{"id":137,"date":"2024-01-25T13:32:36","date_gmt":"2024-01-25T13:32:36","guid":{"rendered":"https:\/\/learnpython.elegantwallp.com\/?p=137"},"modified":"2024-01-25T13:32:37","modified_gmt":"2024-01-25T13:32:37","slug":"python-class-variables","status":"publish","type":"post","link":"https:\/\/learnpython.elegantwallp.com\/2024\/01\/25\/python-class-variables\/","title":{"rendered":"Python Class Variables"},"content":{"rendered":"\n

Summary<\/strong>: in this tutorial, you\u2019ll learn how the Python class variables (or attributes) work.<\/p>\n\n\n\n

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

Everything in Python is an object including a\u00a0class. In other words, a class is an object in Python.<\/p>\n\n\n\n

When you define a class using the\u00a0class<\/code>\u00a0keyword, Python creates an object with the name the same as the class\u2019s name. For example:<\/p>\n\n\n\n

class HtmlDocument: pass<\/code>Code language: Python (python)<\/small><\/code><\/pre>\n\n\n\n
This example defines the\u00a0HtmlDocument<\/code>\u00a0class and the\u00a0HtmlDocument<\/code>\u00a0object. The\u00a0HtmlDocument<\/code>\u00a0object has the\u00a0__name__<\/code>\u00a0property:<\/p>\n\n\n\n
print(HtmlDocument.__name__) # HtmlDocument<\/em><\/code>Code language: Python (python)<\/small><\/code><\/pre>\n\n\n\n
And the\u00a0HTMLDocument<\/code>\u00a0has the type of\u00a0type<\/code>:<\/p>\n\n\n\n
print(type(HtmlDocument)) # <class 'type'><\/em><\/code>Code language: Python (python)<\/small><\/code><\/pre>\n\n\n\n
It\u2019s also an instance of the\u00a0type<\/code>\u00a0class:<\/p>\n\n\n\n
print(isinstance(HtmlDocument, type)) # True<\/em><\/code>Code language: Python (python)<\/small><\/code><\/pre>\n\n\n\n
Class variables are bound to the class. They\u2019re shared by all instances of that class.<\/p>\n\n\n\n
The following example adds the\u00a0extension<\/code>\u00a0and\u00a0version<\/code>\u00a0class variables to the\u00a0HtmlDocument<\/code>\u00a0class:<\/p>\n\n\n\n
class HtmlDocument: extension = 'html' version = '5'<\/code>Code language: Python (python)<\/small><\/code><\/pre>\n\n\n\n
Both extension<\/code> and version<\/code> are the class variables of the HtmlDocument<\/code> class. They\u2019re bound to the HtmlDocument<\/code> class.<\/p>\n\n\n\n
Get the values of class variables<\/h2>\n\n\n\n
To get the values of class variables, you use the dot notation (.<\/code>). For example:<\/p>\n\n\n\n
print(HtmlDocument.extension) # html<\/em> print(HtmlDocument.version) # 5<\/em><\/code>Code language: Python (python)<\/small><\/code><\/pre>\n\n\n\n
If you access a class variable that doesn\u2019t exist, you\u2019ll get an\u00a0AttributeError<\/code>\u00a0exception. For example:<\/p>\n\n\n\n
HtmlDocument.media_type<\/code>Code language: Python (python)<\/small><\/code><\/pre>\n\n\n\n
Error:<\/p>\n\n\n\n
AttributeError: type object 'HtmlDocument' has no attribute 'media_type'<\/code>Code language: Python (python)<\/small><\/code><\/pre>\n\n\n\n
Another way to get the value of a class variable is to use the\u00a0getattr()<\/code>\u00a0function. The\u00a0getattr()<\/code>\u00a0function accepts an object and a variable name. It returns the value of the class variable. For example:<\/p>\n\n\n\n
extension = getattr(HtmlDocument, 'extension') version = getattr(HtmlDocument, 'version') print(extension) # html<\/em> print(version) # 5<\/em><\/code>Code language: Python (python)<\/small><\/code><\/pre>\n\n\n\n
If the class variable doesn\u2019t exist, you\u2019ll also get an\u00a0AttributeError<\/code>\u00a0exception. To avoid the exception, you can specify a default value if the class variable doesn\u2019t exist like this:<\/p>\n\n\n\n
media_type = getattr(HtmlDocument, 'media_type', 'text\/html') print(media_type) # text\/html<\/em><\/code>Code language: Python (python)<\/small><\/code><\/pre>\n\n\n\n
Set values for class variables<\/h2>\n\n\n\n
To set a value for a class variable, you use the dot notation:<\/p>\n\n\n\n
HtmlDocument.version = 10<\/code>Code language: Python (python)<\/small><\/code><\/pre>\n\n\n\n
or you can use the\u00a0setattr()<\/code>\u00a0built-in function:<\/p>\n\n\n\n
setattr(HtmlDocument, 'version', 10)<\/code>Code language: Python (python)<\/small><\/code><\/pre>\n\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\u00a0media_type<\/code>\u00a0class variable to the\u00a0HtmlDocument<\/code>\u00a0class:<\/p>\n\n\n\n
HtmlDocument.media_type = 'text\/html' print(HtmlDocument.media_type)<\/code>Code language: Python (python)<\/small><\/code><\/pre>\n\n\n\n
Similarly, you can use the\u00a0setattr()<\/code>\u00a0function:<\/p>\n\n\n\n
