[issue10544] yield expression inside generator expression does nothing

[Serhiy Storchaka]

Serhiy Storchaka  added the comment:


New changeset 07ca9afaa8768b44baf816b4998d209ed3e0088f by Serhiy Storchaka in 
branch 'master':
bpo-10544: Disallow "yield" in comprehensions and generator expressions. 
(GH-4564)
https://github.com/python/cpython/commit/07ca9afaa8768b44baf816b4998d209ed3e0088f


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[issue10544] yield expression inside generator expression does nothing

[Serhiy Storchaka]

Change by Serhiy Storchaka :


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[issue10544] yield expression inside generator expression does nothing

[Serhiy Storchaka]

Serhiy Storchaka  added the comment:


New changeset 65d1887170fb278c10a836e9e4319cae4707f524 by Serhiy Storchaka in 
branch '2.7':
[2.7] bpo-10544: Deprecate "yield" in comprehensions and generator expressions 
in Py3k mode. (GH-4579) (#4676)
https://github.com/python/cpython/commit/65d1887170fb278c10a836e9e4319cae4707f524


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[issue10544] yield expression inside generator expression does nothing

[Serhiy Storchaka]

Serhiy Storchaka  added the comment:

PR 4676 backports warnings to 2.7 in Py3k mode.

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[issue10544] yield expression inside generator expression does nothing

[Serhiy Storchaka]

Change by Serhiy Storchaka :


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[issue10544] yield expression inside generator expression does nothing

[Nick Coghlan]

Nick Coghlan  added the comment:

With Serhiy's patch merged, I'm marking this as resolved. Thanks all!

https://bugs.python.org/issue32189 is the follow-up issue to turn the warning 
into an unconditional SyntaxError in 3.8.

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status: open -> closed
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[issue10544] yield expression inside generator expression does nothing

[Nick Coghlan]

Nick Coghlan  added the comment:


New changeset 73a7e9b10b2ec9636e3c6396cf7b3695f8ed1856 by Nick Coghlan (Serhiy 
Storchaka) in branch 'master':
bpo-10544: Deprecate "yield" in comprehensions and generator expressions. 
(GH-4579)
https://github.com/python/cpython/commit/73a7e9b10b2ec9636e3c6396cf7b3695f8ed1856


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[issue10544] yield expression inside generator expression does nothing

[Guido van Rossum]

Guido van Rossum  added the comment:

OK, great.

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[issue10544] yield expression inside generator expression does nothing

[Nick Coghlan]

Nick Coghlan  added the comment:

As far as I'm aware, there's nothing that specifically promises these 
constructs will do anything in Py3 at all - the existing behaviour is just an 
accident of implementation arising from the way nested scopes and yield 
expressions interact in general.

Tinkering with "await" in comprehensions and generator expressions would be 
different, since PEP 530 actually does make promises about how we expect that 
to work.

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[issue10544] yield expression inside generator expression does nothing

[Guido van Rossum]

Guido van Rossum  added the comment:

After the tiresome debate I am happy to see this just as a "what's new" entry 
rather than soliciting more debate with a PEP. (However there may be some 
existing PEP that suggests it should work? That PEP should be amended with a 
note that this is being deprecated. But I don't know if there is such a PEP.)

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[issue10544] yield expression inside generator expression does nothing

[Serhiy Storchaka]

Serhiy Storchaka  added the comment:

Note that the DeprecationWarning exception is replaced with a SyntaxError to 
get a more accurate error report.

$ cat yield-gen.py 
def f():
return ((yield x) for x in range(3))

$ ./python -Wd yield-gen.py
yield-gen.py:2: DeprecationWarning: 'yield' inside generator expression
  return ((yield x) for x in range(3))

$ ./python -We yield-gen.py
  File "yield-gen.py", line 2
return ((yield x) for x in range(3))
   ^
SyntaxError: 'yield' inside generator expression


Without this replacement the result would be:

$ ./python -We yield-gen.py
DeprecationWarning: 'yield' inside generator expression

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[issue10544] yield expression inside generator expression does nothing

[Nick Coghlan]

Nick Coghlan  added the comment:

Guido, should we write this change up as a PEP, or are you happy to just cover 
it as a section in the What's New document for 3.7?

