Things like the Counter in python collections. Actually, the python collections library in general. You don't need that stuff to do the job, but it makes the code a lot clearer.

It probably took a couple of months full time in python before I had enough understanding of the std lib to know where to find everything (I sat down and read through a description of most of the functions in there).

There are still plenty of libraries that would have made my life easier if I knew about them. I'm sure there are plenty more.

Having said all that, that's just for example. When it comes to hiring I'd still pick a good experienced developer over a specific language expert for a full time position. For me it's not so much about the understanding of a specific language as it is the appetite for understanding computers / systems in general that makes truely good developers.

Ultimately the task is going to dictate the best type of hire. Short term contract, get a specific expert. Long term hire, find someone who actually enjoys developing software :)