Where do I put jinja2 helpers in a django 1.8 project?

I'm currently upgrading django from 1.7.10 with jingo to 1.8.5 (actually from 1.6.x but I can now run the site with 1.7)

jingo no longer works with django 1.8 so I'm switching to the new Jinja2 support in django 1.8. I've reached a point where my templates are found and they are attempting to render using jinja2 but my helpers (contextfunction) are not found.

My TEMPLATES config looks like this:

TEMPLATES = [
    {
        'BACKEND': 'django.template.backends.jinja2.Jinja2',
        'DIRS': [
            os.path.join(PROJECT_ROOT, 'mysite', 'html'),
            os.path.join(PROJECT_ROOT, 'mysite', 'html', 'site')
        ],
        'APP_DIRS': False,
        'OPTIONS': {
            'extensions': [
                'pipeline.templatetags.ext.PipelineExtension',
                'mysite.site.extensions.CacheTag',
                'mysite.site.extensions.CmsBlockTag',
            ],
            'extensions': [
                'jinja2.ext.autoescape',
                'jinja2.ext.do',
                'jinja2.ext.with_'
            ],
        },
    }
]

I currently have a file mysite/site/helpers.py that contains (amongst other functions and filters):

from jinja2 import contextfunction

@contextfunction
def upcase(text):
    return text.upper()

And a template:

<h3>{{ upcase('asdf') }}</h3>

I get the error Exception Value: 'upcase' is undefined.

How do I make the contents of the helpers file available?

Copyright Notice：Content Author:「edruid」，Reproduced under the CC 4.0 BY-SA copyright license with a link to the original source and this disclaimer.
Link to original article：https://stackoverflow.com/questions/32992593/where-do-i-put-jinja2-helpers-in-a-django-1-8-project