This snippet shows how to isolate a pytest test using pytest fixture.

test_iso.py

"""
This snippet shows how to isolate a pytest test using pytest fixture.

It requires flask, sqlalchemy, pytest, pytest-order to be installed:

> pip install flask sqlalchemy pytest pytest-order

You can run the script by

> pytest -s test_iso.py

(the "-s" is to display prints)
"""
import pytest
from flask import Flask
from sqlalchemy import create_engine
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.orm import scoped_session
from sqlalchemy.orm import sessionmaker
import sqlalchemy as sa

# <<<< SQLAlchemy setup >>>
# use in memory DB
DB_URI = "sqlite:///:memory:"
engine = create_engine(DB_URI)
connection = engine.connect()

Session = scoped_session(sessionmaker(bind=connection))


Base = declarative_base()


class Cat(Base):
    __tablename__ = "cat"
    id = sa.Column(sa.Integer, primary_key=True, autoincrement=True)
    name = sa.Column(sa.String(128), unique=True, nullable=False)


# create all tables
Base.metadata.create_all(engine)
# END <<<< SQLAlchemy setup >>>

# <<< Flask app setup >>>
def create_app() -> Flask:
    app = Flask(__name__)
    app.config["SQLALCHEMY_DATABASE_URI"] = DB_URI

    @app.route("/")
    def index():
        cats = Session.query(Cat).all()
        return {"cats": [cat.name for cat in cats]}

    return app


app = create_app()
# END <<< Flask app setup >>>


@pytest.fixture
def flask_client():
    """
    A test should use flask_client as argument to isolate the DB transaction
    :return:
    """
    transaction = connection.begin()

    with app.app_context():
        try:
            client = app.test_client()
            yield client
        finally:
            # roll back all commits made during a test
            transaction.rollback()
            Session.rollback()
            Session.close()


@pytest.mark.order(1)
def test_index(flask_client):
    print("start test_index")
    Session.add(Cat(name="european"))
    Session.add(Cat(name="egypt"))
    Session.commit()
    r = flask_client.get("/")

    assert r.json["cats"] == ["european", "egypt"]

    assert r.status_code == 200


@pytest.mark.order(2)
def test_2():
    print("start test_2")
    # make sure no Cat was added during test_index
    assert Session.query(Cat).count() == 0