code: def best_match(
              self, req, working_set, installer=None, replace_conflicting=False):
          
          try:
              dist = working_set.find(req)
          except VersionConflict:
              if not replace_conflicting:
                  raise
              dist = None
          if dist is not None:
              return dist
          for dist in self[req.key]:
              if dist in req:
                  return dist
          # try to download/install
          return self.obtain(req, installer)
docs: Find distribution best matching `req` and usable on `working_set`
  
          This calls the ``find(req)`` method of the `working_set` to see if a
          suitable distribution is already active.  (This may raise
          ``VersionConflict`` if an unsuitable version of the project is already
          active in the specified `working_set`.)  If a suitable distribution
          isn't active, this method returns the newest distribution in the
          environment that meets the ``Requirement`` in `req`.  If no suitable
          distribution is found, and `installer` is supplied, then the result of
          calling the environment's ``obtain(req, installer)`` method will be
          returned.