Friday, 28 November 2025

Mypy 1.19 released

We’ve just uploaded mypy 1.19.0 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 on Read the Docs.

Python 3.9 Support Ending Soon

This is the last mypy feature release that supports Python 3.9, which reached end of life in October 2025.

Performance Improvements

  • Switch to a more dynamic SCC processing logic (Ivan Levkivskyi, PR 20053)
  • Speed up type aliases (Ivan Levkivskyi, PR 19810)

Fixed‑Format Cache Improvements

Mypy uses a cache by default to speed up incremental runs by reusing partial results from earlier runs. Mypy 1.18 added a new binary fixed-format cache representation as an experimental feature. The feature is no longer experimental, and we are planning to enable it by default in a future mypy release (possibly 1.20), since it's faster and uses less space than the original, JSON-based cache format. Use --fixed-format-cache to enable the fixed-format cache.

Mypy now has an extra dependency on the librt PyPI package, as it's needed for cache serialization and deserialization.

Mypy ships with a tool to convert fixed-format cache files to the old JSON format. Example of how to use this: 

$ python -m mypy.exportjson .mypy_cache/.../my_module.data.ff

This way existing use cases that parse JSON cache files can be supported when using the new format, though an extra conversion step is needed.

This release includes these improvements:

  • Force-discard cache if cache format changed (Ivan Levkivskyi, PR 20152)
  • Add tool to convert binary cache files to JSON (Jukka Lehtosalo, PR 20071)
  • Use more efficient serialization format for long integers in cache files (Jukka Lehtosalo, PR 20151)
  • More robust packing of floats in fixed-format cache (Ivan Levkivskyi, PR 20150)
  • Use self-descriptive cache with type tags (Ivan Levkivskyi, PR 20137)
  • Use fixed format for cache metas (Ivan Levkivskyi, PR 20088)
  • Make metas more compact; fix indirect suppression (Ivan Levkivskyi, PR 20075)
  • Use dedicated tags for most common cached instances (Ivan Levkivskyi, PR 19762)

PEP 747: Annotating Type Forms

Mypy now recognizes TypeForm[T] as a type and implements PEP 747. The feature is still experimental, and it's disabled by default. Use --enable-incomplete-feature=TypeForm to enable type forms. A type form object captures the type information provided by a runtime type expression. Example: 

from typing_extensions import TypeForm

def trycastT -> T | None: ...

def example(o: object) -> None:
    # 'int | str' below is an expression that represents a type.
    # Unlike type[T], TypeForm[T] can be used with all kinds of types,
    # including union types.
    x = trycast(int | str, o)
    if x is not None:
        # Type of 'x' is 'int | str' here
        ...

This feature was contributed by David Foster (PR 19596).

Fixes to Crashes

  • Do not push partial types to the binder (Stanislav Terliakov, PR 20202)
  • Fix crash on recursive tuple with Hashable (Ivan Levkivskyi, PR 20232)
  • Fix crash related to decorated functions (Stanislav Terliakov, PR 20203)
  • Do not abort constructing TypeAlias if only type parameters hold us back (Stanislav Terliakov, PR 20162)
  • Use the fallback for ModuleSpec early if it can never be resolved (Stanislav Terliakov, PR 20167)
  • Do not store deferred NamedTuple fields as redefinitions (Stanislav Terliakov, PR 20147)
  • Discard partial types remaining after inference failure (Stanislav Terliakov, PR 20126)
  • Fix an infinite recursion bug (Stanislav Terliakov, PR 20127)
  • Fix IsADirectoryError for namespace packages when using --linecoverage-report (wyattscarpenter, PR 20109)
  • Fix an internal error when creating cobertura output for namespace package (wyattscarpenter, PR 20112)
  • Allow type parameters reusing the name missing from current module (Stanislav Terliakov, PR 20081)
  • Prevent TypeGuardedType leak from narrowing declared type as part of type variable bound (Stanislav Terliakov, PR 20046)
  • Fix crash on invalid unpack in base class (Ivan Levkivskyi, PR 19962)
  • Traverse ParamSpec prefix where we should (Ivan Levkivskyi, PR 19800)
  • Fix daemon crash related to imports (Ivan Levkivskyi, PR 20271)

Mypyc: Support for __getattr__, __setattr__, and __delattr__

Mypyc now has partial support for __getattr__, __setattr__ and __delattr__ methods in native classes.

Note that native attributes are not stored using __dict__. Setting attributes directly while bypassing __setattr__ is possible by using super().__setattr__(...) or object.__setattr__(...), but not via __dict__.

Example: 

class Demo:
    _data: dict[str, str]

    def __init__(self) -> None:
        # Initialize data dict without calling our __setattr__
        super().__setattr__("_data", {})

    def __setattr__(self, name: str, value: str) -> None:
        print(f"Setting {name} = {value!r}")

        if name == "_data":
            raise AttributeError("'_data' cannot be set")

        self._data[name] = value

    def __getattr__(self, name: str) -> str:
        print(f"Getting {name}")

        try:
            return self._data[name]
        except KeyError:
            raise AttributeError(name)

d = Demo()
d.x = "hello"
d.y = "world"

print(d.x)
print(d.y)

Related PRs:

  • Generate __setattr__ wrapper (Piotr Sawicki, PR 19937)
  • Generate __getattr__ wrapper (Piotr Sawicki, PR 19909)
  • Support deleting attributes in __setattr__ wrapper (Piotr Sawicki, PR 19997)