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[issue10544] yield expression inside generator expression does nothing

[Serhiy Storchaka]

Change by Serhiy Storchaka :


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[issue10544] yield expression inside generator expression does nothing

[Guido van Rossum]

Guido van Rossum  added the comment:

Great. It should be a DeprecationWarning, since we're planning to disallow
it completely, right? IIRC SyntaxWarning is for syntax that we can't
deprecate.

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[issue10544] yield expression inside generator expression does nothing

[Nick Coghlan]

Nick Coghlan  added the comment:

Cool, if you're OK with that behaviour, it actually makes this a lot easier, 
since it means:

1. Serhiy's patch is already sufficient for the final hard compatibility break
2. It can be readily adapted to emit either DeprecationWarning or SyntaxWarning 
for 3.7

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[issue10544] yield expression inside generator expression does nothing

[Guido van Rossum]

Guido van Rossum  added the comment:

To clarify, the outermost iterator is marked here:

[func(x, y) for x in  for y in ]

and it is evaluated before the comprehension proper is started. This is
just part of how the language works (it's a similar rule as "in an
assignment the RHS is evaluated before it is assigned to the LHS").

I see no benefit in banning `yield` in .

On Sat, Nov 25, 2017 at 10:22 PM, Guido van Rossum 
wrote:

>
> Guido van Rossum  added the comment:
>
> No.
>
> On Nov 25, 2017 18:01, "Nick Coghlan"  wrote:
>
> >
> > Nick Coghlan  added the comment:
> >
> > Serhiy's PR now implements the "Prohibit yield & yield from in generator
> > expressions and comprehensions" approach discussed on python-dev
> (currently
> > as a hard SyntaxError, but it could be amended to be a warning instead
> > without too much difficulty).
> >
> > The PR does highlight an interesting subtlety though: the easiest way to
> > implement this still allows yield expressions in the outermost iterator,
> > since that gets compiled in a different scope from the rest of the
> > comprehension body (it's evaluated eagerly and passed in to the implicit
> > nested function).
> >
> > I'm thinking we probably don't want to expose that detail to end users,
> > and will instead want to include a second check that prohibits yield
> > expressions in the outermost iterator as well. However, I'm not entirely
> > sure how we could implement such a check, short of adding a new "yield
> > expression prohibited" counter in the AST generation and/or symbol
> analysis
> > pass.
> >
> > --
> >
> > ___
> > Python tracker 
> > 
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> >
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[issue10544] yield expression inside generator expression does nothing

[Guido van Rossum]

Guido van Rossum  added the comment:

No.

On Nov 25, 2017 18:01, "Nick Coghlan"  wrote:

>
> Nick Coghlan  added the comment:
>
> Serhiy's PR now implements the "Prohibit yield & yield from in generator
> expressions and comprehensions" approach discussed on python-dev (currently
> as a hard SyntaxError, but it could be amended to be a warning instead
> without too much difficulty).
>
> The PR does highlight an interesting subtlety though: the easiest way to
> implement this still allows yield expressions in the outermost iterator,
> since that gets compiled in a different scope from the rest of the
> comprehension body (it's evaluated eagerly and passed in to the implicit
> nested function).
>
> I'm thinking we probably don't want to expose that detail to end users,
> and will instead want to include a second check that prohibits yield
> expressions in the outermost iterator as well. However, I'm not entirely
> sure how we could implement such a check, short of adding a new "yield
> expression prohibited" counter in the AST generation and/or symbol analysis
> pass.
>
> --
>
> ___
> Python tracker 
> 
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[issue10544] yield expression inside generator expression does nothing

[Nick Coghlan]

Nick Coghlan  added the comment:

Serhiy's PR now implements the "Prohibit yield & yield from in generator 
expressions and comprehensions" approach discussed on python-dev (currently as 
a hard SyntaxError, but it could be amended to be a warning instead without too 
much difficulty).

The PR does highlight an interesting subtlety though: the easiest way to 
implement this still allows yield expressions in the outermost iterator, since 
that gets compiled in a different scope from the rest of the comprehension body 
(it's evaluated eagerly and passed in to the implicit nested function).

I'm thinking we probably don't want to expose that detail to end users, and 
will instead want to include a second check that prohibits yield expressions in 
the outermost iterator as well. However, I'm not entirely sure how we could 
implement such a check, short of adding a new "yield expression prohibited" 
counter in the AST generation and/or symbol analysis pass.

