Mypy 1.9 Released

We’ve just uploaded mypy 1.9 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.

Breaking Changes

Because the version of typeshed we use in mypy 1.9 doesn't support 3.7, neither does mypy 1.9. (Jared Hance, PR 16883)

We are planning to enable local partial types (enabled via the --local-partial-types flag) later this year by default. This change was announced years ago, but now it's finally happening. This is a major backward-incompatible change, so we'll probably include it as part of the upcoming mypy 2.0 release. This makes daemon and non-daemon mypy runs have the same behavior by default.

Local partial types can also be enabled in the mypy config file:

    local_partial_types = True

We are looking at providing a tool to make it easier to migrate projects to use --local-partial-types, but it's not yet clear whether this is practical. The migration usually involves adding some explicit type annotations to module-level and class-level variables.

Basic Support for Type Parameter Defaults (PEP 696)

This release contains new experimental support for type parameter defaults (PEP 696). Please try it out! This feature was contributed by Marc Mueller.

Since this feature will be officially introduced in the next Python feature release (3.13), you will need to import TypeVar, ParamSpec or TypeVarTuple from typing_extensions to use defaults for now.

This example adapted from the PEP defines a default for BotT:

    from typing import Generic
    from typing_extensions import TypeVar
    
    class Bot: ...
    
    <p>BotT = TypeVar("BotT", bound=Bot, default=Bot)
    
    class Context(Generic[BotT]):
        bot: BotT
    
    class MyBot(Bot): ...
    
    # type is Bot (the default)
    reveal_type(Context().bot)
    # type is MyBot
    reveal_type(ContextMyBot.bot)

Type-checking Improvements

• Fix missing type store for overloads (Marc Mueller, PR 16803)
• Fix 'WriteToConn' object has no attribute 'flush' (Charlie Denton, PR 16801)
• Improve TypeAlias error messages (Marc Mueller, PR 16831)
• Support narrowing unions that include type[None] (Christoph Tyralla, PR 16315)
• Support TypedDict functional syntax as class base type (anniel-stripe, PR 16703)
• Accept multiline quoted annotations (Shantanu, PR 16765)
• Allow unary + in Literal (Jelle Zijlstra, PR 16729)
• Substitute type variables in return type of static methods (Kouroche Bouchiat, PR 16670)
• Consider TypeVarTuple to be invariant (Marc Mueller, PR 16759)
• Add alias support to field() in attrs plugin (Nikita Sobolev, PR 16610)
• Improve attrs hashability detection (Tin Tvrtković, PR 16556)

Performance Improvements

• Speed up finding function type variables (Jukka Lehtosalo, PR 16562)

Documentation Updates

• Document supported values for --enable-incomplete-feature in "mypy --help" (Froger David, PR 16661)
• Update new type system discussion links (thomaswhaley, PR 16841)
• Add missing class instantiation to cheat sheet (Aleksi Tarvainen, PR 16817)
• Document how evil --no-strict-optional is (Shantanu, PR 16731)
• Improve mypy daemon documentation note about local partial types (Makonnen Makonnen, PR 16782)
• Fix numbering error (Stefanie Molin, PR 16838)
• Various documentation improvements (Shantanu, PR 16836)

Stubtest Improvements

• Ignore private function/method parameters when they are missing from the stub (private parameter names start with a single underscore and have a default, PR 16507)
• Ignore a new protocol dunder (Alex Waygood, PR 16895)
• Private parameters can be omitted (Sebastian Rittau, PR 16507)
• Add support for setting enum members to "..." (Jelle Zijlstra, PR 16807)
• Adjust symbol table logic (Shantanu, PR 16823)
• Fix posisitional-only handling in overload resolution (Shantanu, PR 16750)

Stubgen Improvements

• Fix crash on star unpack of TypeVarTuple (Ali Hamdan, PR 16869)
• Use PEP 604 unions everywhere (Ali Hamdan, PR 16519)
• Do not ignore property deleter (Ali Hamdan, PR 16781)
• Support type stub generation for staticmethod (WeilerMarcel, PR 14934)

Acknowledgements

​Thanks to all mypy contributors who contributed to this release:

• Aleksi Tarvainen
• Alex Waygood
• Ali Hamdan
• anniel-stripe
• Charlie Denton
• Christoph Tyralla
• Dheeraj
• Fabian Keller
• Fabian Lewis
• Froger David
• Ihor
• Jared Hance
• Jelle Zijlstra
• Jukka Lehtosalo
• Kouroche Bouchiat
• Lukas Geiger
• Maarten Huijsmans
• Makonnen Makonnen
• Marc Mueller
• Nikita Sobolev
• Sebastian Rittau
• Shantanu
• Stefanie Molin
• Stephen Morton
• thomaswhaley
• Tin Tvrtković
• WeilerMarcel
• Wesley Collin Wright
• zipperer

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