What is the purpose of the single underscore “_” variable in Python?

Question

What is the meaning of _ after for in this code?

if tbh.bag:
   n = 0
   for _ in tbh.bag.atom_set():
      n += 1

Answer 1

It's just a variable name, and it's conventional in python to use _ for throwaway variables. It just indicates that the loop variable isn't actually used.