The Mypy Blog: Mypy 0.940 Released

We’ve just uploaded mypy 0.940 to the Python Package Index (PyPI). Mypy is a static type checker for Python. This release includes new features 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.

Match Statement

Mypy now has experimental support for type checking match statements introduced in Python 3.10.

Mypy can perform exhaustiveness checking to detect whether a match statement covers all possible values, if the target value has a union type or an enum type:

    def empty_value(x: int | str) -> str:
        match x:
            case str():
                # Type of "x" is "str" here
                return '""'
            case int():
                # Type of "x" is "int" here
                return "0"
        # No error about missing return statement (unreachable)

Note that exhaustiveness checking is still not generally supported across class hierarchies, since mypy doesn’t support sealed classes with a fixed set of subclasses. You can use an union type or assert False as a workaround:

    class Base(metaclass=ABCMeta): ...

    class A(Base): ...
    class B(Base): ...

    def process(x: A | B) -> bool:
        match x:
            case A():
                ...
            case B():
                ...
        # Mypy knows that this is unrachable.

    def process2(x: Base) -> bool:
        match x:
            case A():
                ...
            case B():
                ...
        # Use "assert False" to silence error about missing import,
        # since mypy can't prove that code is unreachable.
        assert False

You can also use an “assert never” function for more explicit exhaustiveness checks with match statements, as discussed in the documentation.

This feature was contributed by Adrian Freund (PR 12267, PR 10191), with additional updates by Jukka Lehtosalo.

Python 2 End-of-Life Schedule

After this release, Python 2 support is in feature freeze. Mypy won’t add new features or fixes specific to type checking Python 2 code, expect for fixes of significant regressions.

Mypy will drop Python 2 support in the second half of 2022.

Important note: Since typeshed is in the process of removing Python 2 specific stubs, you should pin all installed typeshed stub packages for third-party libraries to a version from Feb 2022 or earlier if you want to type check Python 2 code.

Miscellaneous New Features

Add support for conditionally defined overloads (Marc Mueller, PR 10712)
Give "as" variables in with statements separate scopes when it is safe to do so (Jukka Lehtosalo, PR 12254)
Add an optional error code ignore-without-code to require ignore comments to have error codes (Peter Law, PR 11633)
Add support for typing.Never and typing_extensions.Never as alternative spellings of NoReturn (Jelle Zijlstra, PR 12153)
Add support for typing.reveal_type (Jelle Zijlstra, PR 12117)
Support universal2 macOS wheels (97littleleaf11, PR 10651)
Add match_args support to attr.s() (Nikita Sobolev, PR 12111)

Enum Improvements

Check Enum definition for invalid base classes (Nikita Sobolev, PR 12026)
Understand the self-destructing nature of Enum._ignore_ (Kenny Stauffer, PR 12128)
Add StrEnum support for Python 3.11 (Nikita Sobolev, PR 12035)
Make enum values final (joey-laminar, PR 11962)
Improve final detection for Enum (Nikita Sobolev, PR 11984)
Fix Enum final properties and writable special members (Nikita Sobolev, PR 11945)
Enum now accepts String literals and final values as 2nd argument (Vincent Perez, PR 8664)
Fix false positive about member name reuse in enum (Max Rossmannek, PR 11972)
Fix enum inheritance regression (Nikita Sobolev, PR 12260)

Mypyc Fixes and Improvements

Use Py_TYPE and Py_IsNone (97littleleaf11, PR 12233)
Implement additional internal consistency checks (Jared Hance, PR 12191)
Raise AttributeError also for non-refcounted types (Jukka Lehtosalo, PR 11940)
Fix invalid unlikely() in certain rare branches (Jukka Lehtosalo, PR 11939)
Skip no-op super calls to object.__init__() (Jukka Lehtosalo, PR 11938)
Use latest pythoncapi_compat (97littleleaf11, PR 12188)
Add helpful message to assert (Joshua Cannon, PR 12119)

Documentation Updates

Add documentations about Enum types (Nikita Sobolev, PR 11805)
Update Enum documentation (Nikita Sobolev, PR 12238)
Improve documentation of allow_redefinition (KotlinIsland, PR 11951)
Fix intelligent indexing example (Chris Keefe, PR 11973)
Explain generic Protocol[T1, T2, ...] shorthand (Matt Bogosian, PR 12047)
Clarify that stub-only packages need to be installed (Gustav Gränsbo, PR 9958)
Small documentation improvements for conditional overloads (Marc Mueller, PR 12283)

