title: What is verbosity level exactly in python?(difference between each level)
body: <p>What is the verbosity level in python.
  I see it in unittest.</p>
  
  <p>In documentation it just say simply that higher verbosity level, more info print out. but 
  <strong>more</strong> means what? That is to say, which message will be printed out in a higher level and which will not?</p>
  
  <p>Also find verbosity in logging.</p>
  
  <p>I think they are different due to the reason that logging verbosity level is in [0, 50] and unittest's is just a unit number.
  I just want to find out the difference between each verbosity level in unittests.</p>