Django email templates where subject and text/html body are in the same file

email.py

from django.conf import settings
from django.core import mail
from django.template.context import make_context
from django.template.loader import get_template
from django.template.loader_tags import BlockNode, ExtendsNode


class BaseEmailMessage(mail.EmailMultiAlternatives):
    template_name = None
    context = {}
    html = None
    _node_map = {
        "subject": "subject",
        "text": "body",
        "html": "html",
        # TODO: support extends "email/base.html"
        # "text_body": "body",
        # "html_body": "html",
    }

    def __init__(self, template_name=None, context=None, *args, **kwargs):
        super(BaseEmailMessage, self).__init__(*args, **kwargs)

        if template_name is not None:
            self.template_name = template_name

        if context is not None:
            self.context = context

    def get_template_name(self):
        return self.template_name

    def get_context_data(self):
        return self.context

    def send(self, *args, **kwargs):
        self.render()
        super(BaseEmailMessage, self).send(*args, **kwargs)

    def render(self):
        template = get_template(self.get_template_name())
        context = make_context(self.get_context_data())
        with context.bind_template(template.template):
            blocks = self._get_blocks(template.template.nodelist, context)
            for block_node in blocks.values():
                self._process_block(block_node, context)
        self._attach_body()

    def _process_block(self, block_node, context):
        attr = self._node_map.get(block_node.name)
        if attr is not None:
            setattr(self, attr, block_node.render(context).strip())

    def _get_blocks(self, nodelist, context):
        blocks = {}
        for node in nodelist:
            if isinstance(node, ExtendsNode):
                parent = node.get_parent(context)
                blocks.update(self._get_blocks(parent.nodelist, context))
        blocks.update(
            {node.name: node for node in nodelist.get_nodes_by_type(BlockNode)}
        )
        return blocks

    def _attach_body(self):
        if self.body and self.html:  # when both text_body and html_body provided
            self.attach_alternative(self.html, "text/html")
        elif self.html:  # when only html_body provided
            self.body = self.html
            self.content_subtype = "html"


def send_email(
    template_name,
    to,
    context=None,
    attachments=None,
    cc=settings.EMAIL_DEFAULT_CC,
    bcc=settings.EMAIL_DEFAULT_BCC,
    reply_to=settings.EMAIL_DEFAULT_REPLY_TO,
    from_email=settings.DEFAULT_FROM_EMAIL,
    headers=settings.EMAIL_DEFAULT_HEADERS,
    fail_silently=settings.EMAIL_DEFAULT_FAIL_SILENTLY,
):
    return BaseEmailMessage(
        template_name=template_name,
        to=to,
        context={**settings.EMAIL_DEFAULT_TEMPLATE_CONTEXT, **context},
        attachments=attachments,
        cc=cc,
        bcc=bcc,
        reply_to=reply_to,
        from_email=from_email,
        headers=headers,
    ).send(fail_silently=fail_silently)

template.html

{% block subject %}[Action Required] ⚠️ {{user.first_name}}, review your information{% endblock %}

{% block text %}
Hi {{user.first_name}},
{% endblock %}

{% block html %}
<p>Hi {{user.first_name}},</p>
{% endblock %}