SQLAlchemy with asyncpg crashing with error: asyncpg.InterfaceError - cannot perform operation: another operation is in progress

Question

I am developing a fastapi server using sqlalchemy and asyncpg to work with a postgres database. For each request, a new session is created (via fastapi dependency injection, as in the documentation). I used sqlite+aiosqlite before postgres+asyncpg and everything worked perfectly. After I switched from sqlite to postgres, every fastapi request crashed with the error:

sqlalchemy.dialects.postgresql.asyncpg.InterfaceError - cannot perform operation: another operation is in progress

This is how I create the engine and sessions:

from typing import Generator
import os

from sqlalchemy import create_engine
from sqlalchemy.orm import sessionmaker, Session
from sqlalchemy.ext.asyncio import AsyncSession, create_async_engine


user = os.getenv('PG_USER')
password = os.getenv('PG_PASSWORD')
domain = os.getenv('PG_DOMAIN')
db = os.getenv('PG_DATABASE')

# db_async_url = f'sqlite+aiosqlite:///database.sqlite3'
db_async_url = f'postgresql+asyncpg://{user}:{password}@{domain}/{db}'

async_engine = create_async_engine(
    db_async_url, future=True, echo=True
)

create_async_session = sessionmaker(
    async_engine, class_=AsyncSession, expire_on_commit=False
)

async def get_async_session() -> Generator[AsyncSession]:
    async with create_async_session() as session:
        yield session

Answer 1

The error disappeared after adding poolclass=NullPool to create_async_engine, so here's what engine creation looks like now:

from sqlalchemy.pool import NullPool

...

async_engine = create_async_engine(
    db_async_url, future=True, echo=True, poolclass=NullPool
)

I spent more than a day to solve this problem. I hope my answer will save a lot of time for other developers. Perhaps there are other solutions, and if so, I will be glad to see them here.