The Bug I Hit When I Forgot a return in Python

I once hit a sneaky Python bug: my function kept returning None when I expected a value.
The mistake? I simply forgot to add the return keyword.

Here’s a simplified example similar to my bug:

1def get_data():
2    # Intended to return a list
3    [1, 2, 3]  # Oops! Missing 'return' here

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Why Python Functions Return None

At first glance, it looks like get_data() returns the list [1, 2, 3]. But actually, Python function returned None! Why?

• In Python, no return, return, and return None are all equivalent — they all result in None.
• Writing an expression alone (like [1, 2, 3]) without return does nothing with the value — it’s just evaluated and discarded.

This behavior is obvious once you know it, but it’s easy to slip up if you also use languages like Rust, where the last expression in a function is implicitly returned.

That muscle memory can lead straight into this bug in Python.

Functions can have multiple return points:

1def example(x):
2    if x > 0:
3        return x
4    else:
5        return -x

Python exits at the first encountered return statement during execution.

If any execution path reaches the end of the function without hitting a return, Python returns None:

1def maybe_return(x):
2    if x > 0:
3        return x
4    # no else branch and no return at the end
5
6print(maybe_return(5))   # prints 5
7print(maybe_return(-1))  # prints None

This can lead to bugs if your code expects a non-None value but gets None unexpectedly.

1def maybe_return(x: int) -> int | None:
2    if x > 0:
3        return x
4    # no return if x <= 0

Hope this helps you avoid the tiny but tricky mistake of a Python function returning None when you forgot return!