[issue32113] Strange behavior with await in a generator expression

[Bryan Hu]

Change by Bryan Hu :


--
versions: +Python 3.8

___
Python tracker 

___
___
Python-bugs-list mailing list
Unsubscribe: 
https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com

[issue32113] Strange behavior with await in a generator expression

[Ivan Levkivskyi]

Ivan Levkivskyi  added the comment:

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

--

___
Python tracker 

___
___
Python-bugs-list mailing list
Unsubscribe: 
https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com

[issue32113] Strange behavior with await in a generator expression

[Yury Selivanov]

Yury Selivanov  added the comment:

> ... 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.

--

___
Python tracker 

___
___
Python-bugs-list mailing list
Unsubscribe: 
https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com

[issue32113] Strange behavior with await in a generator expression

[Ivan Levkivskyi]

New submission from Ivan Levkivskyi :

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?

--
components: Interpreter Core
messages: 306732
nosy: levkivskyi, yselivanov
priority: normal
severity: normal
status: open
title: Strange behavior with await in a generator expression
type: behavior
versions: Python 3.6, Python 3.7

___
Python tracker 

___
___
Python-bugs-list mailing list
Unsubscribe: 
https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com