code: def deduce_helpful_msg(req):
      # type: (str) -> str
      
      msg = ""
      if os.path.exists(req):
          msg = " It does exist."
          # Try to parse and check if it is a requirements file.
          try:
              with open(req, 'r') as fp:
                  # parse first line only
                  next(parse_requirements(fp.read()))
                  msg += " The argument you provided " + \
                      "(%s) appears to be a" % (req) + \
                      " requirements file. If that is the" + \
                      " case, use the '-r' flag to install" + \
                      " the packages specified within it."
          except RequirementParseError:
              logger.debug("Cannot parse '%s' as requirements \
              file" % (req), exc_info=True)
      else:
          msg += " File '%s' does not exist." % (req)
      return msg
docs: Returns helpful msg in case requirements file does not exist,
      or cannot be parsed.
  
      :params req: Requirements file path