Return the subset of `parsed_flags` that are found in the list `valid_flags`

def filter_flags(parsed_flags, valid_flags):
    """Return the subset of `parsed_flags` that are found in the list `valid_flags`"""
    def valid_argv(argv):
        """Figures out if a flag parsed from the flagfile matches a flag in
        the command about to be run."""
        flagname_match = FLAG_RE.match(argv)
        if not flagname_match:
            return True
        flagname = flagname_match.group()
        return flagname in valid_flags
    return list(filter(valid_argv, parsed_flags))