如何在类的构造函数中使用异步代码？

Question

I have a fastAPI project that talks to a Cosmos database.
All of my fastAPI routes are async (async def ...).
I need an async class that will perform CRUD operations of the Cosmos DB.
The problem I'm having is figuring out a constructor for the class.

I want the constructor to:

Take in the CosmosClient as an arg (azure.cosmos.aio.CosmosClient)

Use the client to make sure the database is created

Use the client to make sure the container is created

Something like this:

class CosmosCRUD:

    def init(self, client: CosmosClient):

        self.client = CosmosClient

        self.database = await self.client.create_database_if_not_exists("MY_DATABASE_NAME"😉

        self.container = await self.database.create_container_if_not_exists("MY_CONTAINER_NAME", partition_key=...)

Unfortunately, you can only await inside of async functions, and init can't be async, so the code above doesn't work.

As far as I can tell, there's a few solutions:

Create a new event loop inside of init and run the async code that gets the db and container within that
- As far as I can tell, that means that fastAPI will stop processing all requests any time an instance of this class is being created, which is probably going to be on every request. This will absolutely destroy performance of the entire app

Ignore init and create a new async constructor function like create that users of this class need to call
- The IDE (PyCharm) doesn't know that create is the constructor that will always be called first. Any instance variables created in this new constructor that you try to use in other methods will be flagged as non-existing by the IDE, because they weren't created in init
- I tried having both a create async classmethod that is the actual constructor, and an init method where I just added declarations for any instance variables created in create, with the types (ex: self.client: CosmosClient) but the IDE still complains they don't exist

Use dependency injection somehow
- Something like this: https://stackoverflow.com/a/65247387/6423456">https://stackoverflow.com/a/65247387/6423456>
- Unfortunately, my init method is taking the client as an argument, and needs to pass it to the function being injected
- In the example referenced above, that would be like needing to pass the a arg to the async_dep being injected.
- Is that even possible? I'm guessing not

Is there a good way to have async code inside of a constructor, without making the IDE unhappy about missing instance variables?

Adam Franco · Answer

You can inherit your class from AsyncMixin and implement ainit method, there you can use async functions:

import asyncio


class AsyncMixin:

    def init(self, *args, **kwargs):

        """

        Standard constructor used for arguments pass

        Do not override. Use ainit instead

        """

        self.__storedargs = args, kwargs

        self.async_initialized = False

async def __ainit__(self, *args, **kwargs):
    """Async constructor, you should implement this"""

async def __initobj(self):
    """Crutch used for __await__ after spawning"""
    assert not self.async_initialized
    self.async_initialized = True
    # pass the parameters to __ainit__ that passed to __init__
    await self.__ainit__(*self.__storedargs[0], **self.__storedargs[1])
    return self

def __await__(self):
    return self.__initobj().__await__()


class CosmosClient():

    pass


class CosmosCRUD(AsyncMixin):

    def init(self, client: CosmosClient):

        self.client = client

        super().init()    

async def __ainit__(self):
    await asyncio.sleep(1)
    print("it works!")
    # self.database = await self.client.create_database_if_not_exists("MY_DATABASE_NAME")
    # self.container = await self.database.create_container_if_not_exists("MY_CONTAINER_NAME", partition_key=...)

def some_method(self):
    assert self.async_initialized == True



async def main():

    c = await CosmosCRUD(client=CosmosClient())

    c.some_method()

cc = CosmosCRUD(client=CosmosClient())
cc.some_method() # Will fail, because it was not awaited during the instance construction


if name == "main":

    asyncio.run(main())

kennebec · Answer

It is generally a bad idea to run asyc code inside a constructor. You should consider a redesign of your program flow. Classes should be trivially constructible. Instead consider a init, run and done kind of program flow and put your logic inside the run part.