Скачать или смотреть Understanding Confusing Class Behavior in Python: Why Are You Getting Unexpected Results?

Discover why your Python classes are displaying unexpected behavior when looping over arrays. Learn how to create instance-specific attributes to get the desired output.
---
This video is based on the question https://stackoverflow.com/q/74853498/ asked by the user 'meclondrej' ( https://stackoverflow.com/u/19619690/ ) and on the answer https://stackoverflow.com/a/74853573/ provided by the user 'chepner' ( https://stackoverflow.com/u/1126841/ ) 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: Confusing class behavior in python

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.
---
Understanding Confusing Class Behavior in Python: Why Are You Getting Unexpected Results?

When working with classes in Python, especially when dealing with lists or arrays, it can be puzzling to see unexpected output. A common confusion arises when multiple instances of a class seem to share the same data. This guide aims to clarify why this happens and how to solve the problem effectively.

The Problem

Let’s say you have a class defined as follows:

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

What’s Happening Here?

When you run the code, you might expect to see the output showing five 69s, one for each instance of the class example. However, what actually prints are 25 instances of 69. Here's why this occurs:

Shared Class Attribute: The array arr is defined as a class attribute. This means that every instance of the class shares this same list. When you append 69 to arr, you are adding it to the same list across all instances.

Instance Misunderstanding: Since you did not create a separate instance attribute for arr, every instance of example modifies the same list, accumulating values in a single shared array.

Analyzing the Output

As a result:

When you create five instances of the example class, the shared arr list accumulates 69 five times.

When you loop through each instance and print the contents of arr, you are effectively printing the contents of a list that has five 69s. Since there are five instances, you see 25 total prints (5 instances with 5 elements each).

The Solution

To fix this behavior, you need to change the definition of the class so that each instance has its own separate instance of arr. This can be done by initializing arr inside the _init_ method:

Revised Code

Here’s the corrected version of the code:

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

Updated Object Creation

Now, when you create the objects:

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

In this configuration:

Each instance of Example has its own arr, initialized as an empty list at the time of instance creation.

When each instance adds 69, it does so to its unique list.

As a result, there will be five lines printed, each showing 69, as intended.

Final Thoughts

Understanding the difference between class attributes and instance attributes is crucial in Python. This distinction helps prevent unintended data sharing between instances. Always remember to initialize instance-specific data within the _init_ method to avoid such confusions in your future projects. Happy coding!