Emails from file-based templates

from textwrap import fill, dedent

from django.template import Context
from django.template.loader import get_template
from django.template.loader_tags import BlockNode, ExtendsNode
from post_office.mail import send


def send_mail_from_file(recipients, name, context={}, **kwargs):
    subject, message, html_message = render_email(name, context)
    return send(recipients=recipients, subject=subject, html_message=html_message,
                message=message, template=None, context=None, **kwargs)


# https://github.com/BradWhittington/django-templated-email/blob/master/templated_email/backends/vanilla_django.py
def render_email(template_name, context={}, template_dir='mails/'):
    context = Context(context, autoescape=False)
    full_template_name = '%s%s.email' % (template_dir, template_name)
    template = get_template(full_template_name)

    subject = render_template_block(template, 'subject', context).strip()
    plain = render_template_block(template, 'plain', context).strip()
    html = render_template_block(template, 'html', context).strip()

    # strip leading whitespace
    plain = dedent(plain)
    # wrap lines individually
    new_plain = '\n'.join([fill(line) for line in plain.splitlines()])

    return subject, new_plain, html


# https://github.com/uniphil/Django-Block-Render/blob/master/block_render.py
class BlockNotFound(Exception):
    """The requested block did not exist."""
    pass


def render_template_block(template, block, context):
    """
    Renders a single block from a template.
    This template should have previously been rendered.

    """
    template._render(context)
    return _render_template_block_nodelist(template.nodelist, block, context)


def _render_template_block_nodelist(nodelist, block, context):
    for node in nodelist:
        if isinstance(node, BlockNode) and node.name == block:
            return node.render(context)
        for key in ('nodelist', 'nodelist_true', 'nodelist_false'):
            if hasattr(node, key):
                try:
                    rendered = _render_template_block_nodelist(
                        getattr(node, key), block, context)
                except:
                    pass
                else:
                    return rendered
    for node in nodelist:
        if isinstance(node, ExtendsNode):
            try:
                rendered = render_template_block(
                    node.get_parent(context), block, context)
            except BlockNotFound:
                pass
            else:
                return rendered
    raise BlockNotFound