Manually Constructed Engines

Due to the dynamic nature of the creation of the databases themselves, its non-trivial for a user to know what the connection string, for example, would be for the database ahead of time. Which makes testing code which manually constructs its own sqlalchemy.Engine objects internally more difficult.

Therefore, generally preferable way to use the fixtures is that you will be yielded a preconstructed engine pointing at the database to which your test is intended to run against; and to write your code such that it accepts the engine as a function/class parameter.

However, this is not always possible for all classes of tests, nor does it help for code which might already be written with a tightly coupled mechanism for engine creation.

For (contrived) example:

import psycopg2
import sqlalchemy

def psycopg2_main(**config):
    conn = psycopg2.connect(**config)

def sqlalchemy_main(**config):
    conn = sqlalchemy.create_engine(**config)

As you can see, in order to test these functions, we must pass in valid credentials rather than an engine itself.


Each of the fixtures you might create will attach a pmr_credentials attribute onto the engine it yields to the test which will be an instance of a Credentials class.

Attributes on this class include all the credentials required to connect to the particular database. Additionally, there are convenience methods specifically meant to coerce the credentials into a form directly accepted by common connection mechanisms like psycopg2.connect or sqlalchemy.engine.url.URL.

from pytest_mock_resources import (

from package import entrypoint

postgres = create_postgres_fixture()
redshift = create_redshift_fixture()

def test_psycopg2_main_postgres(postgres):
    credentials = postgres.pmr_credentials
    result = entrypoint.psycopg2_main(**credentials.as_psycopg2_connect_args())
    assert result ...

def test_sqlalchemy_main_postgres(postgres):
    credentials = postgres.pmr_credentials
    result = entrypoint.sqlalchemy_main(**credentials.as_url())
    assert result ...