Instance Variables in Python

In this tutorial, you’ll learn about Python instance variables including data variables and function variables.

Introduction to the Python instance variables

In Python, class variables are bound to a class while instance variables are bound to a specific instance of a class. The instance variables are also called instance attributes.

The following defines a HtmlDocument class with two class variables:

from pprint import pprint


class HtmlDocument:
    version = 5
    extension = 'html'


pprint(HtmlDocument.__dict__)

print(HtmlDocument.extension)
print(HtmlDocument.version)

Output

mappingproxy({'__dict__': <attribute '__dict__' of 'HtmlDocument' objects>,
              '__doc__': None,
              '__module__': '__main__',
              '__weakref__': <attribute '__weakref__' of 'HtmlDocument' objects>,
              'extension': 'html',
              'version': 5})

The HtmlDocument class has two class variables: extension and version. Python stores these two variables in the __dict__ attribute.

When you access the class variables via the class, Python looks them up in the __dict__ of the class.

The following creates a new instance of the HtmlDocument class:

home = HtmlDocument()

The home is an instance of the HtmlDocument class. It has its own __dict__ attribute:

pprint(home.__dict__)

The home.__dict__ is now empty:

{}

The home.__dict__ stores the instance variables of the home object like the HtmlDocument.__dict__ stores the class variables of the HtmlDocument class.

Unlike the __dict__ attribute of a class, the type of the __dict__ attribute of an instance is a dictionary. For example:

print(type(home.__dict__))

Output

<class 'dict'>

Since a dictionary is mutable, you can mutate it e.g., adding a new element to the dictionary.

Python allows you to access the class variables from an instance of a class. For example:

print(home.extension)
print(home.version)

In this case, Python looks up the variables extension and version in home.__dict__ first. If it doesn’t find them there, it’ll go up to the class and look up in the HtmlDocument.__dict__.

However, if Python can find the variables in the __dict__ of the instance, it won’t look further in the __dict__ of the class.

The following defines the version variable in the home object:

home.version = 6

Python adds the version variable to the __dict__ attribute of the home object:

print(home.__dict__)

The __dict__ now contains one instance variable:

{'version': 6}

If you access the version attribute of the home object, Python will return the value of the version in the home.__dict__ dictionary:

print(home.version)

Output:

6

If you change the class variables, these changes also reflect in the instances of the class:

HtmlDocument.media_type = 'text/html'
print(home.media_type)

Output:

text/html

Initializing instance variables

In practice, you initialize instance variables for all instances of a class in the __init__ method.

For example, the following redefines the HtmlDocument class that has two instance variables name and contents

class HtmlDocument:
    version = 5
    extension = 'html'

    def __init__(self, name, contents):
        self.name = name
        self.contents = contents

When creating a new instance of the HtmlDocument, you need to pass the corresponding arguments like this:

blank = HtmlDocument('Blank', '')