Kata says function "takes a list of non-negative integers and strings"

This is just for simplification, so as not to rewrite the alphabet.

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You can't.

That's not what the description says. The input consists of non-negative ints and strings and you should throw out all the strings. No need to check for negative ints.

There are solutions in Python that use list comprehension methods without timing out.

They are columns in a table, not a kata issue.

https://www.sqlcourse.com/beginner-course/table-basics/