For an introduction to slicing, please view the article How to Slice Lists/Arrays and Tuples in Python.

Slicing Your Own Python Objects

Alright, that's all cool, now we know how to slice. But how do we slice our own objects? If I implement an object that has a list, or custom data structure, how do I make it slicable?

First, we define our custom data structure class.

Here, we declare a class with a list as the back-end of our structure. MyStructure doesn't really do that much, but you can add, remove, and get items from it, so it's still useful in a sense. The only method we need to pay attention to in particular here is the __getitem__ method. This method is called whenever we try to get an item from our structure. When we call structure[0], this actually calls the __getitem__ method in the background and returns whatever that method returns. It's very useful when implementing a list style object.

Let's create our custom object, add an item to it, and try to get it.

Yes! We did it! We assigned our element, and properly got it back. Nice. Now for the slicing part!

Awesome! We basically get all the slicing for free because of using list's functionality.

That's pretty amazing. Python is amazing. How about something a little more complex? What about using a dictionary as our main data structure?

Alright. Let's define another class for using a dictionary.

Ok, so what are we doing here? Basically the same stuff that we did in the MyStructure, but this time we're using a dictionary as our main structure, and the __getitem__ method definition has changed. Let me explain what changed in __getitem__.

First, we get the slice object that is passed in to __getitem__ and use it to slice the dictionary's key values. Then, using dictionary comprehension, we put all of the sliced keys and values into a new dictionary using our sliced keys. This gives us a nice sliced dictionary that we can use for whatever we like.

Here it is in action:

And that's it. Simple, elegant, and powerful.

If you want to know how to slice strings, that's covered in another article titled How to Get a Sub-string From a String in Python – Slicing Strings.

That's all for now. Hope you enjoyed learning about slicing and I hope it will assist you in your quest.

Peace out.

To Practice: Try this interactive course on the basics of Lists, Functions, Packages and NumPy in Python.

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