code: def _setVirtualEnv():
      
      try:
          activate = options.virtualenv.activate_cmd
      except AttributeError:
          activate = None
  
      if activate is None:
          virtualenv = path(os.environ.get('VIRTUAL_ENV', ''))
          if not virtualenv:
              virtualenv = options.paved.cwd
          else:
              virtualenv = path(virtualenv)
  
          activate = virtualenv / 'bin' / 'activate'
  
          if activate.exists():
              info('Using default virtualenv at %s' % activate)
              options.setdotted('virtualenv.activate_cmd', 'source %s' % activate)
docs: Attempt to set the virtualenv activate command, if it hasn't been specified.