Understanding Why Your Python Recursive Function Returns None Instead of a Value

Explore common pitfalls in Python recursion, specifically why a recursive function might return None, and learn how to fix it effectively.
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Understanding Why Your Python Recursive Function Returns None Instead of a Value

Python is a powerful programming language that allows developers to harness the power of recursion—a method where a function calls itself to solve smaller instances of a problem. However, if you are new to Python and recursion, you might encounter some common pitfalls, such as a function unexpectedly returning None. This article will break down one such scenario while providing a clear solution.

The Problem

Let's say you are experimenting with recursion in Python, and you write a function intended to split a number into four equal pieces recursively until the number is smaller than four. The goal is for an input like 20 to return an array full of values, each being 1.25. However, instead of getting the expected output, you find that the function returns None. What could be going wrong?

Example Code

Here’s a snippet of your current function:

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

The Issue

In the above code, when the function detects that the input number is greater than 4, it does split the input and extends it into the full_result. However, when it makes a recursive call to break_up_number_into_fours(full_result), it does not return the value from this call. Hence, while you are able to see the intermediate print statement outputting the final list, your test variable ends up being None.

The Solution

To fix this issue, you need to ensure that you return the result of the recursive call. Here’s how you can modify the function to incorporate the necessary changes:

Revised Code

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

What Changed?

Instead of just calling break_up_number_into_fours(full_result) within the if condition, the revised code now includes return. This allows the function to pass back the final list properly through each recursive cycle, providing the expected output.

Final Output

When you run the revised function with the input of 20, you’ll get the following output:

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

Conclusion

Recursion can be a complex topic for beginners in Python, especially regarding how data is returned through recursive calls. The key takeaway here is to remember to always return the result from your recursive calls. This basic principle will help you avoid a common pitfall that results in your function returning None unexpectedly. Happy coding!

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