Demo: Fibonacci function, in-browser editor, external grading

The Fibonacci numbers are 1, 1, 2, 3, 5, 8, ..., where each number is the sum of the two before it. These are numbered $F_1, F_2, \ldots$, and \[ F_n = F_{n-1} + F_{n-2}. \]

Write a Python function fib that takes a number n and returns the n^th Fibonacci number, $F_n$.

Your code snippet should define the following variables:

Note: In the above example, the attribute source-file-name='initial_code.py' was used to add existing code to the in-browser file editor from a source file. Another option is omit the source-file-name attribute and instead write the existing code directly inside the html element, between the pl-file-editor tags.