def generateChecker(self, argstring):
        """
        This checker factory expects to get the location of a file.
        The file should conform to the format required by
        L{FilePasswordDB} (using defaults for all
        initialization parameters).
        """
        from twisted.python.filepath import FilePath
        if not argstring.strip():
            raise ValueError('%r requires a filename' % self.authType)
        elif not FilePath(argstring).isfile():
            self.errorOutput.write('%s: %s\n' % (invalidFileWarning, argstring))
        return FilePasswordDB(argstring)

        This checker factory expects to get the location of a file.
        The file should conform to the format required by
        L{FilePasswordDB} (using defaults for all
        initialization parameters).
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/plugins/cred_file.py