Issue 32113: Strange behavior with await in a generator expression (original) (raw)

Issue32113

Created on 2017-11-22 14:42 by levkivskyi, last changed 2022-04-11 14:58 by admin.

Messages (3)
msg306732 - (view)	Author: Ivan Levkivskyi (levkivskyi) *	Date: 2017-11-22 14:42
PEP 530 is not very clear about `await` in generator expressions. But when I try it, the error is a bit confusing: >>> async def g(i): ... print(i) ... >>> async def f(): ... result = list(await g(i) for i in range(3)) ... print(result) ... >>> f().send(None) Traceback (most recent call last): File "", line 1, in File "", line 2, in f TypeError: 'async_generator' object is not iterable At the same time a (seemingly) equivalent list comprehension works fine: >>> async def f(): ... result = [await g(i) for i in range(3)] ... print(result) ... >>> f().send(None) 0 1 2 [None, None, None] Traceback (most recent call last): File "", line 1, in StopIteration I would say that the first case should either behave as a second one, or raise a syntax error. Or is it actually an intended behavior?
msg306733 - (view)	Author: Yury Selivanov (yselivanov) *	Date: 2017-11-22 15:46
> ... result = list(await g(i) for i in range(3)) This is equivalent to this code: async def ait(): for i in range(3): v = await g(i) yield v result = list(ait()) Where 'ait' is an async generator function. You can't iterate it with the regular 'for x in ...' syntax, and you can't pass it to functions that expect a synchronous iterator (such as 'list'). Similarly, with synchronous code: a = (i for i in range(3)) a[0] Traceback (most recent call last): File "", line 1, in TypeError: 'generator' object is not subscriptable where '(' for ... ')' is another syntax for defining a synchronous generator. > ... result = [await g(i) for i in range(3)] This is equivalent to this code: result = [] for i in range(3): v = await g(i) result.append(v) I agree that PEP 530 is a bit vague about this and can be updated. I'll take a look into that. Perhaps we can make the "TypeError: 'async_generator' object is not iterable" error message a bit clearer. Any ideas to improve it are welcome. > I would say that the first case should either behave as a second one, or raise a syntax error. No, but we can improve error messages.
msg306736 - (view)	Author: Ivan Levkivskyi (levkivskyi) *	Date: 2017-11-22 16:50
A first simple idea that comes to my mind is special-case async generators/iterators in PyObject_GetIter to say something like: TypeError: asynchronous iterable can't be used where an iterable is expected If it is possible to detect that an async generator is resulting from a generator expression, then we can say: TypeError: asynchronous generator expression can't be used as an iterable