Mystery Code

What is the value which cause the program to run forever?
I do not know the answer please help me. An explanation is GREATLY appreciated. I need a explanation for this.

Let's see here.

x is a function argument.

We could try calling the function with one parameter.
>>> mystery('hello world')

Ooh! Failure.


Let's look at the function guts.
It compares x with 0. Maybe x should be a number.
And we see that if x has to be positive or the program will quit. Now we can try


>>> mystery(2.718)

Oooph. Failure!



OH! x needs to be a positive integer.
Greater than 0 but less than the length of the short list.

Now we can trace the program execution for values in range(1,len(a))