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[issue10544] yield expression inside generator expression does nothing

[Serhiy Storchaka]

Change by Serhiy Storchaka :


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stage: needs patch -> patch review

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[issue10544] yield expression inside generator expression does nothing

[Ivan Levkivskyi]

Change by Ivan Levkivskyi :


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nosy:  -levkivskyi

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[issue10544] yield expression inside generator expression does nothing

[Serhiy Storchaka]

Serhiy Storchaka  added the comment:

Here is a sample of the implementation of the Nick's idea.

--
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Added file: https://bugs.python.org/file47296/yield-in-comprehensions.diff

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[issue10544] yield expression inside generator expression does nothing

[Guido van Rossum]

Guido van Rossum  added the comment:

No to both. See python-dev.

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[issue10544] yield expression inside generator expression does nothing

[Nick Coghlan]

Nick Coghlan  added the comment:

I realised that even without modifying the compiler first, I could illustrate 
the proposed `yield from` based resolution for the comprehension case by way of 
explicit yield from clauses:

```
def get_gen_result(gen, inputs):
try:
yield_value = next(gen)
for send_value in inputs:
print(f"Received: {yield_value}; Sending: {send_value}")
yield_value = gen.send(send_value)
except StopIteration as exc:
return exc.value
raise RuntimeError("Failed to exhaust generator")

def example():
comp1 = yield from [str((yield x)) for x in ('1st', '2nd')]
comp2 = yield from [int((yield x)) for x in ('3rd', '4th')]
return comp1, comp2

>>> result = get_gen_result(example(), range(4))
Received: 1st; Sending: 0
Received: 2nd; Sending: 1
Received: 3rd; Sending: 2
Received: 4th; Sending: 3
>>> result
(['0', '1'], [2, 3])
```

So if we decided to make yield-in-a-comprehension imply the use of yield from, 
we'd only need:

- DeprecationWarning in 3.7 to say "this is going to imply 'yield from 
(comprehension)' in 3.8+"
- making the 'yield from' implicit in 3.8 (thus ensuring that comprehensions 
always return the correct container type, even when they include yield 
expressions)

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[issue10544] yield expression inside generator expression does nothing

[Nick Coghlan]

Nick Coghlan  added the comment:

Given the direction of the python-dev thread, should we split this question 
into two issues?

Issue 1: a yield expression inside a comprehension changes the type of the 
expression result (returning a generator-iterator instead of the expected 
container type)

Issue 2: a yield expression inside a generator expression interacts weirdly 
with the genexp's implicit yield expression

I ask, as it seems to me that issue 1 can be addressed by wrapping the affected 
cases in an implicit 'yield from' expression, which will both fix the return 
type of the expression and turn the outer function into a generator (if it 
isn't one already). (I'm going to put together a proof-of-concept for that idea 
this weekend)

By contrast, the interaction between generator expressions and explicit yield 
expressions seems intrinsically confusing, so I'm not sure we can do any better 
than declaring it a syntax error to try to combine them.

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[issue10544] yield expression inside generator expression does nothing

[Armin Rigo]

Change by Armin Rigo :


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[issue10544] yield expression inside generator expression does nothing

[Ivan Levkivskyi]

Ivan Levkivskyi  added the comment:

> How about not posting about this topic for 24 hours.

OK, makes sense :-)

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[issue10544] yield expression inside generator expression does nothing

[Guido van Rossum]

Guido van Rossum  added the comment:

I think we all need to calm down a bit. How about not posting about this
topic for 24 hours.

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[issue10544] yield expression inside generator expression does nothing

[Ivan Levkivskyi]

Ivan Levkivskyi  added the comment:

Guido, I am not sure about the complete removal, this is probably to radical. 
What I think we are missing more detailed docs that would be clear about the 
corner cases. (I already mentioned this in https://bugs.python.org/issue32113)

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[issue10544] yield expression inside generator expression does nothing

[Yury Selivanov]

Yury Selivanov  added the comment:

> Yury OK, sorry then this is a misunderstanding from my side.

NP.  Again, sorry if I sounded that way to you.

