Python: py.test fixture for SQLAlchemy test in a transaction, create tables only once!

sqlalchemy_conftest.py

from sqlalchemy import create_engine
from sqlalchemy.orm import Session
from myapp.models import BaseModel
import pytest


@pytest.fixture(scope="session")
def engine():
    return create_engine("postgresql://localhost/test_database")


@pytest.fixture(scope="session")
def tables(engine):
    BaseModel.metadata.create_all(engine)
    yield
    BaseModel.metadata.drop_all(engine)


@pytest.fixture
def dbsession(engine, tables):
    """Returns an sqlalchemy session, and after the test tears down everything properly."""
    connection = engine.connect()
    # begin the nested transaction
    transaction = connection.begin()
    # use the connection with the already started transaction
    session = Session(bind=connection)

    yield session

    session.close()
    # roll back the broader transaction
    transaction.rollback()
    # put back the connection to the connection pool
    connection.close()

Thanks for the answer, I tried that too but that would not help.
Altho I found the problematic part but I don't know how to solve it yet. The issue is that I have a lot of Core CPUs and I had pytest --numprocesses/-n on auto. My assumption is that 2 (or more) of the tests from the same class - which are using these db fixtures - were running on different processes which would create new test session. Setting it to 1 (or not adding it) solved the problem.

With pytest-xdist, you should make a different test database for every process.