I found a very strange bug in asyncio where whether exception handlers are called depends on whether a task is stored.

To illustrate, the following code works as expected, printing out that it made it to the exception handler:

import asyncio
async def run():
    raise RuntimeError
def exception_handler(loop, context):
    print('Made it to exception handler.')
loop = asyncio.get_event_loop()
loop.set_exception_handler(exception_handler)
loop.create_task(run())
loop.run_forever()

However, if you take that exact same code but store the task that is returned from create_task, the exception handler will NOT be called:

import asyncio
async def run():
    raise RuntimeError
def exception_handler(loop, context):
    print('Made it to exception handler.')
loop = asyncio.get_event_loop()
loop.set_exception_handler(exception_handler)
task = loop.create_task(run())
loop.run_forever()

This is completely bizarre, and I have been unable to track down the reason.

In the first case, once the loop has run the task it no longer has a reference to it, and it gets GCed.  The __del__ method of the task calls your exception handler.  In the second case, you have a reference to it, so __del__ does not get called.

If you want the exception to be realized in the second case, you have to yield from it somewhere in your program.