Why Does My Python Recursive Function Return None?

Why Python Recursive Function Returns None

In Python, recursive functions can encounter an issue where they return None unexpectedly. Let's explore a specific example to understand the cause and solution.

Consider the following code snippet:

<code class="python">def gcdIter(a, b):
    a, b = min(a, b), max(a, b)  # Assign smaller value to 'a' and larger value to 'b'

    if b % a == 0:
        print(a)
        return a
    gcdIter(a, b % a)</code>

This code is intended to calculate the greatest common divisor (GCD) using the iterative approach. However, it incorrectly returns None in some cases.

To understand why, let's examine the recursive call:

<code class="python">gcdIter(a, b % a)</code>

This call makes another recursive call to the gcdIter function with updated values of a and b. However, the return value of this recursive call is ignored, which results in None being returned by the original gcdIter function.

The solution is to return the result of the recursive call. The correct version of the function looks like this:

<code class="python">def gcdIter(a, b):
    a, b = min(a, b), max(a, b)  # Assign smaller value to 'a' and larger value to 'b'

    if b % a == 0:
        return a
    return gcdIter(a, b % a)</code>

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