctly updating the class attribute. class Student: count = 0 def __init__(self): self.count += 1 tries to increment self.count, which creates an instance attribute instead. Options C and D have syntax or scope errors.

3. Final Answer:

   class Student: count = 0 def __init__(self): Student.count += 1 -> Option B

4. Quick Check:

   Update class attribute via ClassName.attribute [OK]

Hint: Use ClassName.attribute to update class attributes inside methods [OK]

Common Mistakes:

• Using self.attribute to update class attribute
• Defining count inside __init__ instead of class
• Forgetting to use class name to access class attribute