{"id":440,"date":"2020-10-10T14:00:01","date_gmt":"2020-10-10T14:00:01","guid":{"rendered":"https:\/\/www.pythontutorial.net\/?page_id=440"},"modified":"2020-11-13T06:39:10","modified_gmt":"2020-11-13T06:39:10","slug":"python-class-attributes","status":"publish","type":"page","link":"https:\/\/www.pythontutorial.net\/python-oop\/python-class-attributes\/","title":{"rendered":"Python Class Attributes"},"content":{"rendered":"\n

Summary<\/strong>: in this tutorial, you’ll learn about the Python class attributes and when to use them appropriately.<\/p>\n\n\n\n

Introduction to class attributes #<\/a><\/h2>\n\n\n\n

Let’s start with a simple Circle<\/code> class<\/a>:<\/p>\n\n\n

class<\/span> Circle<\/span>:<\/span>\n    def<\/span> __init__<\/span>(self, radius)<\/span>:<\/span>\n        self.pi = 3.14159<\/span>\n        self.radius = radius\n\n    def<\/span> area<\/span>(self)<\/span>:<\/span>\n        return<\/span> self.pi * self.radius**2<\/span>\n\n    def<\/span> circumference<\/span>(self)<\/span>:<\/span>\n        return<\/span> 2<\/span>*self.pi * self.radius\n<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
The Circle<\/code> class has two attributes pi<\/code> and radius<\/code>. It also has two methods that calculate the area and circumference of a circle.<\/p>\n\n\n\n
Both pi<\/code> and radius<\/code> are called instance attributes<\/strong>. In other words, they belong to a specific instance of the Circle<\/code> class. If you change the attributes of an instance, it won’t affect other instances.<\/p>\n\n\n\n
Besides instance attributes, Python also supports class attributes<\/strong>. The class attributes don’t associate with any specific instance of the class. But they’re shared by all instances of the class.<\/p>\n\n\n\n
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.<\/p>\n\n\n\n
To define a class attribute, you place it outside of the __init__()<\/code> method. For example, the following defines pi<\/code> as a class attribute:<\/p>\n\n\n
class<\/span> Circle<\/span>:<\/span>\n    pi = 3.14159<\/span>\n\n    def<\/span> __init__<\/span>(self, radius)<\/span>:<\/span>\n        self.radius = radius\n\n    def<\/span> area<\/span>(self)<\/span>:<\/span>\n        return<\/span> self.pi * self.radius**2<\/span>\n\n    def<\/span> circumference<\/span>(self)<\/span>:<\/span>\n        return<\/span> 2<\/span> * self.pi * self.radius\n<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
After that, you can access the class attribute via instances of the class or via the class name:<\/p>\n\n\n
object_name.class_attribute\nclass_name.class_attribute<\/code><\/span>Code language:<\/span> Oracle Rules Language<\/span> (<\/span>ruleslanguage<\/span>)<\/span><\/small><\/pre>\n\n\n
In the area()<\/code> and circumference()<\/code> methods, we access the pi<\/code> class attribute via the self<\/code> variable.<\/p>\n\n\n\n
Outside the Circle<\/code> class, you can access the pi<\/code> class attribute via an instance of the Circle<\/code> class or directly via the Circle<\/code> class. For example:<\/p>\n\n\n
c = Circle(10<\/span>)\nprint(c.pi)\nprint(Circle.pi)<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Output:<\/p>\n\n\n
3.14159<\/span>\n3.14159<\/span><\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
How Python class attributes work #<\/a><\/h2>\n\n\n\n
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.<\/p>\n\n\n\n
However, if you access an attribute, Python directly searches for the attribute in the class attribute list. <\/p>\n\n\n\n
The following example defines a Test<\/code> class to demonstrate how Python handles instance and class attributes.<\/p>\n\n\n
