Скачать или смотреть Understanding the RecursionError in Python: Overriding __iter__ Explained

Explore why overriding the `__iter__` method in Python can lead to a `RecursionError` and learn how to implement it correctly to avoid pitfalls.
---
This video is based on the question https://stackoverflow.com/q/63381216/ asked by the user 'Muhammed Hasan Celik' ( https://stackoverflow.com/u/4673198/ ) and on the answer https://stackoverflow.com/a/63381569/ 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: Why does overriding _iter_ throws RecursionError?

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 the RecursionError in Python: Overriding _iter_ Explained

In Python, the _iter_ method is a fundamental part of making your objects iterable. As developers, we often find ourselves in situations where we need to override this method, particularly when dealing with classes that inherit from external libraries. However, doing so can lead to unexpected errors, such as RecursionError. In this guide, we'll explore why this happens, using a specific example, and how you can avoid running into this issue when overriding __iter__.

The Problem: Why Does RecursionError Occur?

In our scenario, we are working with two classes, MyIter2 and MyIter3, that attempt to override the _iter_ method from the ExternalIter class. Here’s a brief overview of the implementation:

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

Testing the Original Behavior

The original class, ExternalIter, works perfectly:

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

The output is:

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

The Issue with MyIter3

When we attempt to override the _iter_ method in MyIter3 like so:

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

Running:

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

Results in a RecursionError. This is because calling super().__iter__() leads to a cycle. The _iter_ method on ExternalIter is called, which in turn, refers back to self. This call does not yield control out of the for loop, resulting in infinite recursive calls.

The Solution: Avoiding Recursion with while True

To properly override the _iter_ method without encountering recursion issues, we can replace the for loop with a while True loop:

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

Why This Works

In this fixed version:

Using next(self): Instead of invoking super().__iter__(), we directly use next(self), treating our instance as an iterator. This avoids making a recursive call to _iter_ again.

Control Flow: The while True loop manages the iteration flow without entering into an infinite recursive cycle.

Key Takeaways

Understand the for Loop: In Python, a for loop implicitly calls iter(self) on each iteration, which mandates using an iterable, leading to recursion if self is an instance of the current class.

Use Iterators over Iterables: When overriding __iter__, if you want to yield values directly, use the iterator's methods (like next()) instead of trying to yield from another __iter__() call.

Avoid Infinite Recursion: Always ensure your overrides lead to valid control flow that doesn't attempt to call the same method in an unending loop.

Conclusion

Overriding the _iter_ method in Python can be tricky, especially when dealing with inheritance. By understanding how Python's iteration protocol works and adjusting your approach accordingly, you can avoid common pitfalls like RecursionError. Remember, when in doubt, opting for a while loop to handle iteration manually often leads to cleaner and safer code.