Others have mentioned this, but I'm raising it as an issue.

Kata should have us raise exception, not return error strings. Specify which type of exception to raise and what custom message to use for each error you want to check.

You created a new array instead of swapping the values of the original array. In this case, args[0] and args[1] still have the same values.

My bad, after another review this solution does not pass all test cases.

Thanks @kazk, noted.

CC BY-SA 4.0 requires redistributing with the same license, but Codewars uses BSD 2-Clause implicitly, so we need to make that explicit (these are incompatible licenses).

The problem description was derived from freeCodeCamp's Arithmetic Formatter (CC BY-SA 4.0).

Technically, you can't give us (Qualified) the rights listed in our ToS, but I think it's fine as long as we don't use this anywhere else. If it's a problem, we'll remove it.

If we start to see more kata based on their problems, I might make some adjustments to rewards, but I'll worrry about it when that happens.

test.assert_equals(actual, expected, error_message) is the way to go.

@hobovsky and @kazk, received answer from fCC. We are allowed to share and adapt content from fCC under the Creative Commons 4.0 International license where stated in their LICENSE.md. Edit to kata description have been made, feel free to check. Human-readable summary of license here.

Just asked clarification from fCC, waiting for answer, thanks @hobovsky.

I cannot retire the kata, but I can unpublish it and put it in draft state. Since people seem to like your kata you can always republish it after clarification of the copyright issues, if you want.
If you are sure you do not want to give it another try, or that you do not want to ask FCC if it's OK to redistribute their problems, and you think it trashes your list of kata then let me know and I will ask Kazk to click the "Retire" button.

It's a nice kata, why don't you ask for clarification about redistribution rights at freecodecamp?
Regarding the concerns of the admin, just use original ideas next time.

My bad, if anyone can help to retire this kata? I don't know how to.

@kazk, I understand your concern. I created this kata, not because of honor points, but because it was interesting and thought others would think the same. Anyways, thank you for your help :). Kata will be retired, I don't mind, I learnt a lot about making one anyways.

I read through the Terms of Service from their website as well as their GitHub repo but couldn't find any way that I am breaking a license or notice.

Not finding violation doesn't mean you have the right to use it. I skimmed through FCC's terms, but I don't think it says anything about redistribution (making it available for others). They allow teachers to use it (redistributing to students), but I think that assumes local use.

Also, the source repo has no license: https://github.com/freeCodeCamp/boilerplate-arithmetic-formatter

If you want, you can email FCC and ask.

If it's allowed, I'm concerned that users will start copying a lot from them. Some are motivated by honor points and this will be much easier than coming up with originals. I wouldn't want our authors publishing originals to be demotivated by this. But that's a separate issue.

@Kacarott and @dfhwze, thanks a lot for the help! Kata looks more polished now, will certainly apply all these when making future katas.

Alright I added the newline, and applied that to all tests, including the example tests. @__str__ if you aren't happy with it, feel free to revert or change it.