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bpo-33786: Fix asynchronous generators to handle GeneratorExit in athrow() #7467

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Merged
merged 3 commits into from
Jun 8, 2018
Merged

bpo-33786: Fix asynchronous generators to handle GeneratorExit in athrow() #7467

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4a1e660
bpo-33786: Fix asynchronous generators to handle GeneratorExit in ath…
1st1 Jun 7, 2018
e1aa019
Add a contextlib-specific regression test
1st1 Jun 7, 2018
81827b3
Make compare_generators test helper use async iter protocol correctly
1st1 Jun 7, 2018
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2 changes: 1 addition & 1 deletion Lib/contextlib.py
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Original file line number Diff line number Diff line change
Expand Up @@ -187,7 +187,7 @@ async def __aexit__(self, typ, value, traceback):
# in this implementation
try:
await self.gen.athrow(typ, value, traceback)
raise RuntimeError("generator didn't stop after throw()")
raise RuntimeError("generator didn't stop after athrow()")
except StopAsyncIteration as exc:
return exc is not value
except RuntimeError as exc:
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56 changes: 56 additions & 0 deletions Lib/test/test_asyncgen.py
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Original file line number Diff line number Diff line change
Expand Up @@ -108,6 +108,31 @@ def sync_iterate(g):
res.append(str(type(ex)))
return res

def async_iterate(g):
res = []
while True:
an = g.__anext__()
try:
while True:
try:
an.__next__()
except StopIteration as ex:
if ex.args:
res.append(ex.args[0])
break
else:
res.append('EMPTY StopIteration')
break
except StopAsyncIteration:
raise
except Exception as ex:
res.append(str(type(ex)))
break
except StopAsyncIteration:
res.append('STOP')
break
return res

def async_iterate(g):
res = []
while True:
Expand Down Expand Up @@ -297,6 +322,37 @@ async def gen():
"non-None value .* async generator"):
gen().__anext__().send(100)

def test_async_gen_exception_11(self):
def sync_gen():
yield 10
yield 20

def sync_gen_wrapper():
yield 1
sg = sync_gen()
sg.send(None)
try:
sg.throw(GeneratorExit())
except GeneratorExit:
yield 2
yield 3

async def async_gen():
yield 10
yield 20

async def async_gen_wrapper():
yield 1
asg = async_gen()
await asg.asend(None)
try:
await asg.athrow(GeneratorExit())
except GeneratorExit:
yield 2
yield 3

self.compare_generators(sync_gen_wrapper(), async_gen_wrapper())

def test_async_gen_api_01(self):
async def gen():
yield 123
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22 changes: 22 additions & 0 deletions Lib/test/test_contextlib_async.py
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Expand Up @@ -36,6 +36,28 @@ async def __aexit__(self, *args):
async with manager as context:
self.assertIs(manager, context)

@_async_test
async def test_async_gen_propagates_generator_exit(self):
# A regression test for https://bugs.python.org/issue33786.

@asynccontextmanager
async def ctx():
yield

async def gen():
async with ctx():
yield 11

ret = []
exc = ValueError(22)
with self.assertRaises(ValueError):
async with ctx():
async for val in gen():
ret.append(val)
raise exc

self.assertEqual(ret, [11])

def test_exit_is_abstract(self):
class MissingAexit(AbstractAsyncContextManager):
pass
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1 change: 1 addition & 0 deletions Misc/NEWS.d/next/Core and Builtins/2018-06-06-23-24-40.bpo-33786.lBvT8z.rst
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@@ -0,0 +1 @@
Fix asynchronous generators to handle GeneratorExit in athrow() correctly
15 changes: 7 additions & 8 deletions Objects/genobject.c
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Expand Up @@ -1876,21 +1876,20 @@ async_gen_athrow_send(PyAsyncGenAThrow *o, PyObject *arg)
return NULL;

check_error:
if (PyErr_ExceptionMatches(PyExc_StopAsyncIteration)) {
if (PyErr_ExceptionMatches(PyExc_StopAsyncIteration) ||
PyErr_ExceptionMatches(PyExc_GeneratorExit))
{
o->agt_state = AWAITABLE_STATE_CLOSED;
if (o->agt_args == NULL) {
/* when aclose() is called we don't want to propagate
StopAsyncIteration; just raise StopIteration, signalling
that 'aclose()' is done. */
StopAsyncIteration or GeneratorExit; just raise
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As far as I understand, this will make .aclose() raise StopIteration in the usual case (when generator doesn't handle the GeneratorExit). What about .athrow(), how will it behave? Shouldn't their behavior be similar to that of .close() and .throw() for sync generators, where .close() swallows the GeneratorExit and .throw() doesn't?

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As far as I understand, this will make .aclose() raise StopIteration in the usual case (when generator doesn't handle the GeneratorExit).

Keep in mind, that raising StopIteration from any of a***() methods just means that they are returning. They are all iterators.

What about .athrow(), how will it behave?

The idea here:

  1. athrow() and aclose() share one implementation.

  2. aclose() is basically an athrow(GeneratorExit).

  3. When aclose() is called and the generator propagates GeneratorExit, it means that everything is fine and the aclose() call was successful. We raise StopIteration to make aclose() awaitable return.

  4. When athrow() is called we don't want to trap any exceptions at all. So we raise StopIteration only if the asynchronous generator swallowed the thrown error and yielded (asynchronously).

This is a bit complex, I know. I explained how asynchronous generators work in detail here: https://www.python.org/dev/peps/pep-0525/#implementation-details

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Thanks, now I get it

StopIteration, signalling that this 'aclose()' await
is done.
*/
PyErr_Clear();
PyErr_SetNone(PyExc_StopIteration);
}
}
else if (PyErr_ExceptionMatches(PyExc_GeneratorExit)) {
o->agt_state = AWAITABLE_STATE_CLOSED;
PyErr_Clear(); /* ignore these errors */
PyErr_SetNone(PyExc_StopIteration);
}
return NULL;
}

Expand Down