old_contents: from airflow.models import BaseOperator
  from airflow.utils import send_email
  from airflow.utils import apply_defaults
  
  
  class EmailOperator(BaseOperator):
      '''
      Sends an email.
      :param to: list of emails to send the email to
      :type to: list or string (comma or semicolon delimited)
      :param subject: subject line for the email (templated)
      :type subject: string
      :param html_content: content of the email (templated), html markup
          is allowed
      :type html_content: string
      '''
  
      template_fields = ('to', 'subject', 'html_content')
  
      __mapper_args__ = {
          'polymorphic_identity': 'EmailOperator'
      }
  
      @apply_defaults
      def __init__(
              self,
              to,
              subject,
              html_content,
              *args, **kwargs):
          super(EmailOperator, self).__init__(*args, **kwargs)
          self.to = to
          self.subject = subject
          self.html_content = html_content
  
      def execute(self, execution_date):
          send_email(self.to, self.subject, self.html_content)
  
new_contents: from airflow.models import BaseOperator
  from airflow.utils import send_email
  from airflow.utils import apply_defaults
  
  
  class EmailOperator(BaseOperator):
      '''
      Sends an email.
      :param to: list of emails to send the email to
      :type to: list or string (comma or semicolon delimited)
      :param subject: subject line for the email (templated)
      :type subject: string
      :param html_content: content of the email (templated), html markup
          is allowed
      :type html_content: string
      '''
  
      template_fields = ('subject', 'html_content')
  
      __mapper_args__ = {
          'polymorphic_identity': 'EmailOperator'
      }
  
      @apply_defaults
      def __init__(
              self,
              to,
              subject,
              html_content,
              *args, **kwargs):
          super(EmailOperator, self).__init__(*args, **kwargs)
          self.to = to
          self.subject = subject
          self.html_content = html_content
  
      def execute(self, execution_date):
          send_email(self.to, self.subject, self.html_content)
  
subject: REmove to from templated_fields