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[issue10544] yield expression inside generator expression does nothing

[Ivan Levkivskyi]

Ivan Levkivskyi  added the comment:

Yury OK, sorry then this is a misunderstanding from my side.

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[issue10544] yield expression inside generator expression does nothing

[Yury Selivanov]

Yury Selivanov  added the comment:

>> Do you understand the difference?

> Yury, sorry, but what is your problem? Have I said something about that this 
> is bad. Your tone is clearly insulting, and this is not the first time. Maybe 
> it makes sense to have some respect?

Sorry, I didn't mean to insult anybody.  I asked an honest question with an 
intent to clarify if there's some misunderstanding of the topic that I'm 
partially responsible for in CPython.

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[issue10544] yield expression inside generator expression does nothing

[Ivan Levkivskyi]

Ivan Levkivskyi  added the comment:

> Do you understand the difference?

Yury, sorry, but what is your problem? Have I said something about that this is 
bad. Your tone is clearly insulting, and this is not the first time. Maybe it 
makes sense to have some respect?

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[issue10544] yield expression inside generator expression does nothing

[Yury Selivanov]

Yury Selivanov  added the comment:

> Well, after all I am thinking maybe it is indeed makes sense to ban `yield` 
> inside both sync/async and both comprehensions/generator expressions.

I agree, as I can't imagine a real use case for

   a = ((yield a) for a in as)

which is really equivalent to

   def a():
  for a in as:
 yield (yield a)

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[issue10544] yield expression inside generator expression does nothing

[Guido van Rossum]

Guido van Rossum  added the comment:

I honestly think we went too far here. Asynchronous generators are still
provisional. Though the PEP is silent about this, the acceptance notice is
clear:
https://mail.python.org/pipermail/python-dev/2016-September/146267.html. I
propose to rip them out.

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[issue10544] yield expression inside generator expression does nothing

[Ivan Levkivskyi]

Ivan Levkivskyi  added the comment:

Well, after all I am thinking maybe it is indeed makes sense to ban `yield` 
inside both sync/async and both comprehensions/generator expressions. Since we 
already have a smörgåsbord of intuitive behaviors.

This, _together_ with extensive clarification to the docs and error messages 
can fix the problem.

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[issue10544] yield expression inside generator expression does nothing

[Yury Selivanov]

Yury Selivanov  added the comment:

> ... but [await x for x in xs] is still valid _only_ inside async def.

Yes, because it's computed right where it is defined.

a = [x for x in xs]  # `a` is a list object
a = (x for x in xs)  # `a` is a generator

Do you understand the difference?

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[issue10544] yield expression inside generator expression does nothing

[Ivan Levkivskyi]

Ivan Levkivskyi  added the comment:

... but [await x for x in xs] is still valid _only_ inside async def.

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[issue10544] yield expression inside generator expression does nothing

[Yury Selivanov]

Yury Selivanov  added the comment:

It's also important to note, that in 3.7, this is legal:

  def foo():
return (await x for x in xs)

Note that 'foo' is a regular function, not 'async def'.

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[issue10544] yield expression inside generator expression does nothing

[Yury Selivanov]

Yury Selivanov  added the comment:

> Yury explained why this happens in https://bugs.python.org/issue32113

It happens because '(x for x in xs)' creates a synchronous generator.  So when 
there's an 'await' in it, it creates an asynchronous generator.

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[issue10544] yield expression inside generator expression does nothing

[Ivan Levkivskyi]

Ivan Levkivskyi  added the comment:

(Of course there should be not [1, 2] in the argument, but a list of some 
awaitables, otherwise there will be an error later.)

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[issue10544] yield expression inside generator expression does nothing

[Ivan Levkivskyi]

Ivan Levkivskyi  added the comment:

> Remind me what happens when you use `await` in a generator expression that 
> survives the async function's scope?

Awaiting on f([1, 2]) will result in an async generator (even though yield 
never appears here). Yury explained why this happens in 
https://bugs.python.org/issue32113

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[issue10544] yield expression inside generator expression does nothing

[Guido van Rossum]

Guido van Rossum  added the comment:

Remind me what happens when you use `await` in a generator expression that
survives the async function's scope?

async def f(xs):
return (await x for x in xs)

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[issue10544] yield expression inside generator expression does nothing

[Serhiy Storchaka]

Serhiy Storchaka  added the comment:

I concur with Ivan. It would be nice to me to disallow "yield" in 
comprehensions and generator expressions, but the fact that "await" in 
comprehensions already works right, and this behavior is intentional and 
useful, make me thinking that "yield" should work in comprehensions.

