{"id":2536,"date":"2021-10-16T07:41:18","date_gmt":"2021-10-16T07:41:18","guid":{"rendered":"https:\/\/www.pythontutorial.net\/?page_id=2536"},"modified":"2025-03-31T07:56:08","modified_gmt":"2025-03-31T07:56:08","slug":"python-instance-variables","status":"publish","type":"page","link":"https:\/\/www.pythontutorial.net\/python-oop\/python-instance-variables\/","title":{"rendered":"Python Instance Variables"},"content":{"rendered":"\n

Summary<\/strong>: in this tutorial, you’ll learn about Python instance variables including data variables and function variables.<\/p>\n\n\n\n

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

In Python, class variables<\/a> are bound to a class<\/a> while instance variables are bound to a specific instance of a class. The instance variables are also called instance attributes.<\/p>\n\n\n\n

The following defines a HtmlDocument<\/code> class with two class variables:<\/p>\n\n\n

from<\/span> pprint import<\/span> pprint\n\n\nclass<\/span> HtmlDocument<\/span>:<\/span>\n    version = 5<\/span>\n    extension = 'html'<\/span>\n\n\npprint(HtmlDocument.__dict__)\n\nprint(HtmlDocument.extension)\nprint(HtmlDocument.version)<\/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              'version'<\/span>: 5<\/span>})<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
The HtmlDocument<\/code> class has two class variables: extension<\/code> and version<\/code>. Python stores these two variables in the __dict__<\/code> attribute.<\/p>\n\n\n\n
When you access the class variables via the class, Python looks them up in the __dict__<\/code> of the class. <\/p>\n\n\n\n
The following creates a new instance of the HtmlDocument<\/code> class:<\/p>\n\n\n
home = HtmlDocument()<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
The home<\/code> is an instance of the HtmlDocument<\/code> class. It has its own __dict__<\/code> attribute:<\/p>\n\n\n
pprint(home.__dict__)<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
The home.__dict__<\/code> is now empty:<\/p>\n\n\n
{}<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
The home.__dict__<\/code> stores the instance variables of the home<\/code> object like the HtmlDocument.__dict__<\/code> stores the class variables of the HtmlDocument<\/code> class. <\/p>\n\n\n\n
print(type(home.__dict__))<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Output:<\/p>\n\n\n
<class<\/span> 'dict<\/span>'><\/span><\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Python allows you to access the class variables<\/a> from an instance of a class. For example:<\/p>\n\n\n
print(home.extension)\nprint(home.version)<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
In this case, Python looks up the variables extension and version in home.__dict__<\/code> first. If it doesn’t find them there, it’ll go up to the class and look up in the HtmlDocument.__dict__<\/code>.<\/p>\n\n\n\n
However, if Python can find the variables in the __dict__<\/code> of the instance, it won’t look further in the __dict__<\/code> of the class.<\/p>\n\n\n\n
The following defines the version<\/code> variable in the home<\/code> object:<\/p>\n\n\n
home.version = 6<\/span><\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Python adds the version<\/code> variable to the __dict__<\/code> attribute of the home<\/code> object:<\/p>\n\n\n
print(home.__dict__)<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
The __dict__<\/code> now contains one instance variable:<\/p>\n\n\n
{'version'<\/span>: 6<\/span>}<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
If you access the version attribute of the home<\/code> object, Python will return the value of the version<\/code> in the home.__dict__<\/code> dictionary:<\/p>\n\n\n
print(home.version)<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Output:<\/p>\n\n\n
6<\/span><\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
If you change the class variables, these changes also reflect in the instances of the class:<\/p>\n\n\n
HtmlDocument.media_type = 'text\/html'<\/span>\nprint(home.media_type)<\/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>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Initializing instance variables #<\/a><\/h2>\n\n\n\n
In practice, you initialize instance variables for all instances of a class in the __init__<\/a><\/code> method.<\/p>\n\n\n\n
For example, the following redefines the HtmlDocument<\/code> class that has two instance variables name<\/code> and contents<\/code><\/p>\n\n\n
class<\/span> HtmlDocument<\/span>:<\/span>\n    version = 5<\/span>\n    extension = 'html'<\/span>\n\n    def<\/span> __init__<\/span>(self, name, contents)<\/span>:<\/span>\n        self.name = name\n        self.contents = contents<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
When creating a new instance of the HtmlDocument<\/code>, you need to pass the corresponding arguments like this:<\/p>\n\n\n
blank = HtmlDocument('Blank'<\/span>, ''<\/span>)<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Summary #<\/a><\/h2>\n\n\n\n
Since a dictionary<\/a> is mutable<\/a>, you can mutate it e.g., adding a new element to the dictionary.<\/p>\n\n\n\n
Unlike the __dict__<\/code> attribute of a class, the type of the __dict__<\/code> attribute of an instance is a dictionary<\/a>. For example:<\/p>\n\n\n
Try it<\/a><\/p>\n\n\n\n