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GH-127682: Only call `__iter__` once in generator expressions. by markshannon · Pull Request #132351 · python/cpython · GitHub

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GH-127682: Only call __iter__ once in generator expressions. - #132351

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markshannon merged 6 commits into
python:mainfrom
faster-cpython:fix-127682
Apr 11, 2025
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GH-127682: Only call __iter__ once in generator expressions.#132351
markshannon merged 6 commits into
python:mainfrom
faster-cpython:fix-127682

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markshannon commented Apr 10, 2025 •
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Removes the extra GET_ITER instruction emitted when creating a generator expression.

This:

  • Allows iterables producing iterators than do not have an __iter__ method to be used in generator expressions, as they were up until 3.12.
  • Makes generator expressions behave the same as all other generators in this regard.

markshannon added the needs backport to 3.13 only security fixes label Apr 10, 2025
bedevere-app Bot mentioned this pull request Apr 10, 2025
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markshannon merged commit d87e7f3 into python:main Apr 11, 2025

miss-islington-app Bot commented Apr 11, 2025

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Thanks @markshannon for the PR 🌮🎉.. I'm working now to backport this PR to: 3.13.
🐍🍒⛏🤖

markshannon deleted the fix-127682 branch April 11, 2025 08:37

miss-islington-app Bot commented Apr 11, 2025

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Sorry, @markshannon, I could not cleanly backport this to 3.13 due to a conflict.
Please backport using cherry_picker on command line.

cherry_picker d87e7f35297d34755026173d84a38eedfbed78de 3.13

markshannon added a commit to faster-cpython/cpython that referenced this pull request Apr 11, 2025
pythonGH-127682: Only call __iter__ once in generator expressions. (p…
a226707 
…ythonGH-132351)

Fidget-Spinner commented Apr 11, 2025

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Do you need to bump the magic number here? I forgot if compiler-only changes need that.

brandtbucher commented Apr 11, 2025

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I don’t think so, since the bytecode format and behavior is unchanged.

efimov-mikhail mentioned this pull request Apr 12, 2025
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markshannon added a commit that referenced this pull request Apr 29, 2025
[3.13] GH-127682: Backport GH-132351 (GH-132384)
132bdf6 
Only call `__iter__` once in generator expressions
hugovk removed the needs backport to 3.13 only security fixes label May 22, 2025
This was referenced May 25, 2025
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serhiy-storchaka reviewed Jun 5, 2025
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Comment thread Lib/test/test_genexps.py
>>> list(g)
[0, 1, 4, 9, 16, 25, 36, 49, 64, 81]

Verify that the outermost for-expression makes an immediate check

serhiy-storchaka Jun 5, 2025

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Choose a reason Spam Abuse Off Topic Outdated Duplicate Resolved Low Quality

This is a regression.

Yhg1s commented Jun 5, 2025

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If this change of behaviour is actually desirable (which may require some further discussion to establish), it should be mentioned in the What's New documentation.

markshannon commented Jun 5, 2025

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See #135171 (comment) for a more detailed analysis

I'll make a PR for the what's new

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iritkatriel iritkatriel approved these changes

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