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JukkaL merged 4 commits into from
Sep 11, 2025
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Updates to 1.18 changelog #19826

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d765259
Various updates to 1.18 changelog
JukkaL Sep 11, 2025
6b5a897
Small improvements
JukkaL Sep 11, 2025
110ccc0
Reorder sections
JukkaL Sep 11, 2025
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Minor edits based on review
JukkaL Sep 11, 2025
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@JukkaL
JukkaL committed Sep 11, 2025
commit 6b5a89710b1e3e47dbf755d75389b567108c1386
36 changes: 19 additions & 17 deletions CHANGELOG.md
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Expand Up @@ -16,7 +16,7 @@ You can read the full documentation for this release on [Read the Docs](http://m

Mypy now supports a new cache format used for faster incremental builds. It makes
incremental builds up to twice as fast. The feature is experimental and
currently only supported when using a compiled mypy. Use `--fixed-format-cache`
currently only supported when using a compiled version of mypy. Use `--fixed-format-cache`
to enable the new format, or `fixed_format_cache = True` in a configuration file.

We plan to enable this by default in a future mypy release, and we'll eventually
Expand All @@ -36,17 +36,17 @@ This feature was contributed by Ivan Levkivskyi (PR [19668](https://github.com/p
Mypy 1.18 includes numerous performance improvements, resulting in about 38% speedup
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@ilevkivskyi ilevkivskyi Sep 11, 2025

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IMO this is unnecessarily precise, saying ~40% may be better.

compared to 1.17 when type checking mypy itself. In extreme cases, the improvement
can be 10x or higher. The list below is an overview of the various mypy optimizations.
Many mypyc improvements (discussed in a separate secion below) also improve performance.
Many mypyc improvements (discussed in a separate section below) also improve performance.
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Should we mention --disable-expression-cache just in case, or only advise it when people report bugs?


- 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))
- Optimize fixed format deserialization (Ivan Levkivskyi, PR [19765](https://github.com/python/mypy/pull/19765))
- Use macros to optimize fixed format deserialization (Ivan Levkivskyi, PR [19757](https://github.com/python/mypy/pull/19757))
- Optimize fixed-format deserialization (Ivan Levkivskyi, PR [19765](https://github.com/python/mypy/pull/19765))
- Use macros to optimize fixed-format deserialization (Ivan Levkivskyi, PR [19757](https://github.com/python/mypy/pull/19757))
- Two additional micro‑optimizations (Ivan Levkivskyi, PR [19627](https://github.com/python/mypy/pull/19627))
- Another set of micro‑optimizations (Ivan Levkivskyi, PR [19633](https://github.com/python/mypy/pull/19633))
- Cache common types (Ivan Levkivskyi, PR [19621](https://github.com/python/mypy/pull/19621))
- Skip more method bodies in third‑party libraries for speed (Ivan Levkivskyi, PR [19586](https://github.com/python/mypy/pull/19586))
- Simplify the representation of callable types (Ivan Levkivskyi, PR [19580](https://github.com/python/mypy/pull/19580))
- Add cache for types of call expressions cache (Ivan Levkivskyi, PR [19505](https://github.com/python/mypy/pull/19505))
- Add cache for types of some expressions (Ivan Levkivskyi, PR [19505](https://github.com/python/mypy/pull/19505))
- Use cache for dictionary expressions (Ivan Levkivskyi, PR [19536](https://github.com/python/mypy/pull/19536))
- Use cache for binary operations (Ivan Levkivskyi, PR [19523](https://github.com/python/mypy/pull/19523))
- Cache types of type objects (Ivan Levkivskyi, PR [19514](https://github.com/python/mypy/pull/19514))
Expand All @@ -70,7 +70,7 @@ Many mypyc improvements (discussed in a separate secion below) also improve perf

Mypy 1.16.0 introduced `--allow-redefinition-new`, which allows redefining variables
with different types, and inferring union types for variables from multiple assignments.
The feature is now documented in `--help` output, but the feature is still experimental.
The feature is now documented in the `--help` output, but the feature is still experimental.

We are planning to enable this by default in mypy 2.0, and we will also deprecate the
older `--allow-redefinition` flag. Since the new behavior differs significantly from
Expand Down Expand Up @@ -100,9 +100,10 @@ This feature was contributed by Ivan Levkivskyi (PR [19573](https://github.com/p

### Disjoint Base Classes (@disjoint_base, PEP 800)

Mypy now implements PEP 800 Disjoint bases: it understands the `@disjoint_base`
decorator, and rejects class definitions that combine mutually incompatible base classess,
and exploits the fact that such classes cannot exist in reachability and narrowing logic.
Mypy now understands disjoint bases (PEP 800): it recognizes the `@disjoint_base`
decorator, and rejects class definitions that combine mutually incompatible base classes,
and takes advantage of the fact that such classes cannot exist in reachability and
narrowing logic.

This class definition will now generate an error:

Expand Down Expand Up @@ -138,11 +139,12 @@ This feature was contributed by Jelle Zijlstra (PR [19678](https://github.com/py
### Experimental Free-threading Support for Mypyc

All mypyc tests now pass on free-threading 3.14 release candidate builds. The performance
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Suggested change
All mypyc tests now pass on free-threading 3.14 release candidate builds. The performance
All mypyc tests now pass on free-threading Python 3.14 release candidate builds. The performance

of various micro-benchmarks scale well when using multiple threads.
of various micro-benchmarks scale well across multiple threads.

