One pain point of asynchronous programming is to understand bugs and rebuild the chain of callbacks / coroutines / tasks. I added the source traceback to Handle, Future (Task) and CoroWrapper to help debugging.

Here is a new enhancement to provide more context in debug mode: add BaseEventLoop._current_handle which is the handle currently executed.

The first usage is the call_exception_handler() which logs the source traceback of the current handle in debug mode.

Example:
---
import asyncio

def bug():
    loop.call_exception_handler({'message': 'bug!'})

def schedule_bug():
    bug()

loop = asyncio.get_event_loop()
loop.call_soon(schedule_bug)
loop.call_later(1, loop.stop)
loop.run_forever()
loop.close()
---

Output in debug mode, without the patch:
---
bug!
---

Output in debug mode, with the patch:
---
bug!
handle_traceback: Handle created at (most recent call last):
  File "x.py", line 10, in <module>
    loop.call_soon(schedule_bug)
---

Later, I plan to use the source traceback of the current handle in more places. For example, use it to log messages.

I would like to know "who" logged the "SSL handshake failed". At the beginning, I wanted to add a source traceback to all transports, but it looks simpler to get the source traceback of the current handler. Moreover, this traceback is more useful than the source traceback of the transport.

Previous try to add the source traceback to transports:
https://code.google.com/p/tulip/issues/detail?id=212

Yury Selivanov proposed something different in the past: add a "context" (or a context identifier) to tasks to be able to (indirectly) attach local variables to tasks.

"Add notion of context_id to event loop"
https://code.google.com/p/tulip/issues/detail?id=165

I don't know if BaseEventLoop._current_handle is too specific or might be implemented with a task context. The task context looks to be specific to tasks, whereas handles are very generic in asyncio: almost all functions in asyncio are called in the context of a handle.

Previous discussion related to task context:

"local context in event loop"
https://groups.google.com/forum/#!topic/python-tulip/zix5HQxtElg

"ThreadLocal analogue"
https://groups.google.com/forum/#!topic/python-tulip/j0cSjUGx8qk

See also the tasklocals project:
https://github.com/vkryachko/tasklocals

@Guido, @Yury: What do you think of this feature? Does it make sense to expose (internally) the "handle currently executed"?

> What do you think of this feature? Does it make sense to expose (internally) the "handle currently executed"?

I think it's OK to have something like `loop._current_handle` to work ~only~ in debug mode. Enhancing `loop.call_exception_handler` to use it also makes sense. I would also want to make sure, that this property exists only in debug mode and shouldn't be used outside of asyncio.

New changeset 54d74f954bf9 by Victor Stinner in branch '3.4':
Issue #23208, asyncio: Add BaseEventLoop._current_handle
https://hg.python.org/cpython/rev/54d74f954bf9

I commited  current_handle.patch. It's only a first step, I will also change the logger or calls to the logger to use this traceback of the current handle.

New changeset d61d1e73674f by Victor Stinner in branch '3.4':
asyncio: sync with Tulip
https://hg.python.org/cpython/rev/d61d1e73674f