in a Flask unit-test, how can I mock objects on the request-global `g` object?

Question

I have a flask application that is setting up a database connection in a before_filter, very similar to this:

@app.before_request
def before_request():
    g.db = connect_db()

Now: I am writing some unit-tests and I do not want them to hit the database. I want to replace g.db with a mock object that I can set expectations on.

My tests are using app.test_client(), as is demonstrated in the flask documentation here. An example test looks something like

def test(self):
    response = app.test_client().post('/endpoint', data={..})
    self.assertEqual(response.status_code, 200)
    ...

The tests work and pass, but they are hitting the database and as I said I want to replace db access with mock objects. I do not see any way in test_client to access the g object or alter the before_filters.

Answer 1

This works

test_app.py

from flask import Flask, g

app = Flask(__name__)

def connect_db():
    print 'I ended up inside the actual function'
    return object()

@app.before_request
def before_request():
    g.db = connect_db()


@app.route('/')
def root():
    return 'Hello, World'

test.py

from mock import patch
import unittest

from test_app import app


def not_a_db_hit():
    print 'I did not hit the db'

class FlaskTest(unittest.TestCase):

    @patch('test_app.connect_db')
    def test_root(self, mock_connect_db):
        mock_connect_db.side_effect = not_a_db_hit
        response = app.test_client().get('/')
        self.assertEqual(response.status_code, 200)

if __name__ == '__main__':
    unittest.main()

So this will print out 'I did not hit the db', rather than 'I ended up inside the actual function'. Obviously you'll need to adapt the mocks to your actual use case.