Free-threading support is still experimental. Note that native attribute get and set
operations, list item access and certain other operations are still unsafe when there are
race conditions. This will likely change in the future. You can follow the
Free-threading support is still experimental. Note that native attribute access
(get and set), list item access and certain other operations are still
unsafe when there are race conditions. This will likely change in the future.
You can follow the
[area-free-threading label](https://github.com/mypyc/mypyc/issues?q=is%3Aissue%20state%3Aopen%20label%3Aarea-free-threading)
in the mypyc issues tracker to follow progress.

Expand Down Expand Up @@ -175,8 +177,8 @@ Related PRs:
- Speed up implicit `__ne__` (Jukka Lehtosalo, PR [19759](https://github.com/python/mypy/pull/19759))
- Speed up equality with optional str/bytes types (Jukka Lehtosalo, PR [19758](https://github.com/python/mypy/pull/19758))
- Speed up access to empty tuples (BobTheBuidler, PR [19654](https://github.com/python/mypy/pull/19654))
- Speed up calls with `*args` (BobTheBuidler, PR [19631](https://github.com/python/mypy/pull/19631))
- Speed up calls with `*args` (BobTheBuidler, PR [19623](https://github.com/python/mypy/pull/19623))
- Further speed up calls with `*args` (BobTheBuidler, PR [19631](https://github.com/python/mypy/pull/19631))
- Speed up calls with `**kwargs` (BobTheBuidler, PR [19630](https://github.com/python/mypy/pull/19630))
- Optimize `type(x)`, `x.__class__`, and `<type>.__name__` (Jukka Lehtosalo, PR [19691](https://github.com/python/mypy/pull/19691), [19683](https://github.com/python/mypy/pull/19683))
- Specialize `bytes.decode` for common encodings (Jukka Lehtosalo, PR [19688](https://github.com/python/mypy/pull/19688))
Expand All @@ -185,7 +187,7 @@ Related PRs:
- Add dictionary set item for exact dict instances (BobTheBuidler, PR [19657](https://github.com/python/mypy/pull/19657))
- Cache length when iterating over immutable types (BobTheBuidler, PR [19656](https://github.com/python/mypy/pull/19656))
- Fix name conflict related to attributes of generator classes (Piotr Sawicki, PR [19535](https://github.com/python/mypy/pull/19535))
- Fix segfault from heap type objects with static doc string (Brian Schubert, PR [19636](https://github.com/python/mypy/pull/19636))
- Fix segfault from heap type objects with a static docstring (Brian Schubert, PR [19636](https://github.com/python/mypy/pull/19636))
- Unwrap NewType to its base type for additional optimizations (BobTheBuidler, PR [19497](https://github.com/python/mypy/pull/19497))
- Generate an export table only for separate compilation (Jukka Lehtosalo, PR [19521](https://github.com/python/mypy/pull/19521))
- Speed up `isinstance` with built‑in types (Piotr Sawicki, PR [19435](https://github.com/python/mypy/pull/19435))
Expand All @@ -197,7 +199,7 @@ Related PRs:
- Use per‑type freelists for nested functions (Jukka Lehtosalo, PR [19390](https://github.com/python/mypy/pull/19390))
- Simplify comparison of tuple elements (Piotr Sawicki, PR [19396](https://github.com/python/mypy/pull/19396))
- Generate introspection signatures for compiled functions (Brian Schubert, PR [19307](https://github.com/python/mypy/pull/19307))
- Fix to undefined attribute checking (Jukka Lehtosalo, PR [19378](https://github.com/python/mypy/pull/19378))
- Fix undefined attribute checking special case (Jukka Lehtosalo, PR [19378](https://github.com/python/mypy/pull/19378))
- Fix comparison of tuples with different lengths (Piotr Sawicki, PR [19372](https://github.com/python/mypy/pull/19372))
- Speed up `list.clear` (Jahongir Qurbonov, PR [19344](https://github.com/python/mypy/pull/19344))
- Speed up `weakref.proxy` (BobTheBuidler, PR [19217](https://github.com/python/mypy/pull/19217))
Expand Down Expand Up @@ -254,7 +256,7 @@ Related PRs:
- Support `_value_` as a fallback for ellipsis Enum members (Stanislav Terliakov, PR [19352](https://github.com/python/mypy/pull/19352))
- Sort arguments in TypedDict overlap messages (Marc Mueller, PR [19666](https://github.com/python/mypy/pull/19666))
- Fix handling of implicit return in lambda (Stanislav Terliakov, PR [19642](https://github.com/python/mypy/pull/19642))
- Add ambiguous to uninhabited type identity for better messaging (Stanislav Terliakov, PR [19648](https://github.com/python/mypy/pull/19648))
- Improve behavior of uninhabited types (Stanislav Terliakov, PR [19648](https://github.com/python/mypy/pull/19648))
- Fix overload diagnostics when `*args` and `**kwargs` both match (Shantanu, PR [19614](https://github.com/python/mypy/pull/19614))
- Further fix overload diagnostics for `*args`/`**kwargs` (Shantanu, PR [19619](https://github.com/python/mypy/pull/19619))
- Show type variable name in "Cannot infer type argument" (Brian Schubert, PR [19290](https://github.com/python/mypy/pull/19290))
Expand Down
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