How require that an abstract method is a coroutine?

Question

How can I require that an abstract base class implement a specific method as a coroutine. For example, consider this ABC:

import abc

class Foo(abc.ABC):
    @abc.abstractmethod
    async def func():
        pass

Now when I subclass and instantiate that:

class Bar(Foo):
    def func():
        pass

b = Bar()

This succeeds, although func is not async, as in the ABC. What can I do so that this only succeeds if func is async?

Answer 1

You may use __new__ and check if and how a child class has override parent's coros.

import asyncio
import abc
import inspect


class A:    

    def __new__(cls, *arg, **kwargs):
        # get all coros of A
        parent_coros = inspect.getmembers(A, predicate=inspect.iscoroutinefunction)

        # check if parent's coros are still coros in a child
        for coro in parent_coros:
            child_method = getattr(cls, coro[0])
            if not inspect.iscoroutinefunction(child_method):
                raise RuntimeError('The method %s must be a coroutine' % (child_method,))

        return super(A, cls).__new__(cls, *arg, **kwargs)

    @abc.abstractmethod
    async def my_func(self):
        pass


class B(A):

    async def my_func(self):
        await asyncio.sleep(1)
        print('bb')


class C(A):

    def my_func(self):
        print('cc')

async def main():
    b = B()
    await b.my_func()

    c = C()  # this will trigger the RuntimeError
    await c.my_func()


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

Caveats

• a child class may override __new__ as well to suppress this constraint

• not only async may be awaited. For example

  async def _change_in_db(self, key, value):
      # some db logic
      pass
  
  def change(self, key, value):
      if self.is_validate(value):
          raise Exception('Value is not valid')
      return self._change_in_db(key, value)  
  

  it's ok to call change like

  await o.change(key, value)
  

  Not to mention __await__ in objects, other raw Futures, Tasks...