thank you both, I will investigate

It depends on what's called built-in. It's the standard library. Even though there can always be non-standard implementations of any language, relying on the standard library is normal.
As for non-standard libraries on CW: https://github.com/Codewars/codewars.com/wiki/Language-Python

yep, sure: https://docs.python.org/3/library/ : everything that is not labelled "Built-in..." is a module present by default (may depend on the python version/installer for some of them, I guess)

@B4B, sorry 'bout the flag... what python modules are present by default on CW? is there a list of such defaults for each language? thanks

spoiler flag, guys...

@Ashish8796: they are not "built-ins", but the related module is present by default so they always are at hand, if that's your question.

i tried thats why i asked.

WTF! SPOILER FLAG!! X/

and for your question, there are more cons than pros to the default argument here, because a user can accidentally mess with your function if it's not documented. Other than that, there is no real difference, I'd say (since there is no tail call optimization in python, afaik).

you can do that all you want, you'll just have a solution that either times out or exhausts all its allocated memory. You'd bet that if something like that could cause troubles, CW would already have been down years ago... x)

Very elegant and terse solution. I am wondering that nothing prevents external users to call persistence(31, 5) which may have unwanted consequences elswhere in the codebase.

In Python 2 this solution isn't good because map returns a list, so this version is less memory efficient.

x for x in url[1:]

means each part, let path will be /index/index/index.html you will recieve empty list but only last one should be omitted.

UPD: general rule: every situation not covered in problem statement - possible.

Now that's thorough. Thanks.