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JukkaL merged 11 commits into from
Sep 10, 2025
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Changelog for Release 1.18 #19818

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@KevinRK29
KevinRK29 committed Sep 10, 2025
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47 changes: 18 additions & 29 deletions CHANGELOG.md
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Expand Up @@ -14,20 +14,12 @@ You can read the full documentation for this release on [Read the Docs](http://m

### `--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

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

### Fixed‑Format Cache (experimental)
This feature was contributed by Jukka Lehtosalo.

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.
### Fixed‑Format Cache (experimental)

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@ilevkivskyi ilevkivskyi Sep 10, 2025

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

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@KevinRK29 KevinRK29 Sep 10, 2025 •
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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 😅

- Initial implementation and flag (hidden): --fixed-format-cache
- Tag packing and compaction: 1‑byte type/symbol tags and u8 storage
- 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

This feature was contributed by Ivan Levkivskyi (PR [19668](https://github.com/python/mypy/pull/19668), [19735](https://github.com/python/mypy/pull/19735), [19750](https://github.com/python/mypy/pull/19750), [19681](https://github.com/python/mypy/pull/19681), [19752](https://github.com/python/mypy/pull/19752), [19815](https://github.com/python/mypy/pull/19815))

Expand All @@ -37,7 +29,7 @@ Mypy now implements PEP 800 Disjoint bases: it understands the @disjoint_base

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

### Mypy Improvements
### Mypy Performance Improvements

- Improve self check performance by 1.8% (Jukka Lehtosalo, PR [19768](https://github.com/python/mypy/pull/19768), [19769](https://github.com/python/mypy/pull/19769), [19770](https://github.com/python/mypy/pull/19770))
- Use fast Python wrappers in native_internal (Ivan Levkivskyi, PR [19765](https://github.com/python/mypy/pull/19765))
Expand Down Expand Up @@ -79,23 +71,6 @@ This feature was contributed by Jelle Zijlstra (PR [19678](https://github.com/py
- 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

- Fix crash with variadic tuple arguments to a generic type (Randolf Scholz, PR [19705](https://github.com/python/mypy/pull/19705))
- Fix crash when enable_error_code in pyproject.toml has wrong type (wyattscarpenter, PR [19494](https://github.com/python/mypy/pull/19494))
- Fix dict assignment to a wider context when an incompatible same‑shape TypedDict exists (Stanislav Terliakov, PR [19592](https://github.com/python/mypy/pull/19592))
- Prevent crash for dataclass with PEP 695 TypeVarTuple on Python 3.13+ (Stanislav Terliakov, PR [19565](https://github.com/python/mypy/pull/19565))
- Fix constructor type for subclasses of Any (Ivan Levkivskyi, PR [19295](https://github.com/python/mypy/pull/19295))
- Fix TypeGuard/TypeIs being forgotten when semanal defers (Brian Schubert, PR [19325](https://github.com/python/mypy/pull/19325))
- Fix TypeIs negative narrowing for unions of generics (Brian Schubert, PR [18193](https://github.com/python/mypy/pull/18193))
- dmypy suggest: fix incorrect signature suggestion when a type matches a module name (Brian Schubert, PR [18937](https://github.com/python/mypy/pull/18937))
- dmypy suggest: fix interaction with `__new__` (Stanislav Terliakov, PR [18966](https://github.com/python/mypy/pull/18966))
- dmypy suggest: support Callable / callable Protocols in decorator unwrapping (Anthony Sottile, PR [19072](https://github.com/python/mypy/pull/19072))
- Fix missing error when redeclaring a type variable in a nested generic class (Brian Schubert, PR [18883](https://github.com/python/mypy/pull/18883))
- Fix for overloaded type object erasure (Shantanu, PR [19338](https://github.com/python/mypy/pull/19338))
- Fix TypeGuard with call on temporary object (Saul Shanabrook, PR [19577](https://github.com/python/mypy/pull/19577))
- Fix crash on settable property alias (Ivan Levkivskyi, PR [19615](https://github.com/python/mypy/pull/19615))

### Mypyc Improvements

- Fix subclass processing in detect_undefined_bitmap (Chainfire, PR [19787](https://github.com/python/mypy/pull/19787))
Expand Down Expand Up @@ -153,7 +128,6 @@ This feature was contributed by Jelle Zijlstra (PR [19678](https://github.com/py
### 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))
- Document that object is often preferable to Any in APIs (wyattscarpenter, PR [19103](https://github.com/python/mypy/pull/19103))
- Include a detailed listing of flags enabled by --strict (wyattscarpenter, PR [19062](https://github.com/python/mypy/pull/19062))
- Update “common issues” (reveal_type/reveal_locals; note on orjson) (wyattscarpenter, PR [19059](https://github.com/python/mypy/pull/19059), [19058](https://github.com/python/mypy/pull/19058))
Expand Down Expand Up @@ -209,6 +183,21 @@ This feature was contributed by Jelle Zijlstra (PR [19678](https://github.com/py
- Fix C compiler flags in the profile self‑check script (Jukka Lehtosalo, PR [19326](https://github.com/python/mypy/pull/19326))
- Add a script for profiling self‑check (Linux only) (Jukka Lehtosalo, PR [19322](https://github.com/python/mypy/pull/19322))
- Retry PyPI upload script: skip existing files on retry (Jukka Lehtosalo, PR [19305](https://github.com/python/mypy/pull/19305))
- Update stubinfo for latest typeshed (Shantanu, PR [19771](https://github.com/python/mypy/pull/19771))
- Fix crash with variadic tuple arguments to a generic type (Randolf Scholz, PR [19705](https://github.com/python/mypy/pull/19705))
- Fix crash when enable_error_code in pyproject.toml has wrong type (wyattscarpenter, PR [19494](https://github.com/python/mypy/pull/19494))
- Fix dict assignment to a wider context when an incompatible same‑shape TypedDict exists (Stanislav Terliakov, PR [19592](https://github.com/python/mypy/pull/19592))
- Prevent crash for dataclass with PEP 695 TypeVarTuple on Python 3.13+ (Stanislav Terliakov, PR [19565](https://github.com/python/mypy/pull/19565))
- Fix constructor type for subclasses of Any (Ivan Levkivskyi, PR [19295](https://github.com/python/mypy/pull/19295))
- Fix TypeGuard/TypeIs being forgotten when semanal defers (Brian Schubert, PR [19325](https://github.com/python/mypy/pull/19325))
- Fix TypeIs negative narrowing for unions of generics (Brian Schubert, PR [18193](https://github.com/python/mypy/pull/18193))
- dmypy suggest: fix incorrect signature suggestion when a type matches a module name (Brian Schubert, PR [18937](https://github.com/python/mypy/pull/18937))
- dmypy suggest: fix interaction with `__new__` (Stanislav Terliakov, PR [18966](https://github.com/python/mypy/pull/18966))
- dmypy suggest: support Callable / callable Protocols in decorator unwrapping (Anthony Sottile, PR [19072](https://github.com/python/mypy/pull/19072))
- Fix missing error when redeclaring a type variable in a nested generic class (Brian Schubert, PR [18883](https://github.com/python/mypy/pull/18883))
- Fix for overloaded type object erasure (Shantanu, PR [19338](https://github.com/python/mypy/pull/19338))
- Fix TypeGuard with call on temporary object (Saul Shanabrook, PR [19577](https://github.com/python/mypy/pull/19577))
- Fix crash on settable property alias (Ivan Levkivskyi, PR [19615](https://github.com/python/mypy/pull/19615))

### Typeshed Updates

Expand Down