Python Class Variables

Summary: in this tutorial, you’ll learn how the Python class variables (or attributes) work.

Introduction to the Python class variables #

Everything in Python is an object including a class. In other words, a class is an object in Python.

When you define a class using the class keyword, Python creates an object with the name the same as the class’s name. For example:

class HtmlDocument:
   passCopy

This example defines the HtmlDocument class and the HtmlDocument object. The HtmlDocument object has the __name__ property:

class HtmlDocument:
   pass

print(HtmlDocument.__name__)Copy

Output:

HtmlDocumentCopy

And the HTMLDocument has the type of type:

class HtmlDocument:
   pass

print(type(HtmlDocument))Copy

Try it

Output:

<class 'type'>Copy

It’s also an instance of the type class:

class HtmlDocument:
   pass

print(isinstance(HtmlDocument, type))Copy

Try it

Output:

TrueCopy

Class variables are bound to the class. They’re shared by all instances of that class.

The following example adds the extension and version class variables to the HtmlDocument class:

class HtmlDocument:
    extension = 'html'
    version = '5'Copy

Both extension and version are the class variables of the HtmlDocument class. They’re bound to the HtmlDocument class.

Get the values of class variables #

To get the values of class variables, you use the dot notation (.). For example:

class HtmlDocument:
    extension = 'html'
    version = '5'

print(HtmlDocument.extension) # html
print(HtmlDocument.version) # 5Copy

Try it

Output:

html
5Copy

If you access a class variable that doesn’t exist, you’ll get an AttributeError exception. For example:

class HtmlDocument:
    extension = 'html'
    version = '5'

print(HtmlDocument.media_type)Copy

Try it

Error:

AttributeError: type object 'HtmlDocument' has no attribute 'media_type'Copy

Another way to get the value of a class variable is to use the getattr() function. The getattr() function accepts an object and a variable name. It returns the value of the class variable. For example:

class HtmlDocument:
    extension = 'html'
    version = '5'

extension = getattr(HtmlDocument, 'extension')
version = getattr(HtmlDocument, 'version')

print(extension)  # html
print(version)  # 5Copy

Try it

Output:

html
5Copy

If the class variable doesn’t exist, you’ll also get an AttributeError exception. To avoid the exception, you can specify a default value if the class variable doesn’t exist like this:

class HtmlDocument:
    extension = 'html'
    version = '5'

media_type = getattr(HtmlDocument, 'media_type', 'text/html')
print(media_type) # text/htmlCopy

Try it

Output:

text/htmlCopy

Set values for class variables #

To set a value for a class variable, you use the dot notation:

HtmlDocument.version = 10Copy

or you can use the setattr() built-in function:

setattr(HtmlDocument, 'version', 10)Copy

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 media_type class variable to the HtmlDocument class:

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

Similarly, you can use the setattr() function:

setattr(HtmlDocument, media_type, 'text/html')
print(HtmlDocument.media_type)Copy

Delete class variables #

To delete a class variable at runtime, you use the delattr() function:

delattr(HtmlDocument, 'version')Copy

Or you can use the del keyword:

del HtmlDocument.versionCopy

Class variable storage #

Python stores class variables in the __dict__ attribute. The __dict__ is a mapping proxy, which is a dictionary. For example:

from pprint import pprint


class HtmlDocument:
    extension = 'html'
    version = '5'


HtmlDocument.media_type = 'text/html'

pprint(HtmlDocument.__dict__)Copy

Try it

Output:

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'})Copy

The output indciates that the __dict__ has three class variables: extension, media_type, and version besides other predefined class variables.

Python does not allow you to change the __dict__ directly. For example, the following will result in an error:

HtmlDocument.__dict__['released'] = 2008Copy

Output:

TypeError: 'mappingproxy' object does not support item assignmentCopy

However, you can use the setattr() function or dot notation to indirectly change the __dict__ attribute.

Also, the key of the __dict__ are strings that will help Python speeds up the variable lookup.

Although Python allows you to access class variables through the __dict__ dictionary, it’s not a good practice. Also, it won’t work in some situations. For example:

print(HtmlDocument.__dict__['type']) # BAD CODECopy

Callable class attributes #

Class attributes can be any object such as a function.

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:

from pprint import pprint


class HtmlDocument:
    extension = 'html'
    version = '5'

    def render():
        print('Rendering the Html doc...')


pprint(HtmlDocument.__dict__)Copy

Try it

Output:

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...Copy

In this example, the render is a class attribute of the HtmlDocument class. Its value is a function.

Summary #

• A class is an object which is an instance of the type class.
• Class variables are attributes of the class object.
• Use dot notation or getattr() function to get the value of a class attribute.
• Use dot notation or setattr() function to set the value of a class attribute.
• Python is a dynamic language. Therefore, you can assign a class variable to a class at runtime.
• Python stores class variables in the __dict__ attribute. The __dict__ attribute is a dictionary.

Quiz #