setattr(HtmlDocument, media_type, 'text\/html') print(HtmlDocument.media_type)<\/code>Code language: Python (python)<\/small><\/code><\/pre>\n\n\n\n
Delete class variables<\/h2>\n\n\n\n
To delete a class variable at runtime, you use the\u00a0delattr()<\/code>\u00a0function:<\/p>\n\n\n\n
delattr(HtmlDocument, 'version')<\/code>Code language: Python (python)<\/small><\/code><\/pre>\n\n\n\n
Or you can use the\u00a0del<\/code>\u00a0keyword:<\/p>\n\n\n\n
del HtmlDocument.version<\/code>Code language: Python (python)<\/small><\/code><\/pre>\n\n\n\n
Class variable storage<\/h2>\n\n\n\n
Python stores class variables in the\u00a0__dict__<\/code>\u00a0attribute. The\u00a0__dict__<\/code>\u00a0is a mapping proxy, which is a dictionary. For example:<\/p>\n\n\n\n
from pprint import pprint class HtmlDocument: extension = 'html' version = '5' HtmlDocument.media_type = 'text\/html' pprint(HtmlDocument.__dict__)<\/code>Code language: Python (python)<\/small><\/code><\/pre>\n\n\n\n
Output:<\/p>\n\n\n\n
mappingproxy({'__dict__': <attribute '__dict__' of 'HtmlDocument' objects>, '__doc__': None, '__module__': '__main__', '__weakref__': <attribute '__weakref__' of 'HtmlDocument' objects>, 'extension': 'html', 'media_type': 'text\/html', 'version': '5'})<\/code>Code language: Python (python)<\/small><\/code><\/pre>\n\n\n\n
As clearly shown in the output, 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\u00a0__dict__<\/code>\u00a0directly. For example, the following will result in an error:<\/p>\n\n\n\n
HtmlDocument.__dict__['released'] = 2008<\/code>Code language: Python (python)<\/small><\/code><\/pre>\n\n\n\n
Output:<\/p>\n\n\n\n
TypeError: 'mappingproxy' object does not support item assignment<\/code>Code language: Python (python)<\/small><\/code><\/pre>\n\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\u00a0__dict__<\/code>\u00a0dictionary, it\u2019s not a good practice. Also, it won\u2019t work in some situations. For example:<\/p>\n\n\n\n
print(HtmlDocument.__dict__['type']) # BAD CODE<\/em><\/code>Code language: Python (python)<\/small><\/code><\/pre>\n\n\n\n
Callable class attributes<\/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\n
from pprint import pprint class HtmlDocument: extension = 'html' version = '5' def render(): print('Rendering the Html doc...') pprint(HtmlDocument.__dict__)<\/code>Code language: Python (python)<\/small><\/code><\/pre>\n\n\n\n
Output:<\/p>\n\n\n\n
mappingproxy({'__dict__': <attribute '__dict__' of 'HtmlDocument' objects>, '__doc__': None, '__module__': '__main__', '__weakref__': <attribute '__weakref__' of 'HtmlDocument' objects>, 'extension': 'html', 'render': <function HtmlDocument.render at 0x0000010710030310>, 'version': '5'}) Rendering the Html doc...<\/code>Code language: Python (python)<\/small><\/code><\/pre>\n\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","protected":false},"excerpt":{"rendered":"
Summary: in this tutorial, you\u2019ll learn how the Python class variables (or attributes) work. Introduction to the Python class variables Everything in Python is an object including a\u00a0class. In other words, a class is an object in Python. When you define a class using the\u00a0class\u00a0keyword, Python creates an object with the name the same as […]<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[25],"tags":[],"class_list":["post-137","post","type-post","status-publish","format-standard","hentry","category-1-classes-and-objects"],"_links":{"self":[{"href":"https:\/\/learnpython.elegantwallp.com\/wp-json\/wp\/v2\/posts\/137","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/learnpython.elegantwallp.com\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/learnpython.elegantwallp.com\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/learnpython.elegantwallp.com\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/learnpython.elegantwallp.com\/wp-json\/wp\/v2\/comments?post=137"}],"version-history":[{"count":1,"href":"https:\/\/learnpython.elegantwallp.com\/wp-json\/wp\/v2\/posts\/137\/revisions"}],"predecessor-version":[{"id":138,"href":"https:\/\/learnpython.elegantwallp.com\/wp-json\/wp\/v2\/posts\/137\/revisions\/138"}],"wp:attachment":[{"href":"https:\/\/learnpython.elegantwallp.com\/wp-json\/wp\/v2\/media?parent=137"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/learnpython.elegantwallp.com\/wp-json\/wp\/v2\/categories?post=137"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/learnpython.elegantwallp.com\/wp-json\/wp\/v2\/tags?post=137"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}