The Mypy Blog: November 2023

We’ve just uploaded mypy 1.7 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.

Using TypedDict for `**kwargs` Typing

Mypy now has support for using Unpack[...] with a TypedDict type to annotate **kwargs arguments enabled by default. Example:

    # Or 'from typing_extensions import ...'
    from typing import TypedDict, Unpack
    
    class Person(TypedDict):
        name: str
        age: int
    
    def foo(**kwargs: Unpack[Person]) -> None:
        ...
    
    foo(name="x", age=1)  # Ok
    foo(name=1)  # Error

The definition of foo above is equivalent to the one below, with keyword-only arguments name and age:

    def foo(*, name: str, age: int) -> None:
        ...

Refer to PEP 692 for more information. Note that unlike in the current version of the PEP, mypy always treats signatures with Unpack[SomeTypedDict] as equivalent to their expanded forms with explicit keyword arguments, and there aren't special type checking rules for TypedDict arguments.

This was contributed by Ivan Levkivskyi back in 2022 (PR 13471).

TypeVarTuple Support Enabled (Experimental)

Mypy now has support for variadic generics (TypeVarTuple) enabled by default, as an experimental feature. Refer to PEP 646 for the details.

TypeVarTuple was implemented by Jared Hance and Ivan Levkivskyi over several mypy releases, with help from Jukka Lehtosalo.

Changes included in this release:

Fix handling of tuple type context with unpacks (Ivan Levkivskyi, PR 16444)
Handle TypeVarTuples when checking overload constraints (robjhornby, PR 16428)
Enable Unpack/TypeVarTuple support (Ivan Levkivskyi, PR 16354)
Fix crash on unpack call special-casing (Ivan Levkivskyi, PR 16381)
Some final touches for variadic types support (Ivan Levkivskyi, PR 16334)
Support PEP-646 and PEP-692 in the same callable (Ivan Levkivskyi, PR 16294)
Support new * syntax for variadic types (Ivan Levkivskyi, PR 16242)
Correctly handle variadic instances with empty arguments (Ivan Levkivskyi, PR 16238)
Correctly handle runtime type applications of variadic types (Ivan Levkivskyi, PR 16240)
Support variadic tuple packing/unpacking (Ivan Levkivskyi, PR 16205)
Better support for variadic calls and indexing (Ivan Levkivskyi, PR 16131)
Subtyping and inference of user-defined variadic types (Ivan Levkivskyi, PR 16076)
Complete type analysis of variadic types (Ivan Levkivskyi, PR 15991)

New Way of Installing Mypyc Dependencies

If you want to install package dependencies needed by mypyc (not just mypy), you should now install mypy[mypyc] instead of just mypy:

    python3 -m pip install -U 'mypy[mypyc]'

Mypy has many more users than mypyc, so always installing mypyc dependencies would often bring unnecessary dependencies.

This change was contributed by Shantanu (PR 16229).

New Rules for Re-exports

Mypy no longer considers an import such as import a.b as b as an explicit re-export. The old behavior was arguably inconsistent and surprising. This may impact some stub packages, such as older versions of types-six. You can change the import to from a import b as b, if treating the import as a re-export was intentional.

This change was contributed by Anders Kaseorg (PR 14086).

Improved Type Inference

The new type inference algorithm that was recently introduced to mypy (but was not enabled by default) is now enabled by default. It improves type inference of calls to generic callables where an argument is also a generic callable, in particular. You can use --old-type-inference to disable the new behavior.

The new algorithm can (rarely) produce different error messages, different error codes, or errors reported on different lines. This is more likely in cases where generic types were used incorrectly.

The new type inference algorithm was contributed by Ivan Levkivskyi. PR 16345 enabled it by default.

Narrowing Tuple Types Using len()

Mypy now can narrow tuple types using len() checks. Example:

    def f(t: tuple[int, int] | tuple[int, int, int]) -> None:
        if len(t) == 2:
            a, b = t   # Ok
        ...

This feature was contributed by Ivan Levkivskyi (PR 16237).

More Precise Tuple Lengths (Experimental)

Mypy supports experimental, more precise checking of tuple type lengths through --enable-incomplete-feature=PreciseTupleTypes. Refer to the documentation for more information.

More generally, we are planning to use --enable-incomplete-feature to introduce experimental features that would benefit from community feedback.

This feature was contributed by Ivan Levkivskyi (PR 16237).

