Python Class Attributes

Summary: in this tutorial, you’ll learn about the Python class attributes and when to use them appropriately.

Introduction to class attributes

Let’s start with a simple Circle class:

class Circle: def __init__(self, radius): self.pi = 3.14159 self.radius = radius def area(self): return self.pi * self.radius**2 def circumference(self): return 2*self.pi * self.radius
Code language: Python (python)

The Circle class has two attributes pi and radius. It also has two methods that calculate the area and circumference of a circle.

Both pi and radius are called instance attributes. In other words, they belong to a specific instance of the Circle class. If you change the attributes of an instance, it won’t affect other instances.

Besides instance attributes, Python also supports class attributes. The class attributes don’t associate with any specific instance of the class. But they’re shared by all instances of the class.

If you’ve been programming in Java or C#, you’ll see that class attributes are similar to the static members, but not the same.

To define a class attribute, you place it outside of the __init__() method. For example, the following defines pi as a class attribute:

class Circle: pi = 3.14159 def __init__(self, radius): self.radius = radius def area(self): return self.pi * self.radius**2 def circumference(self): return 2 * self.pi * self.radius
Code language: Python (python)

After that, you can access the class attribute via instances of the class or via the class name:

object_name.class_attribute class_name.class_attribute
Code language: Oracle Rules Language (ruleslanguage)

In the area() and circumference() methods, we access the pi class attribute via the self variable.

Outside the Circle class, you can access the pi class attribute via an instance of the Circle class or directly via the Circle class. For example:

c = Circle(10) print(c.pi) print(Circle.pi)
Code language: Python (python)

Output:

3.14159 3.14159
Code language: Python (python)

How Python class attributes work

When you access an attribute via an instance of the class, Python searches for the attribute in the instance attribute list. If the instance attribute list doesn’t have that attribute, Python continues looking up the attribute in the class attribute list. Python returns the value of the attribute as long as it finds the attribute in the instance attribute list or class attribute list.

However, if you access an attribute, Python directly searches for the attribute in the class attribute list.

The following example defines a Test class to demonstrate how Python handles instance and class attributes.

class Test: x = 10 def __init__(self): self.x = 20 test = Test() print(test.x) # 20 print(Test.x) # 10
Code language: Python (python)

How it works.

The Test class has two attributes with the same name (x) one is the instance attribute and the other is a class attribute.

When we access the x attribute via the instance of the Test class, it returns 20 which is the variable of the instance attribute.

However, when we access the x attribute via the Test class, it returns 10 which is the value of the x class attribute.

When to use Python class attributes

Class attributes are useful in some cases such as storing class constants, tracking data across all instances, and defining default values.

1) Storing class constants

Since a constant doesn’t change from instance to instance of a class, it’s handy to store it as a class attribute.

For example, the Circle class has the pi constant that is the same for all instances of the class. Therefore, it’s a good candidate for the class attributes.

2) Tracking data across of all instances

The following adds the circle_list class attribute to the Circle class. When you create a new instance of the Circle class, the constructor adds the instance to the list:

class Circle: circle_list = [] pi = 3.14159 def __init__(self, radius): self.radius = radius # add the instance to the circle list self.circle_list.append(self) def area(self): return self.pi * self.radius**2 def circumference(self): return 2 * self.pi * self.radius c1 = Circle(10) c2 = Circle(20) print(len(Circle.circle_list)) # 2
Code language: Python (python)

3) Defining default values

Sometimes, you want to set a default value for all instances of a class. In this case, you can use a class attribute.

The following example defines a Product class. All the instances of the Product class will have a default discount specified by the default_discount class attribute:

class Product: default_discount = 0 def __init__(self, price): self.price = price self.discount = Product.default_discount def set_discount(self, discount): self.discount = discount def net_price(self): return self.price * (1 - self.discount) p1 = Product(100) print(p1.net_price()) # 100 p2 = Product(200) p2.set_discount(0.05) print(p2.net_price()) # 190
Code language: Python (python)

Summary

  • A class attribute is shared by all instances of the class. To define a class attribute, you place it outside of the __init__() method.
  • Use class_name.class_attribute or object_name.class_attribute to access the value of the class_attribute.
  • Use class attributes for storing class contants, track data across all instances, and setting default values for all instances of the class.
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