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[issue10544] yield expression inside generator expression does nothing

[Serhiy Storchaka]

Serhiy Storchaka  added the comment:

Consider this as a sort of optimization. A function inlining. In general case 
we can't inline a function because it can be rebinded at any time, but in this 
particular case we call a just created function that doesn't have any 
references and live only on the stack a, and we have all information for 
generating an inlined code. This will allow to get rid of the overhead for 
creating and calling a one-time function.

The naming problem is the hardest problem. We should assign unique names for 
internal variables, like '.0.x'. We already do this for the single parameter of 
a one-time function. Or we could just use a stack (this requires more work but 
has additional benefits).

If you want to think about comprehensions as about calling inner functions, you 
can do this. And you can think about comprehensions as about loops which don't 
leak inner variables. The behavior should be absolutely identical until you 
will use "yield" which would turn your list producing function into a 
generator. If make "yield" turning into a generator not an implicit inner 
function, but an explicit enclosing function, a comprehension could be 
represented via an implicit inner function. Currently there is no way to 
express this in Python syntax, but before introducing "yield" there was no way 
to express its semantic.

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[issue10544] yield expression inside generator expression does nothing

[Ivan Levkivskyi]

Ivan Levkivskyi  added the comment:

Nick,

> Yury took all this into account when designing the interaction between 
> `await` and comprehensions (which is why that's in a better state), but for 
> `yield` we inherited the existing behaviour of any other nested function.

Actually the fact that `await` in comprehensions already works right is one of 
my main arguments that we can also fix `yield`. So I am totally with Serhiy 
here.

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[issue10544] yield expression inside generator expression does nothing

[Serhiy Storchaka]

Serhiy Storchaka  added the comment:

This is straightforward.

__result = []; __i = None
try:
for __i in range(2, 5):
__result2 = []; __x = __y = None
try:
for __x in range(2):
for __y in range(1, __i):
__result2.append(__x + __y)
__result.append(__result2)
finally:
del __result2, __x, __y
finally:
del __i

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[issue10544] yield expression inside generator expression does nothing

[Nick Coghlan]

Nick Coghlan  added the comment:

Sure, you can technically do that, but you'll have to rewrite most of the 
symtable pass in the compiler, since you won't be able to re-use the current 
recursive descent algorithms (which are designed to handle function scopes). 
That ran afoul of "If the implementation is hard to explain, it's a bad idea."

Where that gets particularly nasty is when you nest comprehensions with the 
*same* iteration variable name:

>>> [[i for i in range(2)] for i in range(2)]
[[0, 1], [0, 1]]

Now your name generator needs to be clever enough to account for how many 
levels deep it is. You also run into the lexical scoping problem anyway once a 
lambda expression shows up:

>>> [[(lambda i=i: i)() for i in range(2)] for i in range(2)]
[[0, 1], [0, 1]]
>>> [[(lambda: i)() for i in range(2)] for i in range(2)]
[[0, 1], [0, 1]]

As noted earlier, not impossible to resolve, but for from the "simple" option 
that folks seem to believe it is (and which I thought it would be before I 
actually tried to implement it while respecting Python's lexical scoping rules).

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[issue10544] yield expression inside generator expression does nothing

[Nick Coghlan]

Nick Coghlan  added the comment:

Another slight variant to that test case to make sure the inner comprehension 
actually generates a closure reference in the current implementation:

>>> [[x+y for x in range(2) for y in range(1, i)] for i in range(2, 5)]
[[1, 2], [1, 2, 2, 3], [1, 2, 3, 2, 3, 4]]

These are the kind of challenges that drove of us towards the current 
implementation. While the status quo definitely has its downsides, those 
downsides at least all flow directly from "There's an implicit nested function 
there that you're not expecting".

Yury took all this into account when designing the interaction between `await` 
and comprehensions (which is why that's in a better state), but for `yield` we 
inherited the existing behaviour of any other nested function.

