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[Python-Dev] Generator objects and list comprehensions?

Ivan Levkivskyi levkivskyi at gmail.com
Wed Jan 25 03:14:27 EST 2017 
On 25 January 2017 at 07:01, Chris Angelico <rosuav at gmail.com> wrote:

> >>>  [(yield 1) for x in range(10)]
> > <generator object <listcomp> at 0x10cd210f8>
>

This is an old bug, see e.g. http://bugs.python.org/issue10544
The ``yield`` inside comprehensions is bound to the auxiliary function.
Instead it should be bound to an enclosing function, like it is done for
``await``.
The behaviour of ``await`` in comprehensions is intuitive
(since it is simply equivalent to a for-loop):

>>> async def f(i):
...     return i

>>> async def g_for():
...     lst = []
...     for i in range(5):
...         lst.append(await f(i))
...     print(lst)

>>> g_for().send(None)
[0, 1, 2, 3, 4]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
StopIteration

>>> async def g_comp():
...     print([await f(i) for i in range(5)])

>>> g_comp().send(None)    # exactly the same as g_for
[0, 1, 2, 3, 4]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
StopIteration

While current behaviour of ``yield`` in comprehensions is confusing:

>>> def c_for():
...     lst = []
...     for i in range(5):
...         lst.append((yield i))
...     print(lst)

>>> c_for().send(None)
0
>>> c_for().send(None)
1
# etc.

>>> def c_comp():
...     print([(yield i) for i in range(5)])

>>> c_comp().send(None)  # Naively this should be equivalent to the above,
but...
<generator object c_comp.<locals>.<listcomp> at 0x7f1fd1faa630>
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'NoneType' object has no attribute 'send'

I promised myself to write a patch, but didn't have time for this yet.
I hope I will do this at some point soon.

--
Ivan
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