Mypy Changelog

We now maintain a changelog in the mypy Git repository. It mirrors the contents of mypy release blog posts. We will continue to also publish release blog posts. In the future, release blog posts will be created based on the changelog near a release date.

This was contributed by Shantanu (PR 16280).

Mypy Daemon Improvements

Fix daemon crash caused by deleted submodule (Jukka Lehtosalo, PR 16370)
Fix file reloading in dmypy with --export-types (Ivan Levkivskyi, PR 16359)
Fix dmypy inspect on Windows (Ivan Levkivskyi, PR 16355)
Fix dmypy inspect for namespace packages (Ivan Levkivskyi, PR 16357)
Fix return type change to optional in generic function (Jukka Lehtosalo, PR 16342)
Fix daemon false positives related to module-level __getattr__ (Jukka Lehtosalo, PR 16292)
Fix daemon crash related to ABCs (Jukka Lehtosalo, PR 16275)
Stream dmypy output instead of dumping everything at the end (Valentin Stanciu, PR 16252)
Make sure all dmypy errors are shown (Valentin Stanciu, PR 16250)

Mypyc Improvements

Generate error on duplicate function definitions (Jukka Lehtosalo, PR 16309)
Don't crash on unreachable statements (Jukka Lehtosalo, PR 16311)
Avoid cyclic reference in nested functions (Jukka Lehtosalo, PR 16268)
Fix direct __dict__ access on inner functions in new Python (Shantanu, PR 16084)
Make tuple packing and unpacking more efficient (Jukka Lehtosalo, PR 16022)

Improvements to Error Reporting

Update starred expression error message to match CPython (Cibin Mathew, PR 16304)
Fix error code of "Maybe you forgot to use await" note (Jelle Zijlstra, PR 16203)
Use error code [unsafe-overload] for unsafe overloads, instead of [misc] (Randolf Scholz, PR 16061)
Reword the error message related to void functions (Albert Tugushev, PR 15876)
Represent bottom type as Never in messages (Shantanu, PR 15996)
Add hint for AsyncIterator incompatible return type (Ilya Priven, PR 15883)
Don't suggest stubs packages where the runtime package now ships with types (Alex Waygood, PR 16226)

Performance Improvements

Speed up type argument checking (Jukka Lehtosalo, PR 16353)
Add fast path for checking self types (Jukka Lehtosalo, PR 16352)
Cache information about whether file is typeshed file (Jukka Lehtosalo, PR 16351)
Skip expensive repr() in logging call when not needed (Jukka Lehtosalo, PR 16350)

Attrs and Dataclass Improvements

dataclass.replace: Allow transformed classes (Ilya Priven, PR 15915)
dataclass.replace: Fall through to typeshed signature (Ilya Priven, PR 15962)
Document dataclass_transform behavior (Ilya Priven, PR 16017)
attrs: Remove fields type check (Ilya Priven, PR 15983)
attrs, dataclasses: Don't enforce slots when base class doesn't (Ilya Priven, PR 15976)
Fix crash on dataclass field / property collision (Nikita Sobolev, PR 16147)

Stubgen Improvements

Write stubs with utf-8 encoding (Jørgen Lind, PR 16329)
Fix missing property setter in semantic analysis mode (Ali Hamdan, PR 16303)
Unify C extension and pure python stub generators with object oriented design (Chad Dombrova, PR 15770)
Multiple fixes to the generated imports (Ali Hamdan, PR 15624)
Generate valid dataclass stubs (Ali Hamdan, PR 15625)

Fixes to Crashes

Fix incremental mode crash on TypedDict in method (Ivan Levkivskyi, PR 16364)
Fix crash on star unpack in TypedDict (Ivan Levkivskyi, PR 16116)
Fix crash on malformed TypedDict in incremental mode (Ivan Levkivskyi, PR 16115)
Fix crash with report generation on namespace packages (Shantanu, PR 16019)
Fix crash when parsing error code config with typo (Shantanu, PR 16005)
Fix __post_init__() internal error (Ilya Priven, PR 16080)

Documentation Updates

Make it easier to copy commands from README (Hamir Mahal, PR 16133)
Document and rename [overload-overlap] error code (Shantanu, PR 16074)
Document --force-uppercase-builtins and --force-union-syntax (Nikita Sobolev, PR 16049)
Document force_union_syntax and force_uppercase_builtins (Nikita Sobolev, PR 16048)
Document we're not tracking relationships between symbols (Ilya Priven, PR 16018)

