Seems like you have a XY problem. You want people who know Python's standard library, but you think having them implement grep is the way to go about it.

This is the classic XY problem.

http://xyproblem.info/

If your requirements are to hire some one that can write functional test cases against the Python's standard library. Just ask them to do that.

It's as simple as that.

If someone says write a grep-like thing in Python so we can see how you write something like that in Python and someone else submits anything in Perl that's not an XY problem. It's a basic failure to meet clear requirements.

If some one wrote grep in Perl, and they are considered incapable of writing the same in Python. I have now doubts on the evaluation capabilities of the hiring committee.

Software world is huge. You will have learn, forget, relearn and learn new things all the time. You don't change programmers every time a new requirement on a new technology comes along. Which is why if some can write grep in Perl, they can also with a little yak shaving write grep in Python as well. Then in Golang, Java etc. And if some one wrote grep in Scheme, they can also write it in Clojure, or Common Lisp.

Programming languages of a particular style(C, Lisp based, ML family etc) don't change much in semantic features in their categories.

The point of a take home project is to see if a person can get things done.

I didn't say anything about capabilities.

If a base requirement is the thing is implemented with X, like wanting a table made of teak wood, and you give me a table that doesn't have any teak in it you've ignored half of my requirements. I don't care if you assessed my needs as being better met by oak and you made a fine table, I need to put a teak table somewhere and you completely failed at that task.

The point of a take home test that says build foo in Python so we can see it in Python is not "to see if a person can get things done," it's to see how a person gets things done in Python.

I mean, we're probably not going to ask someone to solve a brand new problem as part of a take-home project. We're intentionally not trying to be tricky, but to cover well-known territory so there's plenty of prior art and examples to fall back on. As another case, if you were hiring as a frontend dev, we might ask you to write a simple interactive web page using vanilla JavaScript and not jquery or such. It's awesome that you know React! Great, you'll fit right in! But first, can you use the stuff built into the browser without loading an extra library? Or put another way, are you capable of extending web frameworks if the need arises and not just using them?

You're kind of right about it being the XY problem, except it's where the misdirected candidate is trying to solve "how do I do thing X" when the real problem is "write a simple program in Y so we can know if you know Y". We aim to be very explicit about the project's requirements so that none of this is a surprise afterward.