Prevent touching db during unit testing with SQLAlchemy

Question

I've been using Django for several years, but have recently decided to try out Flask for a new API. Thanks to Carl Meyers excellent presentation on testing Django at PyCon, I've been using the following technique to prevent touching the database in my Django unit tests:

cursor_wrapper = Mock()
cursor_wrapper.side_effect = RuntimeError("No touching the database!")

@patch('django.db.backends.util.CursorWrapper', cursor_wrapper)
class TestPurchaseModel(TestCase):
   '''Purchase model test suite'''
   ...

My question is can anyone tell me how to do this same basic technique with SQLAlchemy? In other words, I want any time I actually run a query against the database to produce a runtime error.

Answer 1

You can utilize SQLAlchemy's event system for this, which allows you to use a callback when SQLAlchemy performs different events.

In your case, you would probably want to use the before_execute() or before_cursor_execute() events. For example...

from sqlalchemy import event

class TestCase(unittest.TestCase):
    def setUp(self):
        engine = ... # create or access your engine somehow
        event.listen(engine, "before_cursor_execute", self._before_cursor_execute)

    # We can also clean up the event handler after the test if we want to
    def tearDown(self):
        engine = ... # access your engine again
        event.remove(engine, "before_cursor_execute", self._before_cursor_execute)

    def _before_cusor_execute(self, conn, cursor, statement, parameters, context, executemany):
        raise RuntimeError('No touching the database!')