code: def from_signing_credentials(cls, credentials, **kwargs):
          
          kwargs.setdefault('issuer', credentials.signer_email)
          kwargs.setdefault('subject', credentials.signer_email)
          return cls(credentials.signer, **kwargs)
docs: Creates a new :class:`google.auth.jwt.OnDemandCredentials` instance
          from an existing :class:`google.auth.credentials.Signing` instance.
  
          The new instance will use the same signer as the existing instance and
          will use the existing instance's signer email as the issuer and
          subject by default.
  
          Example::
  
              svc_creds = service_account.Credentials.from_service_account_file(
                  'service_account.json')
              jwt_creds = jwt.OnDemandCredentials.from_signing_credentials(
                  svc_creds)
  
          Args:
              credentials (google.auth.credentials.Signing): The credentials to
                  use to construct the new credentials.
              kwargs: Additional arguments to pass to the constructor.
  
          Returns:
              google.auth.jwt.Credentials: A new Credentials instance.