Sessions are empty when testing

This doc expresses that we can access session without any extra context manager.

However the following code does not return any session key (session is empty):

from flask import Flask


import pytest
from flask import session


myapp = Flask('testing')
myapp.secret_key = 'secret'
myapp.debug = True

@myapp.route('/')
def home():
    return session['message']


@pytest.fixture
def app():
    return myapp


def test_home(client):
    session['message'] = 'something'
    response = client.get('/')
    assert response.data == 'something'

Any hint on what we're missing here?

Thanks :)