def deduce_helpful_msg(req):<EOL>
msg = "<STR_LIT>"<EOL>if os.path.exists(req):<EOL><INDENT>msg = "<STR_LIT>"<EOL>try:<EOL><INDENT>with open(req, '<STR_LIT:r>') as fp:<EOL><INDENT>next(parse_requirements(fp.read()))<EOL>msg += "<STR_LIT>" +"<STR_LIT>" % (req) +"<STR_LIT>" +"<STR_LIT>" +"<STR_LIT>"<EOL><DEDENT><DEDENT>except RequirementParseError:<EOL><INDENT>logger.debug("<STR_LIT>" % (req), exc_info=True)<EOL><DEDENT><DEDENT>else:<EOL><INDENT>msg += "<STR_LIT>" % (req)<EOL><DEDENT>return msg<EOL>
Returns helpful msg in case requirements file does not exist,
    or cannot be parsed.

    :params req: Requirements file path