2048 Pythonic Merge Function
I didn’t review or see your old code when it was posted, but this is noticeably improved. However, I can still make a few suggestions:


num and nums are not great variable names. Characters are cheap; just write out the word number in full. I would also suggest giving numbers a slightly different/longer name to make it easier to read the line where you define slide.

Likewise pair/pairs could be a little easier to read, I think.
A few comments on the different branches to explain why you’re behaving in a particular way would be useful – to somebody unfamiliar with the rules of 2048, it might not be immediately obvious why it behaves the way it does.
A description of the doctest doesn’t belong in the function docstring, it probably belongs in a comment on the main() function. If I import your function into another file and then inspect merge.__doc__, the mention of a doctest “below” could be confusing.
Having “no output” mean success could be a problem, because there’s no way for me to distinguish between a success and the test not running. It should be totally clear whether your test passed, failed, or didn’t run. Anything else can hide problems.