Yeah, that's the point. We want to get the right answer as we make in general mathematic, regardless of the Python specification.

When I tried my solution, system told me that "Process was terminated. It took longer than 6000ms to complete
". I doubt that my solution is inefficient.
Then I copy your code, run again, then I got the the time-out question again; I can't submit it.
Are there some questions with the Computer or speed of the Internet?
So strange.

if 'i' is a string, also doesn't exist in the 'string',
what result would return?

I think,
insertting commas every 3 numbers is a method, but not a good one.
format(price, ',d'), or something like that is clever but not a suitable one.
It is the feature of finance respect of every 3 numbers a comma.
format(price, ',d') solves it by chance.

In my opinion, I used the recursion method. If you like it, you can check my code.
Enjoy...