code: def cert(self):
          
          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]
docs: The betfair certificates, by default it looks for the
          certificates in /certs/.
  
          :return: Path of cert files
          :rtype: str