<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def __security_definitions_descriptor(self, issuers):
 """Create a descriptor for the security definitions. Args: issuers: dict, mapping issuer names to Issuer tuples Returns: The dict representing the security definitions descriptor. """

    if not issuers:
      result = {
          _DEFAULT_SECURITY_DEFINITION: {
              'authorizationUrl': '',
              'flow': 'implicit',
              'type': 'oauth2',
              'x-google-issuer': 'https://accounts.google.com',
              'x-google-jwks_uri': 'https://www.googleapis.com/oauth2/v3/certs',
          }
      }
      return result

    result = {}

    for issuer_key, issuer_value in issuers.items():
      result[issuer_key] = {
          'authorizationUrl': '',
          'flow': 'implicit',
          'type': 'oauth2',
          'x-google-issuer': issuer_value.issuer,
      }

      # If jwks_uri is omitted, the auth library will use OpenID discovery
      # to find it. Otherwise, include it in the descriptor explicitly.
      if issuer_value.jwks_uri:
        result[issuer_key]['x-google-jwks_uri'] = issuer_value.jwks_uri

    return result