v1.18.2

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• Fix crash on recursive alias (Ivan Levkivskyi, PR19845)
• Add additional guidance for stubtest errors when runtime isobject.__init__ (Stephen Morton, PR19733)
• Fix handling of None values in f-string expressions in mypyc (BobTheBuidler, PR19846)

v1.18.1

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We’ve just uploaded mypy 1.18.1 to the Python Package Index (PyPI).
Mypy is a static type checker for Python. This release includes new features, performance
improvements and bug fixes. You can install it as follows:

python3 -m pip install -U mypy

You can read the full documentation for this release onRead the Docs.

Mypy Performance Improvements

Mypy 1.18.1 includes numerous performance improvements, resulting in about 40% speedup
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 section below) also improve performance.

Type caching optimizations have a small risk of causing regressions. When
reporting issues with unexpected inferred types, please also check if
--disable-expression-cache will work around the issue, as it turns off some of
these optimizations.

• Improve self check performance by 1.8% (Jukka Lehtosalo, PR19768,19769,19770)
• Optimize fixed-format deserialization (Ivan Levkivskyi, PR19765)
• Use macros to optimize fixed-format deserialization (Ivan Levkivskyi, PR19757)
• Two additional micro‑optimizations (Ivan Levkivskyi, PR19627)
• Another set of micro‑optimizations (Ivan Levkivskyi, PR19633)
• Cache common types (Ivan Levkivskyi, PR19621)
• Skip more method bodies in third‑party libraries for speed (Ivan Levkivskyi, PR19586)
• Simplify the representation of callable types (Ivan Levkivskyi, PR19580)
• Add cache for types of some expressions (Ivan Levkivskyi, PR19505)
• Use cache for dictionary expressions (Ivan Levkivskyi, PR19536)
• Use cache for binary operations (Ivan Levkivskyi, PR19523)
• Cache types of type objects (Ivan Levkivskyi, PR19514)
• Avoid duplicate work when checking boolean operations (Ivan Levkivskyi, PR19515)
• Optimize generic inference passes (Ivan Levkivskyi, PR19501)
• Speed up the default plugin (Jukka Lehtosalo, PRs19385 and19462)
• Remove nested imports from the default plugin (Ivan Levkivskyi, PR19388)
• Micro‑optimize type expansion (Jukka Lehtosalo, PR19461)
• Micro‑optimize type indirection (Jukka Lehtosalo, PR19460)
• Micro‑optimize the plugin framework (Jukka Lehtosalo, PR19464)
• Avoid temporary set creation in subtype checking (Jukka Lehtosalo, PR19463)
• Subtype checking micro‑optimization (Jukka Lehtosalo, PR19384)
• Return early where possible in subtype check (Stanislav Terliakov, PR19400)
• Deduplicate some types before joining (Stanislav Terliakov, PR19409)
• Speed up type checking by caching argument inference context (Jukka Lehtosalo, PR19323)
• Optimize binding method self argument type and deprecation checks (Ivan Levkivskyi, PR19556)
• Keep trivial instance types/aliases during expansion (Ivan Levkivskyi, PR19543)

Fixed‑Format Cache (Experimental)

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 version of mypy. Use--fixed-format-cache
to enable the new format, orfixed_format_cache = True in a configuration file.

We plan to enable this by default in a future mypy release, and we'll eventually
deprecate and remove support for the original JSON-based format.

Unlike the JSON-based cache format, the new binary format is currently
not easy to parse and inspect by mypy users. We are planning to provide a tool to
convert fixed-format cache files to JSON, but details of the output JSON may be
different from the current JSON format. If you rely on being able to inspect
mypy cache files, we recommend creating a GitHub issue and explaining your use
case, so that we can more likely provide support for it. (Using
MypyFile.read(binary_data) to inspect cache data may be sufficient to support
some use cases.)

This feature was contributed by Ivan Levkivskyi (PR19668,19735,19750,19681,19752,19815).

Flexible Variable Definitions: Update

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 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
the older flag, we encourage users of--allow-redefinition to experiment with
--allow-redefinition-new and create a GitHub issue if the new functionality doesn't
support some important use cases.

This feature was contributed by Jukka Lehtosalo.

Inferred Type for Bare ClassVar

A ClassVar without an explicit type annotation now causes the type of the variable
to be inferred from the initializer:

fromtypingimportClassVarclassItem:### Type of 'next_id' is now 'int' (it was 'Any')next_id:ClassVar=1    ...

This feature was contributed by Ivan Levkivskyi (PR19573).

Disjoint Base Classes (@​disjoint_base, PEP 800)

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: