Скачать или смотреть How to Fix the RecursionError When Using Turtle Graphics in Python

Learn how to solve the `RecursionError` encountered in Python's Turtle Graphics while generating complex recursive patterns, like the Sierpenski Triangle.
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This video is based on the question https://stackoverflow.com/q/76478953/ asked by the user 'shan' ( https://stackoverflow.com/u/20780892/ ) and on the answer https://stackoverflow.com/a/76484141/ provided by the user 'cdlane' ( https://stackoverflow.com/u/5771269/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Troubleshooting RecursionError in Turtle Graphics with Python

If you're diving into the world of Turtle Graphics in Python, you might encounter a frustrating issue: your code might crash after a certain number of moves due to a RecursionError. This problem often arises when using recursive functions to generate complex patterns. A common instance is when creating a Sierpenski Triangle with deeper levels of recursion. Let’s explore why this happens and how to properly fix it.

Understanding the Problem

In your case, you reported encountering a RecursionError that states the "maximum recursion depth exceeded." Specifically, this issue arises when the recursion limit in Python is breached. For your code that builds a Sierpenski Triangle, the recursion happens primarily in two functions:

Generating the pattern: The sierpenski(n) function creates a string of commands that direct the turtle on how to draw the triangle.

Executing the commands: The execute(commands) function executes these commands recursively.

When you increase the complexity (depth) of the Sierpenski Triangle by increasing the value of n, the number of moves grows drastically. This can quickly exceed the default recursion limit of Python—usually set around 1000.

Diagnosing the Crash

When using the sierpenski(n) function with larger values:

The command string that gets generated can become extensively long (e.g., for n=10, it produced around 940,685 characters).

Each character of the string is executed recursively in the execute(commands) function, which causes a stack overflow, leading to the RecursionError.

The Recursion Limit

In Python, you can check and modify the recursion limit using the following commands:

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

However, merely increasing the limit is not the ideal solution, especially with such large outputs. Instead, we need to rethink how to execute the commands.

The Solution: Use Iteration Instead of Recursion

To avoid crashing your program and effectively handle long strings of commands, we should switch from a recursive to an iterative approach in the execute function. Here’s how you can do it:

Revised Execute Function

Replace your recursive execute() function with an iterative one:

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

Key Changes Explained

Iteration vs. Recursion: Using a for loop allows us to process each command in the string without hitting the recursion limit. This means that even with a long string of commands, you won’t face a stack overflow.

Performance: Although the performance will depend on how complex the commands get, this method will drastically reduce the likelihood of crashes.

Conclusion

To sum it up, while recursion is a powerful tool for generating complex fractals like the Sierpenski Triangle, it can become a limitation when the depth exceeds Python's inherent capabilities. By transitioning to an iterative solution for executing commands, you can harness the full potential of Python's Turtle Graphics without running into problems.

So, next time when you’re building beautiful patterns, remember: iterate, don't recurse!