code: def install(self, release_id, upgrade=False):
          
          release_path = os.path.join(self._releases, release_id)
          if not self._runner.exists(release_path):
              self._runner.run("{0} '{1}'".format(self._venv_path, release_path))
  
          cmd = [os.path.join(release_path, 'bin', 'pip'), 'install']
          if upgrade:
              cmd.append('--upgrade')
  
          sources = self._get_install_sources()
          if sources:
              cmd.append(sources)
  
          cmd.extend("'{0}'".format(package) for package in self._packages)
          return self._runner.run(' '.join(cmd))
docs: Install target packages into a virtual environment.
  
          If the virtual environment for the given release ID does not
          exist on the remote system, it will be created. The virtual
          environment will be created according to the standard Tunic
          directory structure (see :doc:`design`).
  
          If ``upgrade=True`` is passed, packages will be updated to the
          most recent version if they are already installed in the virtual
          environment.
  
          :param str release_id: Timestamp-based identifier for this
              deployment. If this ID corresponds to a virtual environment
              that already exists, packages will be installed into this
              environment.
          :param bool upgrade: Should packages be updated if they are
              already installed in the virtual environment.
          :return: The results of running the installation command using
              Fabric. Note that this return value is a decorated version
              of a string that contains additional meta data about the
              result of the command, in addition to the output generated.
          :rtype: str