code: def install_package(package,
                      wheels_path,
                      venv=None,
                      requirement_files=None,
                      upgrade=False,
                      install_args=None):
      
      requirement_files = requirement_files or []
  
      logger.info('Installing %s...', package)
      if venv and not os.path.isdir(venv):
          raise WagonError('virtualenv {0} does not exist'.format(venv))
  
      pip_command = _construct_pip_command(
          package,
          wheels_path,
          venv,
          requirement_files,
          upgrade,
          install_args)
  
      if IS_VIRTUALENV and not venv:
          logger.info('Installing within current virtualenv')
  
      result = _run(pip_command)
      if not result.returncode == 0:
          raise WagonError('Could not install package: {0} ({1})'.format(
              package, result.aggr_stderr))
docs: Install a Python package.
  
      Can specify a specific version.
      Can specify a prerelease.
      Can specify a venv to install in.
      Can specify a list of paths or urls to requirement txt files.
      Can specify a local wheels_path to use for offline installation.
      Can request an upgrade.