def cert(self):
        """
        The betfair certificates, by default it looks for the
        certificates in /certs/.

        :return: Path of cert files
        :rtype: str
        """
        if self.cert_files is not None:
            return self.cert_files

        certs = self.certs or '/certs/'
        ssl_path = os.path.join(os.pardir, certs)
        try:
            cert_path = os.listdir(ssl_path)
        except FileNotFoundError:
            raise CertsError(certs)
        except OSError:   # Python 2 compatability
            raise CertsError(certs)

        cert = None
        key = None
        for file in cert_path:
            ext = os.path.splitext(file)[-1]
            if ext in ['.crt', '.cert']:
                cert = os.path.join(ssl_path, file)
            elif ext == '.key':
                key = os.path.join(ssl_path, file)
        if cert is None or key is None:
            raise CertsError(certs)
        return [cert, key]
The betfair certificates, by default it looks for the
        certificates in /certs/.

        :return: Path of cert files
        :rtype: str