Miscellaneous Mypyc Improvements

  • Fix __new__ in native classes with inheritance (Piotr Sawicki, PR 20302)
  • Fix crash on super in generator (Ivan Levkivskyi, PR 20291)
  • Fix calling base class async method using super() (Jukka Lehtosalo, PR 20254)
  • Fix async or generator methods in traits (Jukka Lehtosalo, PR 20246)
  • Optimize equality check with string literals (BobTheBuidler, PR 19883)
  • Fix inheritance of async defs (Jukka Lehtosalo, PR 20044)
  • Reject invalid mypyc_attr args (BobTheBuidler, PR 19963)
  • Optimize isinstance with tuple of primitive types (BobTheBuidler, PR 19949)
  • Optimize away first index check in for loops if length > 1 (BobTheBuidler, PR 19933)
  • Fix broken exception/cancellation handling in async def (Jukka Lehtosalo, PR 19951)
  • Transform object.__new__ inside __new__ (Piotr Sawicki, PR 19866)
  • Fix crash with NewType and other non-class types in incremental builds (Jukka Lehtosalo, PR 19837)
  • Optimize container creation from expressions with length known at compile time (BobTheBuidler, PR 19503)
  • Allow per-class free list to be used with inheritance (Jukka Lehtosalo, PR 19790)
  • Fix object finalization (Marc Mueller, PR 19749)
  • Allow defining a single-item free "list" for a native class (Jukka Lehtosalo, PR 19785)
  • Speed up unary "not" (Jukka Lehtosalo, PR 19774)

Stubtest Improvements

  • Check _value_ for ellipsis-valued stub enum members (Stanislav Terliakov, PR 19760)
  • Include function name in overload assertion messages (Joren Hammudoglu, PR 20063)
  • Fix special case in analyzing function signature (iap, PR 19822)
  • Improve allowlist docs with better example (sobolevn, PR 20007)

Documentation Updates

  • Update duck type compatibility: mention strict-bytes and mypy 2.0 (wyattscarpenter, PR 20121)
  • Document --enable-incomplete-feature TypeForm (wyattscarpenter, PR 20173)
  • Change the inline TypedDict example (wyattscarpenter, PR 20172)
  • Replace List with built‑in list (PEP 585) (Thiago J. Barbalho, PR 20000)
  • Improve junit documentation (wyattscarpenter, PR 19867)

Other Notable Fixes and Improvements

  • Fix annotated with function as type keyword list parameter (KarelKenens, PR 20094)
  • Fix errors for raise NotImplemented (Shantanu, PR 20168)
  • Don't let help formatter line-wrap URLs (Frank Dana, PR 19825)
  • Do not cache fast container types inside lambdas (Stanislav Terliakov, PR 20166)
  • Respect force-union-syntax flag in error hint (Marc Mueller, PR 20165)
  • Fix type checking of dict type aliases (Shantanu, PR 20170)
  • Use pretty callable formatting more often for callable expressions (Theodore Ando, PR 20128)
  • Use dummy concrete type instead of Any when checking protocol variance (bzoracler, PR 20110)
  • PEP 696: Fix swapping TypeVars with defaults (Randolf Scholz, PR 19449)
  • Fix narrowing of class pattern with union type (Randolf Scholz, PR 19517)
  • Do not emit unreachable warnings for lines that return NotImplemented (Christoph Tyralla, PR 20083)
  • Fix matching against typing.Callable and Protocol types (Randolf Scholz, PR 19471)
  • Make --pretty work better on multi-line issues (A5rocks, PR 20056)
  • More precise return types for TypedDict.get (Randolf Scholz, PR 19897)
  • Prevent false unreachable warnings for @final instances that occur when strict optional checking is disabled (Christoph Tyralla, PR 20045)
  • Check class references to catch non-existent classes in match cases (A5rocks, PR 20042)
  • Do not sort unused error codes in unused error codes warning (wyattscarpenter, PR 20036)
  • Fix [name-defined] false positive in class A[X, Y=X]: case (sobolevn, PR 20021)
  • Filter SyntaxWarnings during AST parsing (Marc Mueller, PR 20023)
  • Make untyped decorator its own error code (wyattscarpenter, PR 19911)
  • Support error codes from plugins in options (Sigve Sebastian Farstad, PR 19719)
  • Allow returning Literals in __new__ (James Hilton-Balfe, PR 15687)
  • Inverse interface freshness logic (Ivan Levkivskyi, PR 19809)
  • Do not report exhaustive-match after deferral (Stanislav Terliakov, PR 19804)
  • Make untyped_calls_exclude invalidate cache (Ivan Levkivskyi, PR 19801)
  • Add await to empty context hack (Stanislav Terliakov, PR 19777)
  • Consider non-empty enums assignable to Self (Stanislav Terliakov, PR 19779)

Typeshed updates

Please see git log for full list of standard library typeshed stub changes.

Acknowledgements

Thanks to all mypy contributors who contributed to this release:

  • A5rocks
  • BobTheBuidler
  • bzoracler
  • Chainfire
  • Christoph Tyralla
  • David Foster
  • Frank Dana
  • Guo Ci
  • iap
  • Ivan Levkivskyi
  • James Hilton-Balfe
  • jhance
  • Joren Hammudoglu
  • Jukka Lehtosalo
  • KarelKenens
  • Kevin Kannammalil
  • Marc Mueller
  • Michael Carlstrom
  • Michael J. Sullivan
  • Piotr Sawicki
  • Randolf Scholz
  • Shantanu
  • Sigve Sebastian Farstad
  • sobolevn
  • Stanislav Terliakov
  • Stephen Morton
  • Theodore Ando
  • Thiago J. Barbalho
  • wyattscarpenter

I’d also like to thank Dropbox for supporting mypy development.

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