[Fixed]-Setting a session variable in django tests

When no cookies are set in the client the session property is a empty dict, hence your error. Here is the relevant source of django.test.client.Client:

def _session(self):
    """
    Obtains the current session variables.
    """
    if 'django.contrib.sessions' in settings.INSTALLED_APPS:
        engine = import_module(settings.SESSION_ENGINE)
        cookie = self.cookies.get(settings.SESSION_COOKIE_NAME, None)
        if cookie:
            return engine.SessionStore(cookie.value)
    return {}
session = property(_session)

Since you are not logged in the cookie with the key matching settings.SESSION_COOKIE_NAME is not found.

However you could manually create a session object like this:

if not self.client.session:
    engine = import_module(settings.SESSION_ENGINE)

    self.client.session = engine.SessionStore()
    self.client.session.save()

This is the way the login handler in Client creates a new session.

EDIT: I realized you also need to save the session key in a cookie so that the next request uses the same session

Here’s a helper function you could put in your Client sub class that creates a new session and a referencing cookie:

def set_session_data(self, key, value):
    """Shortcut for setting session data regardless of being authenticated"""

    if not self.client.session:
        # Save new session in database and add cookie referencing it

        engine = import_module(settings.SESSION_ENGINE)

        self.client.session = engine.SessionStore()
        self.client.session.save()

        session_cookie = settings.SESSION_COOKIE_NAME
        self.client.cookies[session_cookie] = self.client.session.session_key
        cookie_data = {
            'max-age': None,
            'path': '/',
            'domain': settings.SESSION_COOKIE_DOMAIN,
            'secure': settings.SESSION_COOKIE_SECURE or None,
            'expires': None,
        }
        self.client.cookies[session_cookie].update(cookie_data)

    self.client.session[key] = value
    self.client.session.save()

Note: I’m not saying this is the only way to do this, this is one way I found out by reading the django source code. The code in this answer is not tested/run, therefor it might need some fine tuning.

Further reading

To read about how SessionStore works you can look at the django.contrib.sessions module.

To read about how session and cookies are handled in Client you can look at django.test.client.Client.

rzetterberg answer is the more rigorous one so I think it should remain accepted, but this way looks like it will also work

def setUp(self):
    """
    set up sessions for anonymous users
    """
    engine = import_module(settings.SESSION_ENGINE)
    store = engine.SessionStore()
    store.save()  
    self.client.cookies[settings.SESSION_COOKIE_NAME] = store.session_key

It looks like there is a current ticket open on this topic (started 5 years ago… but active within the last few months):
https://code.djangoproject.com/ticket/10899
and
https://code.djangoproject.com/ticket/11475

I used RequestFactory to generate a request object, set the session values manually, and passed it to the view function, as laid out in the docs.

from django.test import TestCase, RequestFactory
from django.urls import reverse

from .views import test_view

class MyTestCase(TestCase):
    def setUp(self):
        self.factory = RequestFactory()

    def test_view_x(self):
        request = self.factory.get(reverse('myapp:test_view'))
        request.session = {'foo': 'bar'}

        response = test_view(request)

        ...

        # Run some test on the response to ensure the view is returning
        # the expected value

Using RequestFactory to generate a request object lets you test a view function as you would test any other function – treating it as a black box, passing certain inputs and checking for the correct output. Note that you’re running the request in isolation, without any of the installed middleware.