When hiring developers, have the candidate read existing code

When hiring developers, have the candidate read existing code
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An example would have been nice though, as I'm not sure how to find a piece of code that does something standalone that is too large to grasp in 20 minutes yet make a reasonable prediction at the output. That combination seems kind of weird.

I wonder how well it would work to modify OP's idea and present a candidate with some code that has a bug in it (you reveal what the bug is to save time, they'll need to understand the code anyhow) and you ask them how they'd fix it. First broadly, like what approach, then actually write part of it.

To take my own advice and make the discussion more concrete, I should add an example. Random script from my github: https://github.com/lucb1e/randomdirectory/blob/master/random...

To prepare for the interview, one could add an argument like -h to show usage (in the same style as -v is currently implemented), then tell the candidate that the bug is that passing -vh executes neither -v nor -h (one would commonly expect it to execute both). Fixing this requires a 'structural' change to this little bit of code, but it's small enough to easily grasp and implement. The candidate might propose to loop over each character after a single hyphen to fix this, or they might propose to throw out this custom reinventing-the-wheel and use a proper arg parsing library. Either is fine, but they should be prepared to write the code (in any language, they can write just the changes in C, Python, JS, or whatever language they're comfortable with).

Has anyone tried such an interview coding question?

> I can quickly train a person to have knowledge in some domain,

This part I have a little bit of trouble with. If you have a trivial domain, sure. In my experience domain knowledge takes a while to truly get your head wrapped around. But I expect that's why there's always other things that people do in an interview.