I was wrong.

from django.conf import settings
settings.configure()

It was as simple as calling settings.configure().

The following is irrelevant:

In a script unrelated to Django

t = Template(open(template_path).read())
c = Context({})
rendered = t.render(c)

I wanted to use the template renderer quickly like this

or use only single features of Django.

If you don't pay attention to PYTHONPATH or define the environment variable DJANGO_SETTINGS_MODULE,

django.core.exceptions.ImproperlyConfigured: Requested setting TEMPLATE_DEBUG, but settings are not configured. You must either define the environment variable DJANGO_SETTINGS_MODULE or call settings.configure() before accessing settings.

you will get this error.

Ideally, it would be better to organize it into a Django management command and run it with ./manage.py hoge, but if you don't want to make it so formal, or if you want to do it outside of a Django project,

from unittest.mock import MagicMock
from django import conf
setattr(conf, 'settings', MagicMock())

how about monkey-patching settings with an instance of MagicMock like this?

If you do this, you can then

t = Template(open(template_path).read())
c = Context({})
rendered = t.render(c)

also execute this.

(Though you might say just use Jinja2)