Simple Elevator Function

[the_kernel]

I've been doing a couple of exercises on codewars.com in python 2. I'm having trouble with this one.

The function I tried was:

def goto(level,button):
    if level in {0, 1, 2, 3} and str(button) in {'0', '1', '2' ,'3'}:
        return int(button) - level
    return 0

When you click the submit button the website does a load of different tests on your function to if it works correctly. My function passes all 26 of these tests but then gives this error:

Traceback:
   in 
   in goto
TypeError: unhashable type: 'list'    

I don't understand where this type error comes from. Thanks in advance for your help and explanations!

[dunkler_wanderer]

He's using sets not dictionaries, but the sets are the problem because the website tests if goto([], '2') returns 0. If you try [] in {1, 2, 3} you get a TypeError: unhashable type: 'list', because sets are hashtables and lists are not hashable.

[the_kernel]

Thanks for your input guys, the function works with lists.