I guess I didn't need to use that if statement for an empty array.

I really like these variations. Maybe I just have neanderthal brain but I find them so easy to read.

Thank you for your reply. Makes sense.

The test cases just never set arr to None, that's why there's no error. What they do though is setting arr to [], which is different than None.
But in general you're right. One should check if arr is None before iterating over it.

Newbie question here. I feel like I just completed a Kata where if you didn't check for if arr is None then it would throw an error. Why doesn't this throw an error if arr is None?

The most correct code that is convenient to debug.

that's why in my solution, identical to this one, i named it amount.

This code seems to be 15% faster than the CrazyMerlyn answer.

Ah, thanks. I thought they were saying that in-built functions and methods should be avoided :D

I guess Acmer77 means that the variable name sum should be avoided because its name is too similar to the function named sum().

why would you avoid built-in functions? I thought they were there to make your life easier.