How to set class attribute with await in __init__

I would recommend a separate factory method. It’s safe and straightforward. However, if you insist on a async version of __init__(), here’s an example:

def asyncinit(cls):
    __new__ = cls.__new__

    async def init(obj, *arg, **kwarg):
        await obj.__init__(*arg, **kwarg)
        return obj

    def new(cls, *arg, **kwarg):
        obj = __new__(cls, *arg, **kwarg)
        coro = init(obj, *arg, **kwarg)
        #coro.__init__ = lambda *_1, **_2: None
        return coro

    cls.__new__ = new
    return cls

Usage:

@asyncinit
class Foo(object):
    def __new__(cls):
        '''Do nothing. Just for test purpose.'''
        print(cls)
        return super().__new__(cls)

    async def __init__(self):
        self.initialized = True

async def f():
    print((await Foo()).initialized)

loop = asyncio.get_event_loop()
loop.run_until_complete(f())

Output:

<class '__main__.Foo'>
True

Explanation:

Your class construction must return a coroutine object instead of its own instance.

Another way to do this, for funsies:

class aobject(object):
    """Inheriting this class allows you to define an async __init__.

    So you can create objects by doing something like `await MyClass(params)`
    """
    async def __new__(cls, *a, **kw):
        instance = super().__new__(cls)
        await instance.__init__(*a, **kw)
        return instance

    async def __init__(self):
        pass

#With non async super classes

class A:
    def __init__(self):
        self.a = 1

class B(A):
    def __init__(self):
        self.b = 2
        super().__init__()

class C(B, aobject):
    async def __init__(self):
        super().__init__()
        self.c=3

#With async super classes

class D(aobject):
    async def __init__(self, a):
        self.a = a

class E(D):
    async def __init__(self):
        self.b = 2
        await super().__init__(1)

# Overriding __new__

class F(aobject):
    async def __new__(cls):
        print(cls)
        return await super().__new__(cls)

    async def __init__(self):
        await asyncio.sleep(1)
        self.f = 6

async def main():
    e = await E()
    print(e.b) # 2
    print(e.a) # 1

    c = await C()
    print(c.a) # 1
    print(c.b) # 2
    print(c.c) # 3

    f = await F() # Prints F class
    print(f.f) # 6

import asyncio
loop = asyncio.get_event_loop()
loop.run_until_complete(main())

Better yet you can do something like this, which is very easy:

import asyncio

class Foo:
    def __init__(self, settings):
        self.settings = settings

    async def async_init(self):
        await create_pool(dsn)

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

loop = asyncio.get_event_loop()
foo = loop.run_until_complete(Foo(settings))

Basically what happens here is __init__() gets called first as usual. Then __await__() gets called which then awaits async_init().

[Almost] canonical answer by @ojii

@dataclass
class Foo:
    settings: Settings
    pool: Pool

    @classmethod
    async def create(cls, settings: Settings, dsn):
        return cls(settings, await create_pool(dsn))

Depending on your needs, you can also use AwaitLoader from:
https://pypi.org/project/async-property/

From the docs:

AwaitLoader will call await instance.load(), if it exists, before loading properties.

I would like to show a much easier way of initiating coroutine based method within the __init__ method.

import asyncio


class Foo(object):

    def __init__(self, settings):
        self.settings = settings
        loop = asyncio.get_event_loop() 
        self.pool = loop.run_until_complete(create_pool(dsn))

foo = Foo(settings)

Important point to be noted is:

• This makes the async code work as sync(blocking)
• This is not the best way to run async code, but when it comes to only initiation via a sync method eg: __init__ it will be a good fit.
• After initiation, you can run the async methods from the object with await. i.e await foo.pool.get(value)
• Do not try to initiate via an await call you will get RuntimeError: This event loop is already running

The AsyncObj class with __ainit__ "async-constructor":

class AsyncObj:
    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
        await self.__ainit__(*self.__storedargs[0], **self.__storedargs[1])  # pass the parameters to __ainit__ that passed to __init__
        return self

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

    def __init_subclass__(cls, **kwargs):
        assert asyncio.iscoroutinefunction(cls.__ainit__)  # __ainit__ must be async

    @property
    def async_state(self):
        if not self.async_initialized:
            return "[initialization pending]"
        return "[initialization done and successful]"

Here is example of "async class":

class MyAsyncObject(AsyncObj):
    async def __ainit__(self, param1, param2=0):
        print("hello!", param1, param2)
        # go something async, e.g. go to db
    

Usage:

async def example():
    my_obj = await MyAsyncObject("test", 123)

This worked for me in Python 3.9

from aiobotocore.session import AioSession
import asyncio




class SomeClass():

    def __init__(self):
        asyncio.run(self.async_init())
        print(self.s3)

    async def async_init(self):
        self.s3 = await AioSession().create_client('s3').__aenter__()

Vishnu shettigar’s answer is so far the simplest, except that his async_init method doesn’t return the object itself so foo isn’t assigned a Foo instance. As for OP’s purpose, the most elegant way to construct the class IMHO is

import asyncio

class Foo:
    def __init__(self, settings):
        self.settings = settings

    def __await__(self):
        self.pool = asyncio.create_task(create_pool(dsn))
        yield from self.pool
        self.pool = self.pool.result()
        return self

To initialize the object, do the following

def main():
    loop = asyncio.get_event_loop()
    foo = loop.run_until_complete(Foo(settings))

Or

async def main():
    foo = await Foo(settings)

we could convert the async call to sync call by running the async code manually through asyncio.run()

class Foo:
    async def __ainit__(self, param):
        self._member = await some_async_func(param)

    def __init__(self, param):
        asyncio.run(self.__ainit__(param))

Everyone can try：
https://pypi.org/project/asyncinit/

• pip install asyncinit

from asyncinit import asyncinit

@asyncinit
class MyClass:
    async def __init__(self, param):
        self.val = await self.deferredFn(param)

    async def deferredFn(self, x):
        # ...
        return x + 2

obj = await MyClass(42)
assert obj.val == 44

I wrote this mixin:

import asyncio


class AsyncMixin:
    """Adds an async_init method to an object which is called when the
    object is awaited.
    Typically the idiom obj = await Object()
    provides the synchronous __init__() and async async_init() calls"""

    async def async_init(self):
        """If an AsyncMixin object is created in an async context (ie await
        Object() then the __init__ method is non-async as normal but
        the async_init() method is called immediately by the
        __await__() magic method.
        """
        pass

    async def _async_init(self):
        task = asyncio.create_task(self.async_init())
        await task
        return self

    def __await__(self):
        return self._async_init().__await__()
    pass

So the OP’s solution becomes:

class Foo(object, AsyncMixin):

    def __init__(self, settings):
        self.settings = settings

    async def async_init(self):
        self.pool = await create_pool(dsn)

foo = await Foo(settings)

Which I think is quite elegant