Скачать или смотреть How to Create a List of Objects Using Values from Another List in Python

Learn how to create a list of objects that encapsulate individual lists in Python, and avoid common pitfalls with class attributes.
---
This video is based on the question https://stackoverflow.com/q/72220468/ asked by the user 'Byron Zhang' ( https://stackoverflow.com/u/19104594/ ) and on the answer https://stackoverflow.com/a/72220516/ provided by the user 'Alexander' ( https://stackoverflow.com/u/17829451/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

Visit these links for original content and any more details, such as alternate solutions, latest updates/developments on topic, comments, revision history etc. For example, the original title of the Question was: How to create a list of objects from with values from another list of values?

Also, Content (except music) licensed under CC BY-SA https://meta.stackexchange.com/help/l...
The original Question post is licensed under the 'CC BY-SA 4.0' ( https://creativecommons.org/licenses/... ) license, and the original Answer post is licensed under the 'CC BY-SA 4.0' ( https://creativecommons.org/licenses/... ) license.

If anything seems off to you, please feel free to write me at vlogize [AT] gmail [DOT] com.
---
Creating a List of Objects in Python from Another List

When working with Python, it's common to need to manage data structures, especially when dealing with lists of lists. One question that often arises is: How can I create a list of objects that each holds a list from another list of values?

In this guide, we will tackle this problem and demonstrate how to correctly create a list of objects while avoiding issues related to class attributes.

The Problem

Let's say we have a list of lists, as shown below:

[[See Video to Reveal this Text or Code Snippet]]

Our goal is to create a list of objects, where each object contains its own list holding the elements from these inner lists. However, when the initial approach is taken, the output turns out to be unexpected:

[[See Video to Reveal this Text or Code Snippet]]

What we really want is:

[[See Video to Reveal this Text or Code Snippet]]

Understanding the Issue

The heart of the problem lies in how class attributes work in Python. Class attributes are shared across all instances of a class, meaning if one instance modifies it, all instances reflect that change. Therefore, in our initial attempt, all created objects were sharing the same temp_list, resulting in the addition of all elements from the original list.

The Solution

To resolve this, we need to change temp_list from a class attribute to an instance attribute. This can be achieved by defining it within the class's _init_ method. Here's how you can do it:

Step 1: Define the Class with an Instance Attribute

[[See Video to Reveal this Text or Code Snippet]]

Step 2: Create Instances and Fill Each temp_list

Now, we can iterate through our original list of lists and create instances of Test, assigning each inner list to its corresponding object:

[[See Video to Reveal this Text or Code Snippet]]

Step 3: Printing to Verify Output

Finally, to see the contents of each temp_list, you can print each object directly. However, for better formatting, consider adding a _str_ method to your class:

[[See Video to Reveal this Text or Code Snippet]]

Full Example Code

Here’s the complete corrected code for your reference:

[[See Video to Reveal this Text or Code Snippet]]

Conclusion

By defining temp_list as an instance attribute, each object retains its own separate list, preventing data from bleeding across instances. This is a fundamental technique in Python that not only applies to this specific problem but can be generalized to many other situations where encapsulation of data in objects is required.

Now you have the knowledge to handle lists of lists effectively with custom objects in Python! Happy coding!