How to properly inherit class method

Question

I have a database connection class that creates a connection pool. Now as the application grows and I'm adding different types of database writers, I want to move database connections to a separate class and inherit from it. So far I have this:

class ServiceDB:
    @classmethod
    async def init(cls, settings):
        self = ServiceDB()
        self.pool = await asyncpg.create_pool(
            database=settings["POSTGRES_DB"],
            user=settings["POSTGRES_USER"],
            password=settings["POSTGRES_PASSWORD"],
            host=settings["DB_HOST"],
            port=settings["DB_PORT"],
        )
        return self



class ChildWriter(ServiceDB):
    async def write_db(self, query):
        # Write to specific table
        pass



if __name__ == "__main__":
    settings = {'info': "some connection settings"}
    query = "SELECT * FROM 'table'"
    connection = await ChildWriter().init(settings)
    await connection.write_db(msg, query)

When I run this I get AttributeError: 'ServiceDB' object has no attribute 'write_db'. How do I properly extend ServiceDB with the write_db method?

Answer 1

Classmethods receive the "current class" as the first argument. Instantiate this cls, not the fixed baseclass.

class ServiceDB:
    @classmethod
    async def init(cls, settings):
        self = cls()  # cls is the *current* class, not just ServiceDB
        self.pool = await asyncpg.create_pool(
            database=settings["POSTGRES_DB"],
            user=settings["POSTGRES_USER"],
            password=settings["POSTGRES_PASSWORD"],
            host=settings["DB_HOST"],
            port=settings["DB_PORT"],
        )
        return self

Note that ideally, all attributes are set via __init__ instead of a separate classmethod constructor. The separate constructor should just pass on any attributes constructed externally.

class ServiceDB:
    def __init__(self, pool):
        self.pool = pool

    @classmethod
    async def init(cls, settings, **kwargs):
        pool = await asyncpg.create_pool(
            database=settings["POSTGRES_DB"],
            user=settings["POSTGRES_USER"],
            password=settings["POSTGRES_PASSWORD"],
            host=settings["DB_HOST"],
            port=settings["DB_PORT"],
        )
        return cls(pool=pool, **kwargs)

class ChildWriter(ServiceDB):
    async def write_db(self, query): ...

if __name__ == "__main__":
    settings = {'info': "some connection settings"}
    query = "SELECT * FROM 'table'"
    # call classmethod on class   V
    connection = await ChildWriter.init(settings)
    await connection.write_db(msg, query)