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[issue10544] yield expression inside generator expression does nothing

[Serhiy Storchaka]

Serhiy Storchaka  added the comment:

I see nothing special in `[[x for x in range(1, i)] for i in range(2, 5)]`. 
This should be equivalent to:

__result = []; __i = None
try:
for __i in range(2, 5):
__result2 = []; __x = None
try:
for __x in range(1, __i)
__result2.append(__x)
__result.append(__result2)
finally:
del __result2, __x
finally:
del __i

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[issue10544] yield expression inside generator expression does nothing

[Nick Coghlan]

Nick Coghlan  added the comment:

Also see https://bugs.python.org/issue1660500 for the original Python 3.0 
change to hide the iteration variable.

While the test suite already covers some interesting scoping edge cases as 
result of that initial patch, I think one we're currently missing is the nested 
comprehension case:

>>> [[x for x in range(1, i)] for i in range(2, 5)]
[[1], [1, 2], [1, 2, 3]]

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[issue10544] yield expression inside generator expression does nothing

[Nick Coghlan]

Nick Coghlan  added the comment:

"Just fix the issue" is easier said than done for the same reason that 
comprehensions were implemented the way they are now: lambda expressions still 
have to work.

That is, we need to maintain the invariant that:

[x for x in iterable]
{x for x in iterable}
(k:v for k, v in iterable)
(x for x in iterable)

give the same results (respectively) as:

[(lambda: x)() for x in iterable]
{(lambda: x)() for x in iterable}
((lambda: k)():(lambda: v)() for k, v in iterable)
((lambda: x)() for x in iterable)

Once you work through the implications of "We need the loop variable to visible 
to lexically nested scopes, but invisible in the containing scope", you're 
going to end up with something that looks enough like a nested function that 
the easiest to implement and explain option is to have it *be* a nested 
function.

I'd be fine with a resolution that forbade yield expressions directly inside 
implicit scopes, though.

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[issue10544] yield expression inside generator expression does nothing

[Nick Coghlan]

Change by Nick Coghlan :


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[issue10544] yield expression inside generator expression does nothing

[Ivan Levkivskyi]

Change by Ivan Levkivskyi :


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[issue10544] yield expression inside generator expression does nothing

[Serhiy Storchaka]

Change by Serhiy Storchaka :


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versions: +Python 3.6, Python 3.7 -Python 3.5

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[issue10544] yield expression inside generator expression does nothing

[Ivan Levkivskyi]

Changes by Ivan Levkivskyi :


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[issue10544] yield expression inside generator expression does nothing

[Jim Fasarakis-Hilliard]

Changes by Jim Fasarakis-Hilliard :


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[issue10544] yield expression inside generator expression does nothing

[Armin Rigo]

Armin Rigo added the comment:

Let's see if the discussion goes anywhere or if this issue remains in limbo for 
the next 7 years.  In the meantime, if I may humbly make a suggestion: whether 
the final decision is to give SyntaxError or change the semantics, one or a few 
intermediate versions with a SyntaxWarning might be a good idea.

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[issue10544] yield expression inside generator expression does nothing

[Ivan Levkivskyi]

Ivan Levkivskyi added the comment:

> How about fixing CPython to raise SyntaxWarning or even SyntaxError?

I think it is better to just fix the issue, i.e. make comprehensions be 
equivalent to for-loops even if they contain `yield`. (In particular this will 
lead to [(yield i) for i in range(5)] be SyntaxError outside function).

The example of `await` shows that it is possible without leaking the loop 
variable into enclosing scope.

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[issue10544] yield expression inside generator expression does nothing

[Armin Rigo]

Armin Rigo added the comment:

Just to add my comment to this 7-years-old never-resolved issue: in PyPy 3.5, 
which behaves like Python 3.x in this respect, I made the following 
constructions give a warning.

def wrong_listcomp():
return [(yield 42) for i in j]
def wrong_gencomp():
return ((yield 42) for i in j)
def wrong_dictcomp():
return {(yield 42):2 for i in j}
def wrong_setcomp():
return {(yield 42) for i in j}

SyntaxWarning: 'yield' inside a list or generator comprehension behaves 
unexpectedly (http://bugs.python.org/issue10544)

The motivation is that none of the constructions above gives the "expected" 
result.  In more details:

- wrong_listcomp() doesn't even return a list at all.  It's possible to have a 
clue about why this occurs, but I would say that it is just plain wrong given 
the ``return [...]`` part of the syntax.  The same is true for wrong_dictcomp() 
and wrong_setcomp().

