Understanding Python's Order of Assignment: A Comparison with JavaScript

Discover how Python's order of assignment differs from JavaScript and why it results in self-referencing objects. Learn how Python handles multiple assignment operations effectively.
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Understanding Python's Order of Assignment: A Comparison with JavaScript

When transitioning from JavaScript to Python, it's not uncommon to encounter confusing behaviors, especially when it comes to assignment operations. One question that arises frequently is: Why does Python's order of assignment behave differently than that in JavaScript? In this post, we'll dissect the differences in the order of assignment between these two popular programming languages, and clarify how Python manages assignments to prevent unexpected results.

The JavaScript Assignment Behavior

In JavaScript, the assignment operator = works from right to left, allowing for the chaining of assignments. Let's examine a simple example:

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

How It Works

Right-to-Left Assignment: In the line a = a['y'] = {};, JavaScript first evaluates a['y'] = {};. This means a['y'] is assigned an empty object {}.

Chained Assignment: Afterward, a is assigned the value of a['y'], which is also {}.

This allows JavaScript to create a reference in x while setting a to the newly created empty object.

The Python Assignment Behavior

Let's now consider the same logic in Python:

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

What's Happening Here?

In Python, the behavior of the assignment operator = is quite different from JavaScript. Here’s what occurs step-by-step:

Single Statement: Python treats = as a dedicated statement rather than an expression. Thus, a = a["y"] = {} is not evaluated as two expressions but as a single statement with multiple targets.

Left-to-Right Storage: When multiple targets are involved, Python stores the references from left to right. This means:

It creates a new dictionary __unnamed_tmp and assigns it to a temporary reference.

It sets a to this temporary reference.

It assigns the same temporary reference to a["y"].

Self-Referencing Object: The result is that a becomes a reference to an object that points back to itself, which is not what we intended.

Equivalent Internal Steps

You could interpret the Python operation a = a["y"] = {} as running through these equivalent internal steps:

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

As you can see, this produces a self-referential structure that can lead to unexpected outcomes if not understood properly.

Conclusion

Python's method of handling assignments, especially with the multiple-target concept, can sometimes lead to behavior that surprises those migrating from JavaScript. While it ensures the stability and integrity of the reference system, understanding the underlying mechanics helps prevent mistakes as you write or transition your codebases.

By grasping these nuances, you can write more effective, predictable code in Python and avoid potential pitfalls related to assignment operations.

Happy coding!