OP solved it, closing

same +1

Your solution works in both 3.6 and 3.8. There should be no reason for it to fail in one and not the other. It is more likely that you have a small error with an edgecase in your solution, and you managed to not receive a random test with that edgecase when you switched to 3.8.

@Lissa-krassa: Plz reset the trainer after switching to 3.8, that should resolve the problem.

don't forget to copy you code first, otherwise you'll lose it in the process... ;o

Can you please post your whole code with a spoiler flag?

The test cases contain letters which are not in abc and keyword. It is not mentioned in the instructions to the kata.

Any characters not provided in the alphabet should be left in situ when encoding or decoding.

The keyword can contain duplicates of letters.

In a keyword cipher, repeats of letters in the keyword are removed and the alphabet is reordered such that the letters in the keyword appear first, followed by the rest of the letters in the alphabet in their otherwise usual order.

For the purpose of this kata, only the first occurence of a letter in a keyword should be used.

That implies that.

If there are restrictions they should be present in the description. Please add a spoiler flag next time you give clues to a solution :)

The first letter is a consonant, the next letter is a vowel, the next is a consonant, the next is a vowel...
or
The first letter is a vowel, the next is a consonant, the next is a vowel, the next is a consonant...

Your function is returning None and the tests are trying to get length of it. Make sure your code returns a list.

AddExtra method adds a new item to the list and returns the list.

The log is quite explicit: you try to use len on None, which does not work. Revise your code, it's impossible to help you without more information. You may want to post your code with a spoiler flag for further help.

Switch to another python version.