class<\/span> Test<\/span>:<\/span>\n    x = 10<\/span>\n\n    def<\/span> __init__<\/span>(self)<\/span>:<\/span>\n        self.x = 20<\/span>\n\n\ntest = Test()\nprint(test.x)  # 20<\/span>\nprint(Test.x)  # 10<\/span><\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
How it works.<\/p>\n\n\n\n
The Test<\/code> class has two attributes with the same name (x<\/code>) one is the instance attribute and the other is a class attribute.<\/p>\n\n\n\n
When we access the x<\/code> attribute via the instance of the Test<\/code> class, it returns 20 which is the variable of the instance attribute.<\/p>\n\n\n\n
However, when we access the x<\/code> attribute via the Test<\/code> class, it returns 10 which is the value of the x<\/code> class attribute.<\/p>\n\n\n\n
When to use Python class attributes #<\/a><\/h2>\n\n\n\n
Class attributes are useful in some cases such as storing class constants, tracking data across all instances, and defining default values.<\/p>\n\n\n\n
1) Storing class constants #<\/a><\/h3>\n\n\n\n
Since a constant doesn’t change from instance to instance of a class, it’s handy to store it as a class attribute.<\/p>\n\n\n\n
For example, the Circle<\/code> class has the pi<\/code> constant that is the same for all instances of the class. Therefore, it’s a good candidate for the class attributes.<\/p>\n\n\n\n
2) Tracking data across of all instances #<\/a><\/h3>\n\n\n\n
The following adds the circle_list<\/code> class attribute to the Circle<\/code> class. When you create a new instance of the Circle<\/code> class, the constructor adds the instance to the list:<\/p>\n\n\n
class<\/span> Circle<\/span>:<\/span>\n    circle_list = []\n    pi = 3.14159<\/span>\n\n    def<\/span> __init__<\/span>(self, radius)<\/span>:<\/span>\n        self.radius = radius\n        # add the instance to the circle list<\/span>\n        self.circle_list.append(self)\n\n    def<\/span> area<\/span>(self)<\/span>:<\/span>\n        return<\/span> self.pi * self.radius**2<\/span>\n\n    def<\/span> circumference<\/span>(self)<\/span>:<\/span>\n        return<\/span> 2<\/span> * self.pi * self.radius\n\n\nc1 = Circle(10<\/span>)\nc2 = Circle(20<\/span>)\n\nprint(len(Circle.circle_list))  # 2<\/span><\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
3) Defining default values #<\/a><\/h3>\n\n\n\n
Sometimes, you want to set a default value for all instances of a class. In this case, you can use a class attribute.<\/p>\n\n\n\n
The following example defines a Product<\/code> class. All the instances of the Product<\/code> class will have a default discount specified by the default_discount<\/code> class attribute:<\/p>\n\n\n
class<\/span> Product<\/span>:<\/span>\n    default_discount = 0<\/span>\n\n    def<\/span> __init__<\/span>(self, price)<\/span>:<\/span>\n        self.price = price\n        self.discount = Product.default_discount\n\n    def<\/span> set_discount<\/span>(self, discount)<\/span>:<\/span>\n        self.discount = discount\n\n    def<\/span> net_price<\/span>(self)<\/span>:<\/span>\n        return<\/span> self.price * (1<\/span> - self.discount)\n\n\np1 = Product(100<\/span>)\nprint(p1.net_price())\n # 100<\/span>\n\np2 = Product(200<\/span>)\np2.set_discount(0.05<\/span>)\nprint(p2.net_price())\n # 190<\/span><\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Summary #<\/a><\/h2>\n\n\n\n
  •  A class attribute is shared by all instances of the class. To define a class attribute, you place it outside of the __init__()<\/code> method.<\/li>
  • Use class_name.class_attribute<\/code> or object_name.class_attribute<\/code> to access the value of the class_attribute<\/code>. <\/li>
  • Use class attributes for storing class contants, track data across all instances, and setting default values for all instances of the class.<\/li><\/ul>\n
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