Python Class Slots

Python class slots are a feature that not many programmers know of. In a slotted class we explicitly define the fields that our class is allowed to have using the magic field name __slots__. This has some advantages:

  • Objects created from the class will take up slightly less memory
  • It’s faster to access class attributes
  • You can’t randomly add new attributes to objects of a slotted class

Here’s an example of how to define a slotted class:

>>> class Card:
...     __slots__ = 'rank', 'suite'
...     def __init__(self, rank, suite):
...             self.rank = rank
...             self.suite = suite
>>> qh = Card('queen', 'hearts')

To me, the biggest advantage is that you can’t randomly add new attributes to a slotted class. It can prevent costly mistakes! To demonstrate: a typo when assigning an attribute to a slotted class will throw an error instead of Python silently creating a new attribute.

For small classes without complex inheritance, using slots can be an advantage. Especially when you need to create many instances of such a class, the savings in memory and faster attribute access can make a difference.

Finally, just so you know, you can combine this technique with data classes as well!

Get certified with our courses

Our premium courses offer a superior user experience with small, easy-to-digest lessons, progress tracking, quizzes to test your knowledge, and practice sessions. Each course will earn you a downloadable course certificate.

Learn more

This article is part of the free Python tutorial. You can head over to the start of the tutorial here. You can navigate this tutorial using the buttons at the top and bottom of the articles. To get an overview of all articles in the tutorial, please use the fold-out menu at the top.

If you liked this article, you might also like to read the following articles:

Leave a Comment

Python Newsletter

Before you leave, you may want to sign up for my newsletter.

Tips & Tricks
Course discounts

Googler exit intent popup

No spam & you can unsubscribe at any time.

Share to...