New submission from Nathaniel Smith <[email protected]>:
In bpo-39386, the 'aclose' method for async generators was fixed so that the
following broken code would correctly raise an error:
# -- bad code --
async def agen_fn():
yield
async def do_bad_thing():
agen = agen_fn()
aclose_coro = agen.aclose()
await aclose_coro
# Should raise RuntimeError:
await aclose_coro
asyncio.run(do_bad_thing())
# -----------------
However, the merged fix also broke the following correct code, which should
*not* raise an error:
# -- good code --
async def agen_fn():
yield
async def do_good_thing():
agen = agen_fn()
await agen.aclose()
# Should *not* raise an error, but currently does in Python dev branches:
await agen.aclose()
asyncio.run(do_good_thing())
# ----------------
It's not supported to iterate the same coroutine object twice. However, making
two *independent* calls to aclose should return two independent coroutine
objects, and it should be legal to iterate over each object once.
This can also occur even if there's only a single call to 'aclose'. For
example, this is the recommended idiom for making sure that an async generator
correctly closes all its resources:
# -- good code --
async def agen_fn():
yield
async def careful_loop():
agen = agen_fn()
try:
async for _ in agen:
pass
finally:
await agen.aclose()
asyncio.run(careful_loop())
# -------------------
On released Python, the code above runs correctly. On the latest Python dev
branches, it raises a RuntimeError.
So basically the problem is that the fix for bpo-39386 is storing the "was
aclose iterated?" state on the async generator object, but in fact it needs to
be stored on the aclose coroutine object.
----------
keywords: 3.7regression, 3.8regression, 3.9regression
messages: 361783
nosy: asvetlov, lukasz.langa, ned.deily, njs, yselivanov
priority: release blocker
severity: normal
status: open
title: Regression: it should be possible to close async iterators multiple times
versions: Python 3.7, Python 3.8, Python 3.9
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Python tracker <[email protected]>
<https://bugs.python.org/issue39606>
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