django_db_setup does not override database settings at runtime

[coryphoenixxx]

I am using testcontainers to run PostgreSQL in a Docker container and override Django’s database settings dynamically via django_db_setup. However, the tests still attempt to connect to the database defined in settings.py instead of the one from django_db_setup.

Code Snippets

#pyproject.toml

[tool.pytest.ini_options]
DJANGO_SETTINGS_MODULE = "emailcrm.core.settings"
python_files = "tests.py test_*.py *_tests.py"

# settings.py

DATABASES = {
    "default": {
        "ENGINE": "django.db.backends.postgresql",
        "NAME": "emailcrm",
        "USER": "admin",
        "PASSWORD": "password",
        "HOST": "localhost",
        "PORT": "5432",
    }
}

# conftest.py

from typing import Generator
import pytest
from testcontainers.postgres import PostgresContainer

@pytest.fixture(scope="session")
def postgres_container() -> Generator[PostgresContainer, None, None]:
    with PostgresContainer("postgres:17") as postgres:
        yield postgres

@pytest.fixture(scope="session")
def django_db_setup(postgres_container) -> None:
    from django.conf import settings

    settings.DATABASES["default"] = {
        "ENGINE": "django.db.backends.postgresql",
        "NAME": postgres_container.dbname,
        "USER": postgres_container.username,
        "PASSWORD": postgres_container.password,
        "HOST": postgres_container.get_container_host_ip(),
        "PORT": postgres_container.get_exposed_port(5432),
    }

# test_some_test.py

import pytest

@pytest.mark.django_db
def test_can_connect_to_database() -> None:
    assert 1 == 1

Environment

• Python version: 3.13.2
• Django version: 5.1.7
• pytest version: 8.3.5
• pytest-django version: 4.10.0
• testcontainers version: 4.9.2

Expected Behavior

Tests should connect to the database provided by PostgresContainer.

Actual Behavior

Tests attempt to connect to the database from settings.py, leading to

ERROR tests/integration/test_some_test.py::test_can_connect_to_database - django.db.utils.OperationalError: connection failed: connection to server at "127.0.0.1", port 5432 failed: Connection refused

.
.
.
Is this the expected behavior? Should django_db_setup override settings before pytest.mark.django_db initializes the database connection? If not, what is the recommended approach for dynamically setting up test databases with testcontainers?

[kingbuzzman]

To understand this more:

1. You have docker installed, and python installed on your local machine.
2. You install pytest, testcontainers, ... ON your local machine (possibly using venv [a virtual environment])
3. You then run pytest (locally), and you want it to spawn a postgres docker instance while the test is running

Did I understand all this correctly?

---

It has come to my attention that you can run docker inside docker (highly inadvisable from what I read), but this could also be another way...

[coryphoenixxx]

@kingbuzzman,

1. Yes.
2. Yes.
3. Yes. The postgres container is running successfully, but I can't overwrite the settings that django has already set.

I don't use docker inside docker.

A similar approach works without problems on my other project with fastapi.

You can easily reproduce this behavior using my snippets.

[kingbuzzman]

I haven't tested this so don't shoot the messenger:

@pytest.fixture(scope="session")
def django_db_setup(postgres_container, request) -> None:
    from django.conf import settings

    settings.DATABASES["default"] = {
        "ENGINE": "django.db.backends.postgresql",
        "NAME": postgres_container.dbname,
        "USER": postgres_container.username,
        "PASSWORD": postgres_container.password,
        "HOST": postgres_container.get_container_host_ip(),
        "PORT": postgres_container.get_exposed_port(5432),
    }
	yield request.getfixturevalue("django_db_setup")

but something like this should work

[kingbuzzman]

OK, i've tested it, and it does work. BUT I HIGHLY suggest you DON'T override settings.DATABASES["default"] completely. Instead, override field by field such as:

    settings.DATABASES["default"]["ENGINE"] = "django.db.backends.postgresql"
    settings.DATABASES["default"]["NAME"] = postgres_container.dbname
    settings.DATABASES["default"]["USER"] = postgres_container.username
    settings.DATABASES["default"]["PASSWORD"] = postgres_container.password
    settings.DATABASES["default"]["HOST"] = postgres_container.get_container_host_ip()
    settings.DATABASES["default"]["PORT"] = postgres_container.get_exposed_port(5432)

The reason is that when django initializes, it add extra keys to each of the db definitions, and well it will crash if you don't add them... If you feel like debugging this in another version of django when it breaks again, the fields are:

{'default': {'ATOMIC_REQUESTS': False,
             'AUTOCOMMIT': True,
             'CONN_HEALTH_CHECKS': False,
             'CONN_MAX_AGE': 0,
             'ENGINE': 'django.db.backends.postgresql',
             'HOST': 'postgres',
             'NAME': 'db',
             'OPTIONS': {},
             'PASSWORD': 'password',
             'PORT': '5432',
             'TEST': {'CHARSET': None,
                      'COLLATION': None,
                      'MIGRATE': True,
                      'MIRROR': None,
                      'NAME': None},
             'TIME_ZONE': None,
             'USER': 'root'},

don't say i didn't warn you 😇

[coryphoenixxx]

@kingbuzzman,
Thanks a lot!

My approach was based on this: https://pytest-django.readthedocs.io/en/latest/database.html#using-an-existing-external-database-for-tests.

[kingbuzzman]

I'll make a PR to fix this.