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Learn how to fix the issue of overwriting objects in a list when appending in Python with our step-by-step guide.
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Python Appending Objects into a List: Solving the Overwrite Issue

When working with Python, it’s common to store objects in lists. However, if you’re not careful, you might run into an issue where appending objects leads to overwriting previous entries. This can be quite frustrating, especially when you expect each object to remain unique in your list.

In this guide, we’ll explore a common problem faced when appending objects to a list in Python — specifically, the overwrite issue — and provide a solution that ensures each object is correctly instantiated and stored in the list.

The Problem: Overwriting Objects

Consider the scenario below where we attempt to load data from a JSON file and append House objects into a list. The user mentioned that after appending the second object, the first object gets overwritten. Here’s a snippet from the user's code:

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

Analyzing the Issue

Upon inspection, it becomes clear that the problem arises from how the House class is being used:

The code assigns values directly to the attributes of the House class rather than creating a new instance of House each time.

Consequently, when the next iteration occurs, it alters the class attributes instead of creating distinct objects for each item in the list.

Because of this, each time an object is appended, it references the same class and modifies its state rather than storing unique objects.

The Solution: Proper Object Instantiation

To resolve the overwrite issue, you need to create a new instance of the House class for each iteration in the loop. Here’s how you can modify the code:

Instantiate the Object: Instead of modifying the class itself, create a new instance named dahouse for each set of properties.

Append the Instance: Append dahouse to the list.

Updated Code

Below is the corrected implementation:

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

Breakdown of Key Changes

Creating a new instance: The line dahouse = House(o['properties']['id'], o['properties']['name']) creates a new House object for every feature in the JSON file.

Appending the instance: The list houses now contains unique House instances, keeping their attributes intact.

With these modifications, you should be able to append multiple House objects into the houses list without overwriting previous entries.

Conclusion

By understanding the differences between class attributes and instance attributes in Python, you can avoid common pitfalls like overwriting data in lists. Always ensure that when dealing with multiple entries, you create new instances of your classes to maintain the integrity of your data.

If you follow the steps outlined in this post, you should efficiently resolve your appending objects issue in Python. Happy coding!