Where does one even begin to learn how to solve this puzzle if they have only ever done scripting?

I thought so too at first, but since we are iterating over every mark in the inputs it will eventually split the string on the leftmost marker.

If this wasn't true, than some the tests would fail.

You were right. My "sorted" code was wrong.

Also after reviewing other solutions, it was also completely unneeded. :)

It is not, fix your code. Sample tests are just that, a sample.

Unfortunately, this Kata is bugged in python 3.

This solution passes the tests, but fails after submitting the solution.

def solution(string, markers):
    result = []

    for l in string.splitlines():
        marker_index = {}

        for m in markers:
            if m in l:
                marker_index[m] = l.index(m)

        if marker_index:
            m = sorted(marker_index.items())[0][0]
            l = l.split(m, maxsplit=1)[0]

        result.append(l.strip())
    
    return '\n'.join(result)

Voted clever, but would not catch any subdomains besides 'www'.

For example:
domain_name('https://mail.google.com/')