How do I write SQLAlchemy test fixtures for FastAPI applications

I am writing a FastAPI application that uses a SQLAlchemy database. I have copied the example from the FastAPI documentation, simplifying the database schema for concisions' sake. The complete source is at the bottom of this post.

This works. I can run it with uvicorn sql_app.main:app and interact with the database via the Swagger docs. When it runs it creates a test.db in the working directory.

Now I want to add a unit test. Something like this.

from fastapi import status
from fastapi.testclient import TestClient
from pytest import fixture

from main import app


@fixture
def client() -> TestClient:
    return TestClient(app)


def test_fast_sql(client: TestClient):
    response = client.get("/users/")
    assert response.status_code == status.HTTP_200_OK
    assert response.json() == []

Using the source code below, this takes the test.db in the working directory as the database. Instead I want to create a new database for every unit test that is deleted at the end of the test.

I could put the global database.engine and database.SessionLocal inside an object that is created at runtime, like so:

    class UserDatabase:
        def __init__(self, directory: Path):
            directory.mkdir(exist_ok=True, parents=True)
            sqlalchemy_database_url = f"sqlite:///{directory}/store.db"
            self.engine = create_engine(
                sqlalchemy_database_url, connect_args={"check_same_thread": False}
            )
            self.SessionLocal = sessionmaker(autocommit=False, autoflush=False, bind=self.engine)
            models.Base.metadata.create_all(bind=self.engine)

but I don't know how to make that work with main.get_db, since the Depends(get_db) logic ultimately assumes database.engine and database.SessionLocal are available globally.

I'm used to working with Flask, whose unit testing facilities handle all this for you. I don't know how to write it myself. Can someone show me the minimal changes I'd have to make in order to generate a new database for each unit test in this framework?

---

The complete source of the simplified FastAPI/SQLAlchemy app is as follows.

database.py

from sqlalchemy import create_engine
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.orm import sessionmaker

SQLALCHEMY_DATABASE_URL = "sqlite:///./test.db"

engine = create_engine(
    SQLALCHEMY_DATABASE_URL, connect_args={"check_same_thread": False}
)
SessionLocal = sessionmaker(autocommit=False, autoflush=False, bind=engine)

Base = declarative_base()

models.py

from sqlalchemy import Column, Integer, String

from database import Base


class User(Base):
    __tablename__ = "users"

    id = Column(Integer, primary_key=True, index=True)
    name = Column(String)
    age = Column(Integer)

schemas.py

from pydantic import BaseModel


class UserBase(BaseModel):
    name: str
    age: int


class UserCreate(UserBase):
    pass


class User(UserBase):
    id: int

    class Config:
        orm_mode = True

crud.py

from sqlalchemy.orm import Session

import schemas
import models


def get_user(db: Session, user_id: int):
    return db.query(models.User).filter(models.User.id == user_id).first()


def get_users(db: Session, skip: int = 0, limit: int = 100):
    return db.query(models.User).offset(skip).limit(limit).all()


def create_user(db: Session, user: schemas.UserCreate):
    db_user = models.User(name=user.name, age=user.age)
    db.add(db_user)
    db.commit()
    db.refresh(db_user)
    return db_user

main.py

from typing import List

from fastapi import Depends, FastAPI, HTTPException
from sqlalchemy.orm import Session

import schemas
import models
import crud
from database import SessionLocal, engine

models.Base.metadata.create_all(bind=engine)

app = FastAPI()


# Dependency
def get_db():
    try:
        db = SessionLocal()
        yield db
    finally:
        db.close()


@app.post("/users/", response_model=schemas.User)
def create_user(user: schemas.UserCreate, db: Session = Depends(get_db)):
    return crud.create_user(db=db, user=user)


@app.get("/users/", response_model=List[schemas.User])
def read_users(skip: int = 0, limit: int = 100, db: Session = Depends(get_db)):
    users = crud.get_users(db, skip=skip, limit=limit)
    return users


@app.get("/users/{user_id}", response_model=schemas.User)
def read_user(user_id: int, db: Session = Depends(get_db)):
    db_user = crud.get_user(db, user_id=user_id)
    if db_user is None:
        raise HTTPException(status_code=404, detail="User not found")
    return db_user

You need to override your get_db dependency in your tests, see these docs.

Something like this for your fixture:

@fixture
def db_fixture() -> Session:
    raise NotImplementError()  # Make this return your temporary session

@fixture
def client(db_fixture) -> TestClient:

    def _get_db_override():
        return db_fixture

    app.dependency_overrides[get_db] = _get_db_override
    return TestClient(app)