Understanding Mutable Objects in Python to Get Your Expected Results

Learn how to fix issues with mutable objects in Python by understanding how to properly append instance attributes in a loop.
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Understanding Mutable Objects in Python to Get Your Expected Results

In programming, particularly in Python, you may often encounter situations where the behavior of your data structures can be a little puzzling. One common issue arises from the nature of mutable and immutable objects. In this guide, we will discuss a specific example involving appending a dictionary as a list inside a class method, and how to resolve unexpected results that stem from using mutable objects. Let's dive in!

The Problem: Unexpected Output in Code

Consider the following snippet of Python code where a class named Result is trying to accumulate parameters that have errors:

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

When this code runs, you might expect the output to show unique entries in params_with_error corresponding to the specified conditions, such as:

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

However, the actual output includes duplicates and references to the last updated values of qs:

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

Why did this happen?

Understanding the Issue: Mutable Dictionaries

The core of the issue lies in how Python handles mutable objects like dictionaries. In your code, you only created one dictionary object named qs:

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

Whenever you update qs, you are modifying the same object in memory.

Key Takeaway:

Dictionaries are mutable: This means that changing the content of a dictionary affects all its references. There are no unique copies unless explicitly created.

The Solution: Appending a Copy

To resolve this issue, you need to ensure that you're appending a new copy of the dictionary to params_with_error, rather than a reference to the original. You can achieve this by utilizing the copy module from Python's standard library. Here's how you can modify the code:

Revised Code Example

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What Changed?

We added import copy at the beginning of the code.

Expected Output

With these changes, you should now see the expected, distinct results in the output:

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

Conclusion

Understanding mutable vs. immutable objects is crucial when programming in Python. When working with mutable objects like dictionaries or lists, be mindful of how and when you're duplicating or referencing those objects to avoid unexpected behavior. By ensuring you append copies of objects instead of references, you can achieve the results you intend and streamline your code processes. Happy coding!