[issue21964] inconsistency in list-generator comprehension with yield(-from)

[Irit Katriel]

Irit Katriel  added the comment:

I think this issue was resolved by now. This is what happens on 3.11:

>>> l = ["abc", range(3)]
>>> g = [(yield from i) for i in l]
  File "", line 1
SyntaxError: 'yield' inside list comprehension

>>> g2 = ((yield from i) for i in l)
  File "", line 1
SyntaxError: 'yield' inside generator expression

--
nosy: +iritkatriel
resolution:  -> out of date
status: open -> pending

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[issue21964] inconsistency in list-generator comprehension with yield(-from)

[Serhiy Storchaka]

Serhiy Storchaka  added the comment:

Resolved in issue10544.

--
nosy: +serhiy.storchaka
resolution: out of date -> duplicate
stage: test needed -> resolved
status: pending -> closed
superseder:  -> yield expression inside generator expression does nothing

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[issue21964] inconsistency in list-generator comprehension with yield(-from)

[hakril]

New submission from hakril:

Will playing with generators and `yield from` I found some inconsistency.

list comprehension with yield(-from) would return a generator.
generator comprehension would yield some None in the middle of the expected 
values.

Examples:
l = ["abc", range(3)]
g1 = [(yield from i) for i in l]
print(g)
 at 0x7f5ebd58b690>
print(list(g))
['a', 'b', 'c', 0, 1, 2]  # this result is super cool !

g2 = ((yield from i) for i in l)
print(g2)
 at 0x7f5ebd58b6e0>
print(list(g2))
['a', 'b', 'c', None, 0, 1, 2, None]


For `g1`: it returns a generator because the listcomp contains a `yield from`.

For `g2` it append None because it yield the return value of `yield from i`.
It could be rewritten as:
def comp(x):
for i in x:
yield (yield from i)

--
components: Interpreter Core
messages: 222811
nosy: hakril
priority: normal
severity: normal
status: open
title: inconsistency in list-generator comprehension with yield(-from)
type: behavior
versions: Python 3.5

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[issue21964] inconsistency in list-generator comprehension with yield(-from)

[Ezio Melotti]

Ezio Melotti added the comment:

There seem to be two issues here:

> list comprehension with yield(-from) would return a generator.

This is somewhat surprising, and I'm not sure it's expected/documented.
The example you provided seems to behave reasonably, so I don't think we should 
change the behavior (unless there is some actual bug in similar example).  If 
anything, we could document this, but I'm not sure if it's worth doing it and 
where could be added.

> generator comprehension would yield some None in the middle
> of the expected values.

This also seems expected, and the behavior is consistent with the equivalent 
generator function.

--
nosy: +ezio.melotti, gcewing, ncoghlan
versions: +Python 3.4

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[issue21964] inconsistency in list-generator comprehension with yield(-from)

[Nick Coghlan]

Nick Coghlan added the comment:

It's a side effect of the hidden closure that provides the new scope for the 
iteration variable - that's an ordinary function object, so using yield or 
yield from turns it into a generator expression instead. Generator expressions 
are already generators, so using yield or yield from just adds more yield 
points beyond the implied ones.

I've never figured out a good way to document it - it's a natural consequence 
of the comprehension's closure. An explicit mention in the reference docs for 
comprehensions may be worth adding.

--

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[issue21964] inconsistency in list-generator comprehension with yield(-from)

[hakril]

hakril added the comment:

I found something else, I think it worth mentioning it.

This side-effect allows to create generators that return other values that 
None. And the CPython's behavior is not the same for all versions:

>>> {(yield i) : i for i in range(2)}
 at 0x7f0b98f41410>
>>> list(_)
# python3.(3,4,5)
[0, 1]
# python3.2
[0, 1, {None: 1}] #  python3.2 appends the generator's return value.

--
versions: +Python 3.2, Python 3.3

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[issue21964] inconsistency in list-generator comprehension with yield(-from)

[Terry J. Reedy]

Changes by Terry J. Reedy :


--
stage:  -> test needed

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[issue21964] inconsistency in list-generator comprehension with yield(-from)

[Terry J. Reedy]

Changes by Terry J. Reedy :


--
versions:  -Python 3.2, Python 3.3

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[issue21964] inconsistency in list-generator comprehension with yield(-from)

[STINNER Victor]

Changes by STINNER Victor :


--
nosy: +haypo

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[issue21964] inconsistency in list-generator comprehension with yield(-from)

[STINNER Victor]

STINNER Victor added the comment:

> For `g1`: it returns a generator because the listcomp contains a `yield from`.

IMO it's a bug: [... for ... in ...] must create a list.

--

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