问题
I here asked how I can make a version of asyncio.gather that executes the list of tasks sequentially and not in parallel, and good people told me how. But my joy ended as soon as I tried to put two in_sequence() methods into one another:
import asyncio
from typing import Coroutine
async def work_a():
print("Work 'A' start")
await asyncio.sleep(0)
print("Work 'A' finish")
async def work_b():
print("Work 'B' start")
await asyncio.sleep(0)
print("Work 'B' finish")
async def raise_exception():
print("raise_exception executed")
await asyncio.sleep(0)
raise RuntimeError("raise_exception executed")
async def in_sequence(*tasks: Coroutine):
"""Executes coroutines in sequence"""
tasks = iter(tasks)
try:
for task in tasks:
await task
# Terminates all other tasks scheduled for execution in event loop to prevent "RuntimeWarning: coroutine was never awaited" message
finally:
for remaining_task in tasks:
remaining_task.close()
async def main():
try:
await in_sequence( # outer in_sequence()
work_a(),
raise_exception(),
in_sequence( # inner in_sequence()
work_b()
)
)
except Exception as e:
print(f"+++ Quit with exception: {e}")
if __name__ == '__main__':
asyncio.run(main())
As a result, I received the following output with "RuntimeWarning: coroutine 'work_b' was never awaited" error:
Work 'A' start
Work 'A' finish
raise_exception executed
+++ Quit with exception: raise_exception executed
{some path}\terminate_inner_coroutine_issue.py:33: RuntimeWarning: coroutine 'work_b' was never awaited
remaining_task.close()
RuntimeWarning: Enable tracemalloc to get the object allocation traceback
After several experiments, I realized what the problem was: When the raise_exception() method throws an exception in the outer in_sequence() method, the outer in_sequence() method terminates only its child tasks in the finally block, i.e. only inner in_sequence(). In turn, the inner in_sequence() does not terminate its nested task work_b(), because at the moment when an exception occurs, inner in_sequence() is not yet running and the error occurs.
In further experiments, I “invented” the following version of in_sequence() which seems to solve the problem:
async def in_sequence(name: str, *tasks: Coroutine):
"""Executes coroutines in sequence"""
print(f"in_sequence '{name}' start")
tasks = iter(tasks)
try:
for task in tasks:
await task
except asyncio.CancelledError as c:
print(f"+++ in_sequence '{name}' catch CancelledError: {c}")
finally:
# Terminates all other tasks scheduled for execution in event loop
for remaining_task in tasks:
print(f"+++ Cancel {remaining_task}")
remaining_task.send(None)
try:
# Will be raised asyncio.CancelledError in coroutine
remaining_task.throw(asyncio.CancelledError)
except asyncio.CancelledError:
pass
# remaining_task.close() # >>> RuntimeWarning: coroutine 'work_b' was never awaited
# asyncio.create_task(remaining_task).cancel() # >>> RuntimeWarning: coroutine 'work_b' was never awaited
# asyncio.create_task(remaining_task).set_exception(asyncio.CancelledError) # >>> Exception - Task does not support set_exception operation
print(f"in_sequence '{name}' finish")
and returns the following output:
in_sequence '1' start
Work 'A' start
Work 'A' finish
raise_exception executed
+++ Cancel <coroutine object in_sequence at 0x0000024AF58B6AC0>
in_sequence '2' start
Work 'B' start
+++ in_sequence '2' catch CancelledError:
in_sequence '2' finish
+++ Quit with exception: coroutine raised StopIteration
The questions:
- Is this the correct solution or are there better options? How to terminate child tasks in such a situation?
- Is it normal that a 'coroutine raised StopIteration' exception throws in my implementation?
来源：https://stackoverflow.com/questions/61070740/how-to-properly-terminate-nested-asyncio-tasks