- wrong_gencomp() returns a generator as expected.  However, it is a generator 
that yields two elements for each i in j: first 42, and then whatever was 
``send()`` into the generator.  I would say that it is in contradiction with 
the general idea that this syntax should give a generator that yields one item 
for each i in j.  In fact, when the user writes such code he might be expecting 
the "yield" to apply to the function level instead of the genexpr level---but 
none of the functions above end up being themselves generators.

For completeness, I think there is no problem with "await" instead of "yield" 
in Python 3.6.

How about fixing CPython to raise SyntaxWarning or even SyntaxError?

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[issue10544] yield expression inside generator expression does nothing

[Glyph Lefkowitz]

Glyph Lefkowitz added the comment:

OK, cool. So, long term, there will be a way to do this (suspend within a 
generator expression). Thanks for the pointer.

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[issue10544] yield expression inside generator expression does nothing

[Glyph Lefkowitz]

Glyph Lefkowitz added the comment:

(As far as awaiting on int, yes, I know how await works, I was focusing on the 
syntax.)

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[issue10544] yield expression inside generator expression does nothing

[Ivan Levkivskyi]

Ivan Levkivskyi added the comment:

Python 3.5 does not support this, you should use Python 3.6 (plus await x will 
fail when you will run the coroutine, since you cannot await on int).

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[issue10544] yield expression inside generator expression does nothing

[Glyph Lefkowitz]

Glyph Lefkowitz added the comment:

>>> async def foo():
... bar = [await x for x in range(10)]
  File "", line 2
SyntaxError: 'await' expressions in comprehensions are not supported

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[issue10544] yield expression inside generator expression does nothing

[Ivan Levkivskyi]

Ivan Levkivskyi added the comment:

> Is the fact that 'await' produces a syntax error in this context the same bug 
> or a new one?

What kind of SyntaxError? await outside an async function is prohibited, bare 
await is also prohibited.

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[issue10544] yield expression inside generator expression does nothing

[Glyph Lefkowitz]

Glyph Lefkowitz added the comment:

Is the fact that 'await' produces a syntax error in this context the same bug 
or a new one?

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[issue10544] yield expression inside generator expression does nothing

[A. Jesse Jiryu Davis]

Changes by A. Jesse Jiryu Davis je...@emptysquare.net:


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[issue10544] yield expression inside generator expression does nothing

[levkivskyi]

levkivskyi added the comment:

I would like to add that since the introduction of asyncio module that heavily 
uses yield from syntax, binding of yield inside comprehensions/generator 
expressions could lead to unexpected results/confusing behavior. See for 
example this question on SO: 
http://stackoverflow.com/questions/29334054/why-am-i-getting-different-results-when-using-a-list-comprehension-with-coroutin

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[issue10544] yield expression inside generator expression does nothing

[Yury Selivanov]

Changes by Yury Selivanov yselivanov...@gmail.com:


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versions: +Python 3.5 -Python 3.3, Python 3.4

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[issue10544] yield expression inside generator expression does nothing

[Terry J. Reedy]

Changes by Terry J. Reedy tjre...@udel.edu:


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[issue10544] yield expression inside generator expression does nothing

[Meador Inge]

Changes by Meador Inge mead...@gmail.com:


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stage:  - needs patch

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[issue10544] yield expression inside generator expression does nothing

[Daniel Shahaf]

Changes by Daniel Shahaf pyt...@danielsh.fastmail.net:


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[issue10544] yield expression inside generator expression does nothing

[Brett Cannon]

Changes by Brett Cannon br...@python.org:


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[issue10544] yield expression inside generator expression does nothing

[Brett Cannon]

Changes by Brett Cannon br...@python.org:


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[issue10544] yield expression inside generator expression does nothing

[Ed Campbell]

Changes by Ed Campbell drescampb...@gmail.com:


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[issue10544] yield expression inside generator expression does nothing

[Terry J. Reedy]

Terry J. Reedy tjre...@udel.edu added the comment:

#3267 did not expose endless loop possibility and was closed as won't fix.
Rather than reopen that and close this and move nosy list back, I added to nosy 
list here.

