Python Instance Variables

Summary: 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)
Code language: Python (python)

Output:

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

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()
Code language: Python (python)

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

pprint(home.__dict__)
Code language: Python (python)

The home.__dict__ is now empty:

{}
Code language: Python (python)

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__))
Code language: Python (python)

Output:

<class 'dict'>
Code language: Python (python)

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)
Code language: Python (python)

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
Code language: Python (python)

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

print(home.__dict__)
Code language: Python (python)

The __dict__ now contains one instance variable:

{'version': 6}
Code language: Python (python)

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)
Code language: Python (python)

Output:

6
Code language: Python (python)

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)
Code language: Python (python)

Output:

text/html
Code language: Python (python)

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
Code language: Python (python)

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

blank = HtmlDocument('Blank', '')
Code language: Python (python)

Summary

  • Instance variables are bound to a specific instance of a class.
  • Python stores instance variables in the __dict__ attribute of the instance. Each instance has its own __dict__ attribute and the keys in this __dict__ may be different.
  • When you access a variable via the instance, Python finds the variable in the __dict__ attribute of the instance. If it cannot find the variable, it goes up and look it up in the __dict__ attribute of the class.
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