When unittesting with Flask, Flask-SQLAlchemy, and SQLAlchemy, how do I isolate test databases between threads?

Question

Suppose I'm following the Flask-SQLAlchemy quickstart example, and I want to add a couple of unittests.

My model might look something like:

db = SQLAlchemy(app)
Base = db.Model

class Account(Base):
    id = Column(Integer, primary_key=True)
    name = Column(String(1000))

For unittesting, I'll want to create and destroy a database for each test.

def _setup_database():
    db_name = 'test_%s' % random.randint(0,999999)

    # (Code that creates database with db_name and setups the schema)

    app.config['DB_NAME'] = db_name
    app.config['SQLALCHEMY_DATABASE_URI'] = 'postgresql:///{}'.format(db_name)


def _destroy_db():
    db_name = app.config['DB_NAME']

    # Code that destroys the test db


class TestAccounts(unittest.TestCase):
    def setUp(self):
        _setup_database()

    def tearDown(self):
        _destroy_db()

    def test_new_signup(self):
        account = models.Account(...)

    def test_something_else(self):
        account = models.Account(...)

The problem here is that, if I run this code in an environment where unittests are multi-threaded, there is a race condition. Two databases are usually setup simultaneously and app.config is pointed to one of them.

If I were using SQLAlchemy directly, I would create a new session for each test and use that session. But Flask-SQLAlchemy creates sessions for me, and as such, seems to depend on having one global app.config['SQLALCHEMY_DATABASE_URI'] to point to a database.

What's the right way to create test databases and point a test thread to them with Flask-SQLAlchemy?

Don't have an answer but curious as to why you are doing multithreaded tests? Seems like an unnecessary layer of complexity. — Adam
– Adam, Commented Jul 11, 2017 at 0:52
@Adam It results in tests getting done a lot faster. Don't most people use multiple threads for unit tests? — Ken Kinder
– Ken Kinder, Commented Jul 11, 2017 at 17:29
I've never done it. My work doesn't have a robust CI/testing regimen in place. — Adam
– Adam, Commented Jul 11, 2017 at 20:42

Leon Matthews · Accepted Answer · 2023-04-14 17:40:11Z

With unittest TestCase.setUp and TestCase.tearDown are run for each test_ functions.

So running your test in multithreaded process will indeed create a race condition. You need to either run your test in a single thread. Which will fix the race condition, but you'll still create and destroy the all database for each test which is unnecessary and slow.

A better solution is to use the setUpclass and tearDownClass methods, which only run once per test class.

If you need to run this fixtures for all classes in a module there is also setUpModule and tearDownModule.

If you need to set fixtures for the full session now you have a problem... IMHO that is the time to switch to pytest.

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