{"id":2527,"date":"2021-10-16T04:00:25","date_gmt":"2021-10-16T04:00:25","guid":{"rendered":"https:\/\/www.pythontutorial.net\/?page_id=2527"},"modified":"2025-03-31T07:53:34","modified_gmt":"2025-03-31T07:53:34","slug":"python-class-variables","status":"publish","type":"page","link":"https:\/\/www.pythontutorial.net\/python-oop\/python-class-variables\/","title":{"rendered":"Python Class Variables"},"content":{"rendered":"\n

Summary<\/strong>: in this tutorial, you’ll learn how the Python class variables (or attributes) work.<\/p>\n\n\n\n

Introduction to the Python class variables #<\/a><\/h2>\n\n\n\n

Everything in Python is an object including a class<\/a>. In other words, a class is an object in Python. <\/p>\n\n\n\n

When you define a class using the class<\/code> keyword, Python creates an object with the name the same as the class’s name. For example:<\/p>\n\n\n

class<\/span> HtmlDocument<\/span>:<\/span>\n   pass<\/span><\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
This example defines the HtmlDocument<\/code> class and the HtmlDocument<\/code> object. The HtmlDocument<\/code> object has the __name__<\/code> property:<\/p>\n\n\n
class<\/span> HtmlDocument<\/span>:<\/span>\n   pass<\/span>\n\nprint(HtmlDocument.__name__)<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Output:<\/p>\n\n\n
HtmlDocument<\/code><\/span><\/pre>\n\n\n
And the HTMLDocument<\/code> has the type of type<\/code>:<\/p>\n\n\n
class<\/span> HtmlDocument<\/span>:<\/span>\n   pass<\/span>\n\nprint(type(HtmlDocument))<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Output:<\/p>\n\n\n
<class<\/span> 'type<\/span>'><\/span><\/code><\/span>Code language:<\/span> HTML, XML<\/span> (<\/span>xml<\/span>)<\/span><\/small><\/pre>\n\n\n
It’s also an instance of the type<\/code> class:<\/p>\n\n\n
class<\/span> HtmlDocument<\/span>:<\/span>\n   pass<\/span>\n\nprint(isinstance(HtmlDocument, type))<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Output:<\/p>\n\n\n
True<\/span><\/code><\/span>Code language:<\/span> PHP<\/span> (<\/span>php<\/span>)<\/span><\/small><\/pre>\n\n\n
Class variables are bound to the class. They’re shared by all instances of that class.<\/p>\n\n\n\n
The following example adds the extension<\/code> and version<\/code> class variables to the HtmlDocument<\/code> class:<\/p>\n\n\n
class<\/span> HtmlDocument<\/span>:<\/span>\n    extension = 'html'<\/span>\n    version = '5'<\/span><\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Both extension<\/code> and version<\/code> are the class variables of the HtmlDocument<\/code> class. They’re bound to the HtmlDocument<\/code> class.<\/p>\n\n\n\n
Get the values of class variables #<\/a><\/h2>\n\n\n\n
To get the values of class variables, you use the dot notation (.<\/code>). For example:<\/p>\n\n\n
class<\/span> HtmlDocument<\/span>:<\/span>\n    extension = 'html'<\/span>\n    version = '5'<\/span>\n\nprint(HtmlDocument.extension) # html<\/span>\nprint(HtmlDocument.version) # 5<\/span><\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Output:<\/p>\n\n\n
html\n5<\/code><\/span><\/pre>\n\n\n
If you access a class variable that doesn’t exist, you’ll get an AttributeError<\/code> exception. For example:<\/p>\n\n\n
class<\/span> HtmlDocument<\/span>:<\/span>\n    extension = 'html'<\/span>\n    version = '5'<\/span>\n\nprint(HtmlDocument.media_type)<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Error:<\/p>\n\n\n
AttributeError: type object 'HtmlDocument'<\/span> has no attribute 'media_type'<\/span><\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Another way to get the value of a class variable is to use the getattr()<\/code> function. The getattr()<\/code> function accepts an object and a variable name. It returns the value of the class variable. For example:<\/p>\n\n\n
class<\/span> HtmlDocument<\/span>:<\/span>\n    extension = 'html'<\/span>\n    version = '5'<\/span>\n\nextension = getattr(HtmlDocument, 'extension'<\/span>)\nversion = getattr(HtmlDocument, 'version'<\/span>)\n\nprint(extension)  # html<\/span>\nprint(version)  # 5<\/span><\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Output:<\/p>\n\n\n
html\n5<\/code><\/span><\/pre>\n\n\n
If the class variable doesn’t exist, you’ll also get an AttributeError<\/code> exception. To avoid the exception, you can specify a default value if the class variable doesn’t exist like this:<\/p>\n\n\n
class<\/span> HtmlDocument<\/span>:<\/span>\n    extension = 'html'<\/span>\n    version = '5'<\/span>\n\nmedia_type = getattr(HtmlDocument, 'media_type'<\/span>, 'text\/html'<\/span>)\nprint(media_type) # text\/html<\/span><\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Output:<\/p>\n\n\n
text\/html<\/code><\/span><\/pre>\n\n\n
Set values for class variables #<\/a><\/h2>\n\n\n\n
To set a value for a class variable, you use the dot notation: <\/p>\n\n\n
HtmlDocument.version = 10<\/span><\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
