python asyncio - how to wait for a cancelled shielded task?

Question

If I have a coroutine which runs a task which should not be cancelled, I will wrap that task in asyncio.shield().

It seems the behavior of cancel

Answer 1

It would make more sense if asyncio.shield() raised the asyncio.CancelledError after the await-ed task has completed, but obviously there is some other idea going on here that I don't understand.

asyncio.shield

• creates a dummy future, that may be cancelled
• executes the wrapped coroutine as future and bind to it a callback on done to setting a result for the dummy future from the completed wrapped coroutine
• returns the dummy future

You can see the implementation here

What is the proper method to shield a task from cancellation and then wait for it to complete before returning

You should shield count(5) future

async def t():
  c_ft = asyncio.ensure_future(count(5))
  try:
    await asyncio.shield(c_ft)
  except asyncio.CancelledError:
    print('This gets called at 3, not 5')
    await c_ft

  return 42

or t() future

async def t():
  await count(5)
  return 42    

async def m():
  ft = asyncio.ensure_future(t())
  shielded_ft = asyncio.shield(ft)
  ct = asyncio.ensure_future(c(shielded_ft))

  try:
    r = await shielded_ft
  except asyncio.CancelledError:
    print('Shield cancelled')
    r = await ft