Persist and fetch data in with block

Question

I have a situation - I'm using the asyncio package with Python 3.x, and persisting data in a with block, something like this:

test_repo = TestRepository()

with (yield from test_repo):
    res = yield from test_repo.get_by_lim_off(
            page_size=int(length),
            offset=start,
            customer_name=customer_name,
            customer_phone=customer_phone,
            return_type=return_type
        )

I need to get res data in the with block, but persistence and fetching data should happen when I exit from the with block. How can I achieve this?

Answer 1

This behavior is only supported in Python 3.5+, via asynchronous context managers (__aenter__/__aexit__), and async with, both of which were added in PEP 492:

class TestRepository:
   # All your normal methods go here

   async def __aenter__(self):
      # You can call coroutines here
      await self.some_init()

   async def __aexit__(self, exc_type, exc, tb):
      # You can call coroutines here
      await self.do_persistence()
      await self.fetch_data()


async def do_work():
    test_repo = TestRepository()

    async with test_repo:
        res = await test_repo.get_by_lim_off(
                page_size=int(length),
                offset=start,
                customer_name=customer_name,
                customer_phone=customer_phone,
                return_type=return_type
            )

 asyncio.get_event_loop().run_until_complete(do_work())

Prior to 3.5, you have to use a try/finally block with explicit calls to the init/cleanup coroutines, unfortunately:

@asyncio.coroutine
def do_work():
    test_repo = TestRepository()

    yield from test_repo.some_init()
    try:
        res = yield from test_repo.get_by_lim_off(
                page_size=int(length),
                offset=start,
                customer_name=customer_name,
                customer_phone=customer_phone,
                return_type=return_type
            )
    finally:
        yield from test_repo.do_persistence()
        yield from test_repo.fetch_data()