Async SQLAlchemy in pytest fixture: greenlet_spawn has not been called

Hey there, fellow developer! Are you struggling with async SQLAlchemy in pytest fixtures and getting the dreaded “greenlet_spawn has not been called” error? Worry no more, as this article will provide you with a comprehensive guide on how to tackle this issue and get your tests up and running.

What is async SQLAlchemy?

Async SQLAlchemy is a library that allows you to interact with your database asynchronously, using Python’s async/await syntax. This is particularly useful when working with databases that support asynchronous operations, such as PostgreSQL.

Why do we need async SQLAlchemy in pytest fixtures?

When writing tests for your application, you often need to create a test database and populate it with some sample data. However, if your application uses async SQLAlchemy, you’ll need to ensure that your test database is also set up to handle asynchronous operations. This is where pytest fixtures come in.

The Problem: greenlet_spawn has not been called

When trying to use async SQLAlchemy in a pytest fixture, you might encounter the following error:

E       RuntimeError: greenlet_spawn has not been called

This error occurs because pytest doesn’t support async operations out of the box. To fix this, we need to use a special library called pytest-asyncio.

Solution 1: Using pytest-asyncio

Pytest-asyncio is a plugin that allows you to write async tests using pytest. To use it, you’ll need to install it first:

pip install pytest-asyncio

Next, you can write your pytest fixture using the @pytest.fixture marker:

import pytest
import asyncio
from sqlalchemy.ext.asyncio import AsyncSession, create_async_engine
from myapp import models

@pytest.fixture
async def db_session():
    engine = create_async_engine('postgresql+asyncpg://user:password@host/dbname')
    async with engine.begin() as conn:
        await conn.run_sync(models.Base.metadata.create_all)
        async_session = AsyncSession(bind=conn)
        yield async_session
        await async_session.close()

In this example, we’re creating an async engine using create_async_engine, and then creating an async session using AsyncSession. We’re also creating the database tables using run_sync.

Solution 2: Using pytest-asyncio with event_loop

An alternative approach is to use the event_loop fixture provided by pytest-asyncio. This allows you to write your tests using the async/await syntax:

import pytest
import asyncio
from sqlalchemy.ext.asyncio import AsyncSession, create_async_engine
from myapp import models

@pytest.fixture
def event_loop():
    loop = asyncio.get_event_loop()
    yield loop
    loop.close()

@pytest.fixture
async def db_session(event_loop):
    engine = create_async_engine('postgresql+asyncpg://user:password@host/dbname')
    async with engine.begin() as conn:
        await conn.run_sync(models.Base.metadata.create_all)
        async_session = AsyncSession(bind=conn)
        yield async_session
        await async_session.close()

In this example, we’re using the event_loop fixture to get the event loop, and then using it to create the async engine and session.

Solution 3: Using asyncpg

Another approach is to use the asyncpg library, which provides an async interface to PostgreSQL:

import pytest
import asyncpg
from myapp import models

@pytest.fixture
async def db_session():
    conn = await asyncpg.connect('postgresql://user:password@host/dbname')
    try:
        await conn.execute(models.Base.metadata.create_all)
        yield conn
    finally:
        await conn.close()

In this example, we’re using asyncpg to connect to the database, create the tables, and then yield the connection. Finally, we’re closing the connection in the fixture’s cleanup code.

Best Practices

When using async SQLAlchemy in pytest fixtures, there are a few best practices to keep in mind:

• Use async/await syntax: Make sure to use the async/await syntax when writing your tests, as this will ensure that your code is executed asynchronously.
• Use pytest-asyncio: Pytest-asyncio provides a lot of functionality out of the box, including support for async fixtures and tests.
• Use event_loop fixture: The event_loop fixture provided by pytest-asyncio can be very useful when writing async tests.
• Use asyncpg: Asyncpg provides a lightweight and efficient way to interact with PostgreSQL, and can be a good alternative to async SQLAlchemy.

Conclusion

In conclusion, using async SQLAlchemy in pytest fixtures can be a bit tricky, but with the right tools and techniques, it’s definitely possible. By using pytest-asyncio, asyncpg, and following best practices, you can write efficient and effective tests for your asynchronous application.

Remember to always use the async/await syntax, and to choose the solution that best fits your needs. With a little practice and patience, you’ll be writing async tests like a pro!

Solution	Pros	Cons
Pytest-asyncio	Provides async support out of the box, easy to use	Can be slow, requires additional setup
Pytest-asyncio with event_loop	Provides more control over the event loop, easy to use	Can be slow, requires additional setup
Asyncpg	Faster and more lightweight, easy to use	Requires additional setup, limited functionality

I hope this article has been helpful in resolving the “greenlet_spawn has not been called” error and using async SQLAlchemy in pytest fixtures. Happy testing!

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