setattr(HtmlDocument, 'version'<\/span>, 10<\/span>)<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Since Python is a dynamic language, you can add a class variable to a class at runtime after you have created it. For example, the following adds the media_type<\/code> class variable to the HtmlDocument<\/code> class:<\/p>\n\n\n
HtmlDocument.media_type = 'text\/html'<\/span>\nprint(HtmlDocument.media_type)<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Similarly, you can use the setattr()<\/code> function:<\/p>\n\n\n
setattr(HtmlDocument, media_type, 'text\/html'<\/span>)\nprint(HtmlDocument.media_type)<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Delete class variables #<\/a><\/h2>\n\n\n\n
To delete a class variable at runtime, you use the delattr()<\/code> function:<\/p>\n\n\n
delattr(HtmlDocument, 'version'<\/span>)<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Or you can use the del<\/code> keyword:<\/p>\n\n\n
del<\/span> HtmlDocument.version<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Class variable storage #<\/a><\/h2>\n\n\n\n
Python stores class variables in the __dict__<\/code> attribute. The __dict__<\/code> is a mapping proxy, which is a dictionary. For example:<\/p>\n\n\n
from<\/span> pprint import<\/span> pprint\n\n\nclass<\/span> HtmlDocument<\/span>:<\/span>\n    extension = 'html'<\/span>\n    version = '5'<\/span>\n\n\nHtmlDocument.media_type = 'text\/html'<\/span>\n\npprint(HtmlDocument.__dict__)<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Output:<\/p>\n\n\n
mappingproxy({'__dict__'<\/span>: <attribute '__dict__'<\/span> of 'HtmlDocument'<\/span> objects>,\n              '__doc__'<\/span>: None<\/span>,\n              '__module__'<\/span>: '__main__'<\/span>,\n              '__weakref__'<\/span>: <attribute '__weakref__'<\/span> of 'HtmlDocument'<\/span> objects>,\n              'extension'<\/span>: 'html'<\/span>,\n              'media_type'<\/span>: 'text\/html'<\/span>,\n              'version'<\/span>: '5'<\/span>})<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
The output indciates that the __dict__<\/code> has three class variables: extension<\/code>, media_type<\/code>, and version<\/code> besides other predefined class variables.<\/p>\n\n\n\n
Python does not allow you to change the __dict__<\/code> directly. For example, the following will result in an error:<\/p>\n\n\n
HtmlDocument.__dict__['released'<\/span>] = 2008<\/span><\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Output:<\/p>\n\n\n
TypeError: 'mappingproxy'<\/span> object does not<\/span> support item assignment<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
However, you can use the setattr()<\/code> function or dot notation to indirectly change the __dict__<\/code> attribute.<\/p>\n\n\n\n
Also, the key of the __dict__<\/code> are strings that will help Python speeds up the variable lookup.<\/p>\n\n\n\n
Although Python allows you to access class variables through the __dict__<\/code> dictionary, it’s not a good practice. Also, it won’t work in some situations. For example:<\/p>\n\n\n
print(HtmlDocument.__dict__['type'<\/span>]) # BAD CODE<\/span><\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Callable class attributes #<\/a><\/h2>\n\n\n\n
Class attributes can be any object such as a function. <\/p>\n\n\n\n
When you add a function to a class, the function becomes a class attribute. Since a function is callable, the class attribute is called a callable class attribute. For example:<\/p>\n\n\n
from<\/span> pprint import<\/span> pprint\n\n\nclass<\/span> HtmlDocument<\/span>:<\/span>\n    extension = 'html'<\/span>\n    version = '5'<\/span>\n\n    def<\/span> render<\/span>()<\/span>:<\/span>\n        print('Rendering the Html doc...'<\/span>)\n\n\npprint(HtmlDocument.__dict__)<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Output:<\/p>\n\n\n
mappingproxy({'__dict__'<\/span>: <attribute '__dict__'<\/span> of 'HtmlDocument'<\/span> objects>,\n              '__doc__'<\/span>: None<\/span>,\n              '__module__'<\/span>: '__main__'<\/span>,\n              '__weakref__'<\/span>: <attribute '__weakref__'<\/span> of 'HtmlDocument'<\/span> objects>,\n              'extension'<\/span>: 'html'<\/span>,\n              'render'<\/span>: <function HtmlDocument.render at 0x0000010710030310<\/span>>,\n              'version'<\/span>: '5'<\/span>})\nRendering the Html doc...<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
In this example, the render<\/code> is a class attribute of the HtmlDocument<\/code> class. Its value is a function.<\/p>\n\n\n\n
Summary #<\/a><\/h2>\n\n\n\n
Try it<\/a><\/p>\n\n\n\n
Try it<\/a><\/p>\n\n\n\n
or you can use the setattr()<\/a><\/code> built-in function:<\/p>\n\n\n
Try it<\/a><\/p>\n\n\n\n
Try it<\/a><\/p>\n\n\n\n
Try it<\/a><\/p>\n\n\n\n
Try it<\/a><\/p>\n\n\n\n
Try it<\/a><\/p>\n\n\n\n
Try it<\/a><\/p>\n\n\n\n