Unit test a Flask session - cannot reproduce failure with session_transaction - flask

Question

I am testing a Flask application (Flask 0.9), and in particular I have a session fixture that I would like to run in the documented-way, being something like this (as I understand it):
from flask import Flask, session
app = Flask(__name__)
#app.route('/', methods=['POST'])
def m():
logging.error(session) # expect {'x': 1}
return ""
with app.test_request_context() as trc:
with app.test_client() as c:
with c.session_transaction() as sess:
sess['x'] = 1
c.post()
This works as expected, with the output being something like this:
ERROR:root:<SecureCookieSession {'x': 1}>
Unfortunately I am encountering an unexpected result where the session data is not set in the endpoint function, i.e. the output is something like this:
ERROR:root:<SecureCookieSession {}>
This issue exhibits only when run from my unit testing framework. As it stands, I am unable to reproduce this problem with a degenerate case, though I have made a fairly substantial effort with a gist of some of this effort here. The salient points being that I have included itsdangerous and Google App Engine testbed, expecting maybe one of them to have been the cause.
On my own system I have gone further than the gist, and almost completely replicated my unit test framework trying to isolate this. Likewise, I have removed ever-increasing amounts of relevant code from my testing framework. To the point, I am unable to think of differences between the degenerate case and my stripped-down framework that could influence the outcome. I have traversed the c.post() call in pdb to try eek out the cause of this malignity, but have yet to glean any useful insight.
Which is all to say, I would be grateful for a little direction or suggestion as to where the issue may lie. What could possibly be influencing the Werkzeug context in such a way that the session_transaction is not being honoured?

Answer 1

In my case, I was restricting cookies to a specific domain automatically by loading a configuration file. By updating the configuration on-the-fly, I was able to get cookies to work while unit testing. Setting the SESSION_COOKIE_DOMAIN property to None, all domains (namely localhost) were able to set sessions.
app.config.update(
SESSION_COOKIE_DOMAIN = None
)
You may want to fiddle around with the configuration settings described under Configuration Handling in the docs.

Answer 2

I hate to resurrect an old question, but I believe that I figured out the solution to this issue. For testing, try setting your server name to localhost:
app.config['SERVER_NAME'] = 'localhost'
I was originally using Brian's hack, but this solved the problem for me.

Answer 3

Without a testcase that actually fails it's hard to say much anything. All I can think of is that the TestClient instance you're using the make the request is different from the one you use to set up the session. E.g. you could make the gist fail as expected with this:
with self.app.test_client() as c:
with c.session_transaction() as sess:
sess['d'] = 1
self.client.post()
But as this isn't the case here or the gist, go figure.

Answer 4

Here is what I ended up doing:
# Code that modifies the current `session`; we must be in a test context already.
headers = {}
with self.client.session_transaction():
# Get the cookie that is sent to the browser to establish a connection
# via a fake request.
response = make_response()
self.app.session_interface.save_session(self.app, session, response)
headers['Cookie'] = response.headers.get('Set-Cookie', '')
self.client.post(..., headers=headers) # also works for .get, .put, etc.