Not checking more than once if a number is prime could help. :P

Hi @Anton-Kim - I just solved this kata, to be able to give you clearer answer:

Your test/error message occurs when you are given the same URL twice, and you are supposed to therefore identify that you have already assigned a code to that URL and therefore you must return the same code

For example, if you are given as input "google.com" and you assign it code "aaac", then any following query for "google.com" input must return the same code "aaac", rather than assigning it a new one like "bfxz" for example.

In Python the error message format is:

what_your_code_returned should equal what_the_kata_correct_solution_is

So here, your code is returning 'short.ly/aaac' but the correct answer is 'short.ly/aaad'