code: def check_if_exists(self, use_user_site):
          # type: (bool) -> bool
          
          if self.req is None:
              return False
          try:
              # get_distribution() will resolve the entire list of requirements
              # anyway, and we've already determined that we need the requirement
              # in question, so strip the marker so that we don't try to
              # evaluate it.
              no_marker = Requirement(str(self.req))
              no_marker.marker = None
              self.satisfied_by = pkg_resources.get_distribution(str(no_marker))
              if self.editable and self.satisfied_by:
                  self.conflicts_with = self.satisfied_by
                  # when installing editables, nothing pre-existing should ever
                  # satisfy
                  self.satisfied_by = None
                  return True
          except pkg_resources.DistributionNotFound:
              return False
          except pkg_resources.VersionConflict:
              existing_dist = pkg_resources.get_distribution(
                  self.req.name
              )
              if use_user_site:
                  if dist_in_usersite(existing_dist):
                      self.conflicts_with = existing_dist
                  elif (running_under_virtualenv() and
                          dist_in_site_packages(existing_dist)):
                      raise InstallationError(
                          "Will not install to the user site because it will "
                          "lack sys.path precedence to %s in %s" %
                          (existing_dist.project_name, existing_dist.location)
                      )
              else:
                  self.conflicts_with = existing_dist
          return True
docs: Find an installed distribution that satisfies or conflicts
          with this requirement, and set self.satisfied_by or
          self.conflicts_with appropriately.