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[issue10544] yield expression inside generator expression does nothing

[Georg Brandl]

Georg Brandl ge...@python.org added the comment:

FWIW, the endless loop possibility is not of issue here, and is simply an 
artifact of the specific generator function the OP uses.

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[issue10544] yield expression inside generator expression does nothing

[Georg Brandl]

Georg Brandl ge...@python.org added the comment:

While the behavior is confusing, I don't think yield inside comprehensions 
should be disallowed.  Rather, the fact that comprehensions have their own 
scope should be stated clearer.

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[issue10544] yield expression inside generator expression does nothing

[Benjamin Peterson]

Benjamin Peterson benja...@python.org added the comment:

I think I can probably fix it, but it's debatable whether it should be done, 
since it'd make list comps more of quasi functions.

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[issue10544] yield expression inside generator expression does nothing

[Raymond Hettinger]

Raymond Hettinger rhettin...@users.sourceforge.net added the comment:

This discussion should probably be moved to python-dev.  With tools like 
Twisted's inlineDefer or the Monocle package, there is a growing need to be 
able to use yield in complex expressions.  Yet, that goes against the trend 
toward making lists comps more like genexps and less like sugar for a simple 
for-loop accumulator.

Guido, do you have any thoughts on the subject?  Mark it a documentation issue 
or try out Benjamin's fix?

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[issue10544] yield expression inside generator expression does nothing

[Guido van Rossum]

Guido van Rossum gu...@python.org added the comment:

I think it is definitely wrong the way it works in 3.x.  (Especially since it 
works as expected in 2.x.)

I agree with Inyeol's preference of fixes: (1) make it work properly for 
listcomps as well as genexps, (2) if that's not possible, forbid yield in a 
genexp or listcomp.

Note that even though yield in a genexp could be considered as having a 
well-defined meaning, that meaning is not useful and I would consider it as 
merely a coincidence of the specification, not an intentional effect.  So I 
would be fine changing its meaning.  (My assumption is that since it is not 
useful there is -- almost -- no code depending on that meaning.)

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[issue10544] yield expression inside generator expression does nothing

[Guido van Rossum]

Guido van Rossum gu...@python.org added the comment:

PS. Wasn't there a similar issue with something inside a genexp that raises 
StopIteration?  Did we ever solve that?

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[issue10544] yield expression inside generator expression does nothing

[Guido van Rossum]

Changes by Guido van Rossum gu...@python.org:


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[issue10544] yield expression inside generator expression does nothing

[Alexander Belopolsky]

Changes by Alexander Belopolsky belopol...@users.sourceforge.net:


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[issue10544] yield expression inside generator expression does nothing

[Alexander Belopolsky]

Alexander Belopolsky belopol...@users.sourceforge.net added the comment:

Isn't this the same issue as #3267?

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[issue10544] yield expression inside generator expression does nothing

[Guido van Rossum]

Guido van Rossum gu...@python.org added the comment:

Yes it is, but I was never asked about it back then.

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[issue10544] yield expression inside generator expression does nothing

[Raymond Hettinger]

Raymond Hettinger rhettin...@users.sourceforge.net added the comment:

Hmm, what an interesting and unexpected side-effect of the efforts to hide the 
loop induction variable.

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[issue10544] yield expression inside generator expression does nothing

[Inyeol Lee]

New submission from Inyeol Lee inyeol@gmail.com:

Simple coroutine with for loop works:

 def pack_a():
while True:
L = []
for i in range(2):
L.append((yield))
print(L)

 pa = pack_a()
 next(pa)
 pa.send(1)
 pa.send(2)
[1, 2]


If using list comprehension (generator expression), it fails:

 def pack_b():
while True:
L = [(yield) for i in range(2)]
print(L)

 pb = pack_b()
endless loop here


I understand what's going on here - generator expression is converted to nested 
function and there's no way to either stop the execution inside nested function 
(since it's not started yet!) or send() a value to its yield expression. Still 
I think this behavior is a bug and needs fixed.

- best fix would make it behave the same as for loop.
- if it's not fixable, yield expression inside genexp should not be allowed.

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messages: 122475
nosy: Inyeol.Lee
priority: normal
severity: normal
status: open
title: yield expression inside generator expression does nothing
type: behavior
versions: Python 3.1

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