code: def get_issuers(self):
          
          issuers = []
  
          message_issuer_nodes = OneLogin_Saml2_Utils.query(self.document, '/samlp:Response/saml:Issuer')
          if len(message_issuer_nodes) > 0:
              if len(message_issuer_nodes) == 1:
                  issuers.append(OneLogin_Saml2_Utils.element_text(message_issuer_nodes[0]))
              else:
                  raise OneLogin_Saml2_ValidationError(
                      'Issuer of the Response is multiple.',
                      OneLogin_Saml2_ValidationError.ISSUER_MULTIPLE_IN_RESPONSE
                  )
  
          assertion_issuer_nodes = self.__query_assertion('/saml:Issuer')
          if len(assertion_issuer_nodes) == 1:
              issuers.append(OneLogin_Saml2_Utils.element_text(assertion_issuer_nodes[0]))
          else:
              raise OneLogin_Saml2_ValidationError(
                  'Issuer of the Assertion not found or multiple.',
                  OneLogin_Saml2_ValidationError.ISSUER_NOT_FOUND_IN_ASSERTION
              )
  
          return list(set(issuers))
docs: Gets the issuers (from message and from assertion)
  
          :returns: The issuers
          :rtype: list