tests.py

import mock
from django.test import TestCase
from django.contrib.auth.models import User

class EmailTests(TestCase):
    
    def setUp(self):
        self.user = User.objects.create(
            username='theusername', email='the@email.com',
            first_name='Filipe', last_name='Ximenes',
            password='1')
        self.client.login(email=user.email, password='1')
    
    @mock.patch('myapp.views.send_templated_mail')
    def test_email_context_variables(self, send_templated_mail):
        # assume '/complete-signup/' is the URL to our view 
        self.client.post('/complete-signup/') 
        
        kargs = send_templated_mail.call_args[1]
        context = kwargs['context']
        
        self.assertEqual(context['username'], self.user.username)
        self.assertEqual(context['full_name'], self.user.get_full_name())
        

tests2.py

from django.conf import settings

# Get a copy of the default TEMPLATES value
RAISE_EXCEPTION_TEMPLATES = copy.deepcopy(settings.TEMPLATES)

# this is where the magic happens
class InvalidVarException(str):

    def __new__(cls, *args, **kwargs):
        return super().__new__(cls, '%s')

    def __mod__(self, missing):
        try:
            missing_str = str(missing)
        except:
            missing_str = 'Failed to create string representation'
        raise Exception('Unknown template variable %r %s' % (missing, missing_str))

    def __contains__(self, search):
        return True

# set the value of string_if_invalid and template debug to True
RAISE_EXCEPTION_TEMPLATES[0]['OPTIONS']['string_if_invalid'] = InvalidVarException()
RAISE_EXCEPTION_TEMPLATES[0]['OPTIONS']['debug'] = True


from django.test import override_settings

    @override_settings(TEMPLATES=RAISE_EXCEPTION_TEMPLATES)
    def test_passes_all_email_variables(self):
        self.client.post('/complete-signup/')

views.py

from django.http import HttpResponse
from templated_email import send_templated_mail


def complete_signup(request):
    request.user.is_active = True
    request.user.save()
    
    send_templated_mail(
        template_name='welcome',
        from_email='from@example.com',
        recipient_list=[request.user.email],
        context={
            'username': request.user.username,
            'full_name': request.user.get_full_name(),
        },
    )
    return HttpResponse('Signup Completed!')

welcome.email

{# templates/templated_emails/welcome.email #}

{% block subject %}Welcome {{ full_name }}!{% endblock %}

{% block plain %}
Hi {{ full_name }}!

You have successfuly completed the signup proccess. 
Access our site using your username: {{ username }}
{% endblock %}

{% block html %}
<h1>Hi {{full_name}}!</h1>
<p>You have successfuly completed the signup proccess</p>
<p>Access our site using your username: {{ username }}</p>
{% endblock %}