Improved Error Messages

Improve the "Argument must be a mapping" error message (Purna Chandra Mansingh, PR 12222)
Coalesce Literals when printing unions (Marti Raudsepp, PR 12205)
Suggest typing.Callable when using callable as type (Tuomas Siipola, PR 12204)
Suggest typing.Any when using any as type (Tuomas Siipola, PR 12185)
Add note about wrong error code in type: ignore (Jukka Lehtosalo, PR 12067)
Add no-overload-impl error code (Brian Phillips, PR 11944)
Display ellipsis when formatting variadic tuple[T, ...] (Marti Raudsepp, PR 11857)
Deduplicate error codes for ignore-without-code (Marc Mueller, PR 12194)
Tweak ignore-without-code error message (Marc Mueller, PR 12216)
Mention common resolutions for build errors (Shantanu, PR 12154)

Stubtest Improvements

Ignore more dunder positional-only errors (Shantanu, PR 12294)
Fix wrong assumption about relative path (Stanislav Levin, PR 12282)
Catch more getattr errors (Shantanu, PR 12219)
Error if module level dunder is missing, housekeeping (Shantanu, PR 12217)
Ignore __main__ module (Shantanu, PR 12218)
Error if a dunder method is missing from a stub (Alex Waygood, PR 12203)
Error if a function is async at runtime but not in the stub (and vice versa) (Alex Waygood, PR 12212)
Do not error if a stub is async, but runtime is not (Alex Waygood, PR 12234)
Error if a class should be decorated with @final (Akuli, PR 12091)
Use VERSIONS for submodules (Shantanu, PR 12083)
Treat dicts as a subtype of typeddict (Shantanu, PR 12040)
Ignore more exceptions in stubtest (Jelle Zijlstra, PR 11946)

Other Notable Fixes and Improvements

Fix non-default keyword-only argument positioning in stubgen (Štěpán Horáček, PR 12303)
Remove orjson stubs from default list (Shantanu, PR 12264)
Use __truediv__ for Python 2 with __future__ import (Nikita Sobolev, PR 11787)
Fix Python 2 compatibility issue (Shantanu, PR 12244)
Use type variable bound to infer constraints (Jukka Lehtosalo, PR 12230)
Handle raise Exception(), None on Python 2.7 (Nikita Sobolev, PR 11786)
Fix inference of protocol against overloaded function (Jukka Lehtosalo, PR 12227)
Fix an issubclass failure for protocols with overloaded methods (Bas van Beek, PR 9904)
Fix crashes in class scoped imports (PR 12199, PR 12023) (Shantanu)
Fix use of TypeAlias from aliased imports (Shantanu, PR 12180)
Delete open plugin (Shantanu, PR 9275)
Read pyproject.toml with correct encoding on Windows (Dominic Davis-Foster, PR 12105)
Fix issue with implicit type aliases in import cycles (but only for stubs) (Alex Waygood, PR 11915)
Forbid extra ParamSpec arguments (Nikita Sobolev, PR 12024)
Fix crash involving explicit any flag and Required (Mehdi Drissi, PR 12039)
Fix join of Any against a union type (Jukka Lehtosalo, PR 12068)
Simplify unions when erasing last known values (Jukka Lehtosalo, PR 12064)
Fix crash with yield in comprehension (Alexandre Bouayad, PR 12048)
Fix handling of NoReturn in union return types (Jannic Warken, PR 11996)
Fix __init__ in dataclasses inheriting from Any (joey-laminar, PR 11966)
Narrow NamedTuple to bool correctly when __bool__ is defined (Nikita Sobolev, PR 11822)
Improve type of __attrs_attrs__ in attrs classes (Tin Tvrtković, PR 11794)
Install dependencies needed for reports via pip install mypy[reports] (James Braza, PR 11777)
Consider import * to be an explicit re-export (Shantanu, PR 11867)
Fix --no-implicit-reexport inconsistency (Shantanu, PR 11707)
Fix crash if "_" is in builtins (StefanM-TT, PR 11811)
Fixes crash on subclassing Annotated without args (Nikita Sobolev, PR 11814)

Typeshed Updates

Typeshed is now modular and distributed as separate PyPI packages for everything except the standard library stubs. Please see git log for full list of typeshed changes.

Acknowledgements

Thanks to all mypy contributors who contributed to this release:

97littleleaf11
Adrian Freund
Akuli
Alex Waygood
Alexandre Bouayad
Anthony Sottile
Bas van Beek
Brian Phillips
Chris Keefe
Dominic Davis-Foster
Gustav Gränsbo
James Braza
Jannic Warken
Jared Hance
Jelle Zijlstra
joey-laminar
Joshua Cannon
Kenny Stauffer
KotlinIsland
Marc Mueller
Mark Bell
Marti Raudsepp
Matt Bogosian
Max Rossmannek
Mehdi Drissi
Michael R. Crusoe
Nikita Sobolev
Oliver Newman
Peter Law
Purna Chandra Mansingh
Shantanu
Stanislav Levin
StefanM-TT
Tin Tvrtković
Tuomas Siipola
Vincent Perez
Štěpán Horáček

I’d also like to thank my employer, Dropbox, for funding the mypy core team.

Friday, 11 March 2022

Mypy 0.940 Released