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 pprintclass 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 = 6Code 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:
6Code 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/htmlCode 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.