Templated email with {% extends parent_name %} doesn't re-resolve the base template

[Andrew]

In django templating you can use {% extends parent_name %} to indicate that the template is derived from another. We use this feature along with django-templated-email to have different base email templates for white-label purposes.

The issue appears to be that {% extends parent_name %} does not get re-evaluated when the variable changes, eg. from email/app1.email to email/app2.email.

• The Engine uses django.template.loaders.cached.Loader
• The first time the template is loaded, the parent template is loaded based on parent_name is loaded
• Subsequent emails load the template, but do not re-evaluate the extends

---

If this is as-designed, how would one go about clearing out that cache? When setting up my test case I call the code below, but it doesn't appear to clear the cache, even though my print() debugging shows its the same object (<django.template.loaders.cached.Loader object at 0x113e99828>_

for template in engines.all():
    for loader in template.engine.template_loaders:
        loader.reset()
        for x in loader.loaders:
            x.reset()

[Tim Graham]

Please include more explicit steps to reproduce (that don't rely on a third-party library, to rule out an issue there). Based on your report, I tried the following change in Django's test suite but don't see a failure.

tests/template_tests/syntax_tests/test_extends.py

@@ -108 +108 @@
         """
         output = self.engine.render_to_string('inheritance06', {'foo': 'inheritance02'})
         self.assertEqual(output, '1234')
+        output = self.engine.render_to_string('inheritance06', {'foo': 'inheritance01'})
+        self.assertEqual(output, '1&3_')
 
     @setup(inheritance_templates)
     def test_inheritance07(self):

[Andrew]

It would be best to know, going in, if {extends} is actually *expected* to act like a meta-{%include%} or if the template engine actually combines the two.

I'll work on a better test case or sample project, but it'll probably be a few days. I've not had any cause to ever pull down the source before, and certainly don't know how write proper tests for it, i'm just investigating templates for the first time now.

• we use the django-templated-email package to wrap doing it, but it ends up in a DjangoTemplates processor.

The Base Email (object_created.email). The base template includes {%include%} directives which seem to be re-evaluated each time, the opposite of extends. My working theory is that somehow the two just get combined on the *first run* and ever-after are the same template somehow (even though I can't find the cache, or figure out who's doing it)

{% extends base_template %}{% load i18n %}
{% block subject_line %}{% blocktrans %}New Something Happened{% endblocktrans %}{% endblock %}
{% block html_content %}
    <h2 style="font-size:15px;">Dear {{ some_person }},</h2>

In the two tests I set the base_template to something different, and also try to reset() all the loaders including the children of the cached.Loader, to no affect.

context_goog = { base_template="email/google_template.email",  some_person="Wonko the Sane" }
context_amzn = { base_template="email/amazon_template.email",  some_person="Wonko the Sane" }

send_templated_email("object_created.email", context_goog)
...
django.template.loader.get_template('email/object_created.email')
  uses engine: <django.template.backends.django.DjangoTemplates object at 0x108573668>
    DjangoTemplates.get_template('email/object_created.email')
      <django.template.engine.Engine object at 0x108d7a2e8>
      .find_template('email/object_created.email') loader=<django.template.loaders.cached.Loader object at 0x10b957ef0>
      .find_template('email/google_template.email') loader=<django.template.loaders.cached.Loader object at 0x10b957ef0>
      .find_template('email/partials/contents-footer.html') loader=<django.template.loaders.cached.Loader object at 0x10b957ef0>


send_templated_email("object_created.email", context_amzn)
...
django.template.loader.get_template('email/object_created.email')
  uses engine: <django.template.backends.django.DjangoTemplates object at 0x108573668>
    DjangoTemplates.get_template('email/object_created.email')
      <django.template.engine.Engine object at 0x108d7a2e8>
      .find_template('email/object_created.email') loader=<django.template.loaders.cached.Loader object at 0x10b957ef0>
      .find_template('email/partials/contents-footer.html') loader=<django.template.loaders.cached.Loader object at 0x10b957ef0>

I hope that printf_trace stuff helps in evaluating what is going on. It might take me a few days to formulate a test case (if I can) or make a new project (if I can't) demonstrating what is going on.