code: def evaluate(self, context):
          
          headers = {"Subject": self._subject.format(**context)}
  
          md = self._source.format(**context)
          html = markdown.markdown(md)
          parts = {"text/html": html, "text/plain": md}
          return headers, parts
docs: Evaluates the markdown source in the given context, producing HTML
          and (Markdown) plain text. Also interpolates the subject line with
          the given context.