Changelog for Release 1.18 by KevinRK29 · Pull Request #19818 · python/mypy

KevinRK29 · 2025-09-10T07:54:47Z

Added initial changelog for the 1.18 release

for more information, see https://pre-commit.ci

…to changelog-1.18

hauntsaninja

I think it's worth having the top-level section mentioning the overall amount of performance speed up (maybe get a number relative to 1.17 and 1.10)

hauntsaninja · 2025-09-10T08:37:21Z

+
+This feature was contributed by Jelle Zijlstra (PR [19678](https://github.com/python/mypy/pull/19678)).
+
+### Mypy Improvements


Suggested change

### Mypy Improvements

### Mypy Performance Improvements

hauntsaninja · 2025-09-10T08:37:57Z

+### Documentation Updates
+
+- Add idlemypyextension to IDE integrations (CoolCat467, PR [18615](https://github.com/python/mypy/pull/18615))
+- Update stubinfo for latest typeshed (Shantanu, PR [19771](https://github.com/python/mypy/pull/19771))


nit: this should be under "Other Notable Improvements" (it affects the diagnostics users get)

hauntsaninja · 2025-09-10T08:41:09Z

+- More detailed checking of type objects in stubtest (Stephen Morton, PR [18251](https://github.com/python/mypy/pull/18251))
+- Support running stubtest in non-UTF8 terminals (Stanislav Terliakovm, PR [19085](https://github.com/python/mypy/pull/19085))
+
+### Bug Fixes


Not sure what the difference between this section and Other Notable Improvements is (many of the things in the other one would count as bug fixes)

ilevkivskyi

FYI changelog is a very serious document, that is actually read by humans. Please show some respect to mypy users and actually spend some time writing it.

This is not acceptable.

ilevkivskyi · 2025-09-10T11:24:15Z

+
+### `--allow-redefinition-new`
+
+Exposes an (experimental) command‑line flag to control whether subclass `__new__` redefinitions are accepted. This was previously an internal toggle; it’s now publicly documented via --allow-redefinition-new so advanced projects can fine‑tune behavior without internal flags


Sorry, what? This flag has nothing to do with __new__. Also this paragraph should have a brief description of the semantics, and probably an example of use (e.g. with an if statement). Also it probably makes sense to add a section to mypy docs.

Hey, just to confirm then, is this mypy doc about the same flag then, if so do we still need a section in the mypy docs? or is it different
https://mypy.readthedocs.io/en/latest/command_line.html#cmdoption-mypy-allow-redefinition-new

That documentation is pretty good, yes. Maybe it should be edited to remove the "experimental" framing.

It certainly has nothing to do with __new__.

ilevkivskyi · 2025-09-10T11:24:40Z

+
+Exposes an (experimental) command‑line flag to control whether subclass `__new__` redefinitions are accepted. This was previously an internal toggle; it’s now publicly documented via --allow-redefinition-new so advanced projects can fine‑tune behavior without internal flags
+
+This feature was contributed by Ivan Levkivskyi (PR [19796](https://github.com/python/mypy/pull/19796)).


The core feature was contributed by Jukka, not by me.

ilevkivskyi · 2025-09-10T11:26:10Z

+- Denser representation for common scalars (int, str)
+- Lower GC pressure when reading caches
+- Guardrails around constructor cache emission (compatibility)
+- Expose --fixed-format-cache if compiled


All these details are not needed, just remove them.

ilevkivskyi · 2025-09-10T11:27:58Z

+### Fixed‑Format Cache (experimental)
+
+Introduces an optional binary fixed‑format cache that’s both smaller and notably faster to load than the JSON/orjson cache. Local measurements on the PR show ~2.5× faster deserialization than orjson and reviewer benchmarks report a ~35% cached import speedup on a large workload; the flag is currently hidden/opt‑in while we iterate. Follow‑ups further shrink/optimize the format and reduce GC overhead during loads.
+


Please stop using LLMs, sorry, but this looks horrible.

Apologies, I didn't have much context into it, so I wrote something down with the help of it as a placeholder before Jukka could get a look to write something for it. It was also 3 am for me, so I wasn't thinking straight 😅

for more information, see https://pre-commit.ci

JukkaL

Thanks for preparing a draft changelog! I'm going to merge this now and prepare a follow-up PR with various edits. This way is faster than leaving review feedback here.

(cherry picked from commit 9ae3e9a)

KevinRK29 added 3 commits September 10, 2025 03:43

boilerplate

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added typeshed

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added backticks

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KevinRK29 requested a review from JukkaL September 10, 2025 07:54

KevinRK29 mentioned this pull request Sep 10, 2025

1.18 Release Planning #19764

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BobTheBuidler reviewed Sep 10, 2025

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KevinRK29 added 3 commits September 10, 2025 04:12

update hyperlinks

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Merge branch 'changelog-1.18' of https://github.com/KevinRK29/mypy in…

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…to changelog-1.18

minor pr link update

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hauntsaninja reviewed Sep 10, 2025

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ilevkivskyi requested changes Sep 10, 2025

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KevinRK29 and others added 4 commits September 10, 2025 11:09

minor log moves

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added TODOs

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added speed up metric

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JukkaL approved these changes Sep 10, 2025

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JukkaL merged commit 9ae3e9a into python:master Sep 10, 2025
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KevinRK29 added a commit that referenced this pull request Sep 11, 2025

Initial changelog for release 1.18 (#19818)

24eed0b

(cherry picked from commit 9ae3e9a)


		This feature was contributed by Jelle Zijlstra (PR [19678](https://github.com/python/mypy/pull/19678)).

		### Mypy Improvements


		### `--allow-redefinition-new`

		Exposes an (experimental) command‑line flag to control whether subclass `__new__` redefinitions are accepted. This was previously an internal toggle; it’s now publicly documented via --allow-redefinition-new so advanced projects can fine‑tune behavior without internal flags


		Exposes an (experimental) command‑line flag to control whether subclass `__new__` redefinitions are accepted. This was previously an internal toggle; it’s now publicly documented via --allow-redefinition-new so advanced projects can fine‑tune behavior without internal flags

		This feature was contributed by Ivan Levkivskyi (PR [19796](https://github.com/python/mypy/pull/19796)).

		### Fixed‑Format Cache (experimental)

		Introduces an optional binary fixed‑format cache that’s both smaller and notably faster to load than the JSON/orjson cache. Local measurements on the PR show ~2.5× faster deserialization than orjson and reviewer benchmarks report a ~35% cached import speedup on a large workload; the flag is currently hidden/opt‑in while we iterate. Follow‑ups further shrink/optimize the format and reduce GC overhead during loads.

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