Other Notable Changes and Fixes

Propagate narrowed types to lambda expressions (Ivan Levkivskyi, PR 16407)
Avoid importing from setuptools._distutils (Shantanu, PR 16348)
Delete recursive aliases flags (Ivan Levkivskyi, PR 16346)
Properly use proper subtyping for callables (Ivan Levkivskyi, PR 16343)
Use upper bound as inference fallback more consistently (Ivan Levkivskyi, PR 16344)
Add [unimported-reveal] error code (Nikita Sobolev, PR 16271)
Add |= and | operators support for TypedDict (Nikita Sobolev, PR 16249)
Clarify variance convention for Parameters (Ivan Levkivskyi, PR 16302)
Correctly recognize typing_extensions.NewType (Ganden Schaffner, PR 16298)
Fix partially defined in the case of missing type maps (Shantanu, PR 15995)
Use SPDX license identifier (Nikita Sobolev, PR 16230)
Make __qualname__ and __module__ available in class bodies (Anthony Sottile, PR 16215)
stubtest: Hint when args in stub need to be keyword-only (Alex Waygood, PR 16210)
Tuple slice should not propagate fallback (Thomas Grainger, PR 16154)
Fix cases of type object handling for overloads (Shantanu, PR 16168)
Fix walrus interaction with empty collections (Ivan Levkivskyi, PR 16197)
Use type variable bound when it appears as actual during inference (Ivan Levkivskyi, PR 16178)
Use upper bounds as fallback solutions for inference (Ivan Levkivskyi, PR 16184)
Special-case type inference of empty collections (Ivan Levkivskyi, PR 16122)
Allow TypedDict unpacking in Callable types (Ivan Levkivskyi, PR 16083)
Fix inference for overloaded __call__ with generic self (Shantanu, PR 16053)
Call dynamic class hook on generic classes (Petter Friberg, PR 16052)
Preserve implicitly exported types via attribute access (Shantanu, PR 16129)
Fix a stubtest bug (Alex Waygood)
Fix tuple[Any, ...] subtyping (Shantanu, PR 16108)
Lenient handling of trivial Callable suffixes (Ivan Levkivskyi, PR 15913)
Add add_overloaded_method_to_class helper for plugins (Nikita Sobolev, PR 16038)
Bundle misc/proper_plugin.py as a part of mypy (Nikita Sobolev, PR 16036)
Fix case Any() in match statement (DS/Charlie, PR 14479)
Make iterable logic more consistent (Shantanu, PR 16006)
Fix inference for properties with __call__ (Shantanu, PR 15926)

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:

Albert Tugushev
Alex Waygood
Ali Hamdan
Anders Kaseorg
Anthony Sottile
Chad Dombrova
Cibin Mathew
dinaldoap
DS/Charlie
Eli Schwartz
Ganden Schaffner
Hamir Mahal
Ihor
Ikko Eltociear Ashimine
Ilya Priven
Ivan Levkivskyi
Jelle Zijlstra
Jukka Lehtosalo
Jørgen Lind
KotlinIsland
Matt Bogosian
Nikita Sobolev
Petter Friberg
Randolf Scholz
Shantanu
Thomas Grainger
Valentin Stanciu

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

The Mypy Blog

Friday 10 November 2023

Mypy 1.7 Released

Using TypedDict for `**kwargs` Typing

TypeVarTuple Support Enabled (Experimental)

New Way of Installing Mypyc Dependencies

New Rules for Re-exports

Improved Type Inference

Narrowing Tuple Types Using len()

More Precise Tuple Lengths (Experimental)

Mypy Changelog

Mypy Daemon Improvements

Mypyc Improvements

Improvements to Error Reporting

Performance Improvements

Attrs and Dataclass Improvements

Stubgen Improvements

Fixes to Crashes

Documentation Updates

Other Notable Changes and Fixes

Typeshed Updates

Acknowledgements

Contributors

Friday 10 November 2023

Mypy 1.7 Released

Using TypedDict for **kwargs Typing

TypeVarTuple Support Enabled (Experimental)

New Way of Installing Mypyc Dependencies

New Rules for Re-exports

Improved Type Inference

Narrowing Tuple Types Using len()

More Precise Tuple Lengths (Experimental)

Mypy Changelog

Mypy Daemon Improvements

Mypyc Improvements

Improvements to Error Reporting

Performance Improvements

Attrs and Dataclass Improvements

Stubgen Improvements

Fixes to Crashes

Documentation Updates

Other Notable Changes and Fixes

Typeshed Updates

Acknowledgements

Using TypedDict for `**kwargs` Typing