I've reverted all generator expressions related changes in codegen.c.
And for now I can see no problems with tests, included those which are added in related PR's.
Could you please review this again?
And backport for 3.13 is also neccessary.

@markshannon Do you want to take a look at this fix? Do you have time to do?

@efimov-mikhail are you sure that this isn't adding back too many GET_ITER/GET_AITER instructions?
Particularly, for inlined list comprehensions, this might add an extra GET_[A]ITER.

We should have tests for generator expressions and list comprehensions, both sync and async that __iter__ is called only once.
It seems that we only have test for sync gen expressions. The test is called test_genexpr_only_calls_dunder_iter_once, can you extend it to the other cases?

I've a had a quick look at the generated bytecode and looks like this PR is OK.

@Yhg1s if you need this in ASAP, I think it's good to merge.
We can add those tests to main in another PR and backport them later.

Changes in this PR look similar to the full rollback in #135390, so I think it adds the right number of GET_ITER/GET_AITER at right place. The total effect of #135293 and this PR should restore the old bytecode generation.

Changes in this PR look similar to the full rollback in #135390, so I think it adds the right number of GET_ITER/GET_AITER at right place. The total effect of #135293 and this PR should restore the old bytecode generation.

Yes, changes in these two PRs are very similar. Although, there aren't exactly the same.
In fact, on 3.14 after this PR will be merged we will have issue #125038 fixed w/o additional GET_ITER instruction, but with null pointer checks in FOR_ITER.

But on 3.13 with #135390 being merged there is a different solution (from #125846).

So, I have a question why we don't revert #125846?
Do we want to preserve bytecode changes in 3.13.1 vs 3.13.0?

Was not it reverted as the result of all these changes?

Was not it reverted as the result of all these changes?

It was reverted here.
But it wasn't on 3.13 even after #135390 is being merged.

Thank you, this is a great observation.

Do we want to preserve bytecode changes in 3.13.1 vs 3.13.0?

I don't think I was aware of bytecode changes in 31.1. I tested the stdlib's compiled bytecode (including Lib/test) and saw no differences between 13.0, 13.3 and my rollback (and saw many changes between 13.3, 13.4 and 13.4 + the other fix-forward, but I didn't bother trying with this one applied).

In any case it's too late to fix a change in bytecode that slipped into 13.1, if 13.1-13.3 are all the same we should stick with that.

It is not only change in bytecode generation. It is a behavior change in comparison with 3.12, 3.13.0 and 3.13.4.

To be precise, there is a bytecode change:

-> % ./python
Python 3.13.0 (tags/v3.13.0:60403a5409, Jun 11 2025, 19:23:44) [GCC 12.2.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import dis
>>> dis.dis('(x for x in range(10))')
  0           RESUME                   0

  1           LOAD_CONST               0 (<code object <genexpr> at 0x7fa6c309f450, file "<dis>", line 1>)
              MAKE_FUNCTION
              LOAD_NAME                0 (range)
              PUSH_NULL
              LOAD_CONST               1 (10)
              CALL                     1
              GET_ITER
              CALL                     0
              RETURN_VALUE

Disassembly of <code object <genexpr> at 0x7fa6c309f450, file "<dis>", line 1>:
   1           RETURN_GENERATOR
               POP_TOP
       L1:     RESUME                   0
               LOAD_FAST                0 (.0)
       L2:     FOR_ITER                 6 (to L3)
               STORE_FAST_LOAD_FAST    17 (x, x)
               YIELD_VALUE              0
               RESUME                   5
               POP_TOP
               JUMP_BACKWARD            8 (to L2)
       L3:     END_FOR
               POP_TOP
               RETURN_CONST             0 (None)

  --   L4:     CALL_INTRINSIC_1         3 (INTRINSIC_STOPITERATION_ERROR)
               RERAISE                  1
ExceptionTable:
  L1 to L4 -> L4 [0] lasti

and

-> % ./python                  
Python 3.13.1 (tags/v3.13.1:0671451779, Jun 11 2025, 19:25:28) [GCC 12.2.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import dis
>>> dis.dis('(x for x in range(10))')
  0           RESUME                   0

  1           LOAD_CONST               0 (<code object <genexpr> at 0x7f4332397bc0, file "<dis>", line 1>)
              MAKE_FUNCTION
              LOAD_NAME                0 (range)
              PUSH_NULL
              LOAD_CONST               1 (10)
              CALL                     1
              GET_ITER
              CALL                     0
              RETURN_VALUE

Disassembly of <code object <genexpr> at 0x7f4332397bc0, file "<dis>", line 1>:
   1           RETURN_GENERATOR
               POP_TOP
       L1:     RESUME                   0
               LOAD_FAST                0 (.0)
               GET_ITER
       L2:     FOR_ITER                 6 (to L3)
               STORE_FAST_LOAD_FAST    17 (x, x)
               YIELD_VALUE              0
               RESUME                   5
               POP_TOP
               JUMP_BACKWARD            8 (to L2)
       L3:     END_FOR
               POP_TOP
               RETURN_CONST             0 (None)

  --   L4:     CALL_INTRINSIC_1         3 (INTRINSIC_STOPITERATION_ERROR)
               RERAISE                  1
ExceptionTable:
  L1 to L4 -> L4 [0] lasti
>>> 

You can see additional GET_ITER instruction before FOR_ITER.
This change is made in #125846.

IMO, it's better to revert this change and fix regression in #127682.

I've created new PR for 3.13: #135403

I'd like tests to check that __iter__/__aiter__ is only called once in inlined comprehensions, but that can wait for another PR.

There were 3 tests related to this topic, and I've added the fourth.
Is it enough?
Or I miss something?

We should remove backport-3.13 label.

And what about changes on main branch?
Maybe some thread on d.p.o. should be started?

Thanks, this reverts the 3.14 async generator back to 3.12/3.13 behaviour:

>>> (x async for x in None)
Traceback (most recent call last):
  File "<python-input-1>", line 1, in <module>
    (x async for x in None)
                      ^^^^
TypeError: 'async for' requires an object with __aiter__ method, got NoneType

And what about changes on main branch? Maybe some thread on d.p.o. should be started?

Yes, please do for main/3.15.

I've started related d.p.o. thread here: https://discuss.python.org/t/generator-expressions-behavior-with-non-iterable-objects/96329.