Using Markdown in Django

django_markdown.py

# Source code for article:
# https://hakibenita.com/django-markdown

from typing import Optional
import re
import markdown
from markdown.inlinepatterns import LinkInlineProcessor, LINK_RE
from urllib.parse import urlparse

from django.core.exceptions import ValidationError
from django.core.validators import EmailValidator
from django.urls import reverse, resolve, Resolver404, NoReverseMatch


class Error(Exception):
    pass


class InvalidMarkdown(Error):

    def __init__(self, error: str, value: Optional[str] = None) -> None:
        self.error = error
        self.value = value

    def __str__(self) -> str:
        if self.value is None:
            return self.error
        return f'{self.error} "{self.value}"';


def get_site_domain() -> str:
    # TODO: Get your site domain here
    return 'example.com'


def clean_link(href: str, site_domain: str) -> str:
    if href.startswith('mailto:'):
        email_match = re.match('^(mailto:)?([^?]*)', href)
        if not email_match:
            raise InvalidMarkdown('Invalid mailto link', value=href)

        email = email_match.group(2)
        if email:
            try:
                EmailValidator()(email)
            except ValidationError:
                raise InvalidMarkdown('Invalid email address', value=email)

        return href

    # Remove fragments or query params before trying to match the url name
    href_parts = re.search(r'#|\?', href)
    if href_parts:
        start_ix = href_parts.start()
        url_name, url_extra = href[:start_ix], href[start_ix:]
    else:
        url_name, url_extra = href, ''

    try:
        url = reverse(url_name)
    except NoReverseMatch:
        pass
    else:
        return url + url_extra

    parsed_url = urlparse(href)

    if parsed_url.netloc == site_domain:
        try:
            resolver_match = resolve(parsed_url.path)
        except Resolver404:
            raise InvalidMarkdown(
                "Should not use absolute links to the current site.\n"
                "We couldn't find a match to this URL. Are you sure it exists?",
                value=href,
            )
        else:
            raise InvalidMarkdown(
                'Should not use absolute links to the current site.\n'
                'Try using the url name "{}".'.format(resolver_match.url_name),
                value=href,
            )

    if parsed_url.scheme not in ('http', 'https'):
        raise InvalidMarkdown('Must provide an absolute URL (be sure to include https:// or http://)', href)

    return href


class DjangoLinkInlineProcessor(LinkInlineProcessor):
    def getLink(self, data, index):
        href, title, index, handled = super().getLink(data, index)
        site_domain = get_site_domain()
        href = clean_link(href, site_domain)
        return href, title, index, handled


class DjangoUrlExtension(markdown.Extension):
    def extendMarkdown(self, md, *args, **kwrags):
        md.inlinePatterns.register(DjangoLinkInlineProcessor(LINK_RE, md), 'link', 160)

django_markdown_test.py

import markdown

from . import DjangoUrlExtension, get_site_domain


md = markdown.Markdown(extensions=[DjangoUrlExtension()])

html = md.convert("[Help](mailto:support@service.com?subject=I need help!)")
assert html == '<p><a href="mailto:support@service.com?subject=I need help!">Help</a></p>'

html = md.convert("[Help](mailto:?subject=I need help!)")
assert html == '<p><a href="mailto:?subject=I need help!">Help</a></p>'

try:
    md.convert("[Help](mailto:invalidemail?subject=I need help!)")
except InvalidMarkdown as e:
    assert e.error == 'Invalid email address'
    assert e.value == 'invalidemail'
else:
    assert False, 'Expected "InvalidMarkdown" error'


html = md.convert("Go back to [homepage](home)")
assert html == '<p>Go back to <a href="/">homepage</a></p>'

html = md.convert("Go back to [homepage](home#offers)")
assert html == '<p>Go back to <a href="/#offers">homepage</a></p>'

html = md.convert("Go back to [homepage](home?foo=bar)")
assert html == '<p>Go back to <a href="/?foo=bar">homepage</a></p>'

html = md.convert("Go back to [homepage](home?foo=bar#offers)")
assert html == '<p>Go back to <a href="/?foo=bar#offers">homepage</a></p>'


try:
    md.convert(f'[link](https://{get_site_domain()}/foo)')
except InvalidMarkdown as e:
    assert e.value == f'https://{get_site_domain()}/foo'
    assert "We couldn't find a match to this URL." in e.error
else:
    assert False, 'Expected "InvalidMarkdown" error'

try:
    md.convert(f'[link](https://{get_site_domain()}/)')
except InvalidMarkdown as e:
    assert e.value == f'https://{get_site_domain()}/'
    assert 'Try using the url name' in e.error
else:
    assert False, 'Expected "InvalidMarkdown" error'

html = md.convert('[link](https://external.com)')
assert html == '<p><a href="https://external.com">link</a></p>'


try:
    md.convert('[link](external.com)')
except InvalidMarkdown as e:
    assert e.value == 'external.com'
    assert 'Must provide absolute url' in e.error
else:
    assert False, 'Expected "InvalidMarkdown" error'