Current behavior has a subtle pitfall.
The function returns a list of *existing* tasks (not removed by decref or explicit call).
If user's code has a strong reference to some task the task will be in return list for unpredictable amount of time, even if the task was done.

Returning only *alive* tasks can make a function result more predictable (`not task.done()`).

Opinions?

I agree, returning done tasks is pretty useless.

This is a new function so let's fix its behavour.  We need to:

1. Document this difference between new asyncio.all_tasks() and now deprecated Task.all_tasks().

2. Make sure that Task.all_tasks() works in 3.7 in the same way it was working before 3.7.

After re-thinking I come to another idea: let's keep `all_tasks()` behavior as is but add an `active_tasks()` function for returning all non-finished tasks.

It should be done in Python 3.8

After an off-list discussion I think I was able to convince Andrew to change the behaviour of the newly added asyncio.all_tasks() to return only pending tasks.  The logic behind this is that:

1. there's no good known use case for returning a list of all non-GC-ed tasks; if such a case exists, it's easy to implement support for it via 'loop.set_task_factory'.

2. OTOH, whenever one uses Task.all_task() they always want to filter out completed tasks.

3. Since this is a new API, it makes sense to design it right and not strive for backwards compatibility with now deprecated asyncio.Task.all_tasks().

New changeset 416c1ebd9896b394790dcb4f9f035b1a44ebe9ff by Yury Selivanov in branch 'master':
bpo-32610: Fix asyncio.all_tasks() to return only pending tasks. (GH-7174)
https://github.com/python/cpython/commit/416c1ebd9896b394790dcb4f9f035b1a44ebe9ff

Ned, this one would also be nice to have in 3.7.0.  The patch alters the behaviour of new 3.7 api.

New changeset ddc613f20394eba25f2420f6d2fa62b277c95058 by Miss Islington (bot) in branch '3.7':
bpo-32610: Fix asyncio.all_tasks() to return only pending tasks. (GH-7174)
https://github.com/python/cpython/commit/ddc613f20394eba25f2420f6d2fa62b277c95058