[Python] How to use in memory database for pytest without changing code?

How can I use a in memory database for pytest or avoid that data acutally will write in database after the test, like Django TestCase?

I don't want to change my code. I've created a function for creating a user with validation and more. For that function I used SQLAchemy with a asynchronous ContextManager for connection setup.

Like:

class DatabaseConnection:
async def __aenter__(self):
...

async def __aexit__(self, exc_type, exc_val, exc_tb):
...



My user creation looks like this then:

async def create_user(self, username: str, display_name: str, biography: str):
""" Create a new user in the database. """
# Validation with Pydantic ...

# Run the SQLAlchemy session and create the user
async with DatabaseConnection() as session:
...



Now when I start to test this function in pytest, it writes the data in the database but its only a test and I don't want to save useless and unreal data in my database.

conftest.py

import pytest
from sqlalchemy.ext.asyncio import AsyncSession
from initialisier import AsyncSessionLocal
import logging

logger = logging.getLogger(__name__)

@pytest.fixture
async def db_session():
async with AsyncSessionLocal() as session:
try:
yield session
except Exception as e:
logger.error(f"Error during test: {e}", exc_info=True)
finally:
await session.rollback()



test_service.py

@pytest.mark.asyncio
async def test_create_user_success(self, db_session, monkeypatch):
""" Test creating a user successfully. """
async def fake_enter(self):
return db_session

monkeypatch.setattr("connection.DatabaseConnection.__aenter__", fake_enter)

user = await self.service.create(
username=self.username, display_name=self.display_name, biography=self.biography
)

assert user is not None
assert user.username == self.username
assert user.display_name == self.display_name
assert user.biography == self.biography
assert user.id is not None

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