The Mypy Blog: July 2024

We’ve just uploaded mypy 1.11 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.

Support Python 3.12 Syntax for Generics (PEP 695)

Mypy now supports the new type parameter syntax introduced in Python 3.12 (PEP 695).

This feature is still experimental and must be enabled with the --enable-incomplete-feature=NewGenericSyntax flag, or with enable_incomplete_feature = NewGenericSyntax in the mypy configuration file.

We plan to enable this by default in the next mypy feature release.

This example demonstrates the new syntax:

    # Generic function
    def fT -> T: ...
    
    reveal_type(f(1))  # Revealed type is 'int'
    
    # Generic class
    class C[T]:
        def __init__(self, x: T) -> None:
           self.x = x
    
    c = C('a')
    reveal_type(c.x)  # Revealed type is 'str'
    
    # Type alias
    type A[T] = C[list[T]]

This feature was contributed by Jukka Lehtosalo.

Support for `functools.partial`

Mypy now type checks uses of functools.partial. Previously mypy would accept arbitrary arguments.

This example will now produce an error:

    from functools import partial
    
    def f(a: int, b: str) -> None: ...
    
    g = partial(f, 1)
    
    # Argument has incompatible type "int"; expected "str"
    g(11)

This feature was contributed by Shantanu (PR 16939).

Stricter Checks for Untyped Overrides

Past mypy versions didn't check if untyped methods were compatible with overridden methods. This would result in false negatives. Now mypy performs these checks when using --check-untyped-defs.

For example, this now generates an error if using --check-untyped-defs:

    class Base:
        def f(self, x: int = 0) -> None: ...
    
    class Derived(Base):
        # Signature incompatible with "Base"
        def f(self): ...

This feature was contributed by Steven Troxler (PR 17276).

Type Inference Improvements

The new polymorphic inference algorithm introduced in mypy 1.5 is now used in more situations. This improves type inference involving generic higher-order functions, in particular.

This feature was contributed by Ivan Levkivskyi (PR 17348).

Mypy now uses unions of tuple item types in certain contexts to enable more precise inferred types. Example:

    for x in (1, 'x'):
        # Previously inferred as 'object'
        reveal_type(x)  # Revealed type is 'int | str'

This was also contributed by Ivan Levkivskyi (PR 17408).

Improvements to Detection of Overlapping Overloads

The details of how mypy checks if two @overload signatures are unsafely overlapping were overhauled. This both fixes some false positives, and allows mypy to detect additional unsafe signatures.

This feature was contributed by Ivan Levkivskyi (PR 17392).

Better Support for Type Hints in Expressions

Mypy now allows more expressions that evaluate to valid type annotations in all expression contexts. The inferred types of these expressions are also sometimes more precise. Previously they were often object.

This example uses a union type that includes a callable type as an expression, and it no longer generates an error:

    from typing import Callable
    
    print(Callable[[], int] | None)  # No error

This feature was contributed by Jukka Lehtosalo (PR 17404).

Mypyc Improvements

Mypyc now supports the new syntax for generics introduced in Python 3.12 (see above). Another notable improvement is signficantly faster basic operations on int values.

Support Python 3.12 syntax for generic functions and classes (Jukka Lehtosalo, PR 17357)
Support Python 3.12 type alias syntax (Jukka Lehtosalo, PR 17384)
Fix ParamSpec (Shantanu, PR 17309)
Inline fast paths of integer unboxing operations (Jukka Lehtosalo, PR 17266)
Inline tagged integer arithmetic and bitwise operations (Jukka Lehtosalo, PR 17265)
Allow specifying primitives as pure (Jukka Lehtosalo, PR 17263)

Changes to Stubtest

Ignore _ios_support (Alex Waygood, PR 17270)
Improve support for Python 3.13 (Shantanu, PR 17261)

Changes to Stubgen

Gracefully handle invalid Optional and recognize aliases to PEP 604 unions (Ali Hamdan, PR 17386)
Fix for Python 3.13 (Jelle Zijlstra, PR 17290)
Preserve enum value initialisers (Shantanu, PR 17125)

Miscellaneous New Features

Add error format support and JSON output option via --output json (Tushar Sadhwani, PR 11396)
Support enum.member in Python 3.11+ (Nikita Sobolev, PR 17382)
Support enum.nonmember in Python 3.11+ (Nikita Sobolev, PR 17376)
Support namedtuple.__replace__ in Python 3.13 (Shantanu, PR 17259)
Support rename=True in collections.namedtuple (Jelle Zijlstra, PR 17247)
Add support for __spec__ (Shantanu, PR 14739)

Changes to Error Reporting

Mention --enable-incomplete-feature=NewGenericSyntax in messages (Shantanu, PR 17462)
Do not report plugin-generated methods with explicit-override (sobolevn, PR 17433)
Use and display namespaces for function type variables (Ivan Levkivskyi, PR 17311)
Fix false positive for Final local scope variable in Protocol (GiorgosPapoutsakis, PR 17308)
Use Never in more messages, use ambiguous in join (Shantanu, PR 17304)
Log full path to config file in verbose output (dexterkennedy, PR 17180)
Added [prop-decorator] code for unsupported property decorators (Christopher Barber, PR 14461">16571)
Suppress second error message with := and [truthy-bool] (Nikita Sobolev, PR 15941)
Generate error for assignment of functional Enum to variable of different name (Shantanu, PR 16805)
Fix error reporting on cached run after uninstallation of third party library (Shantanu, PR 17420)

Fixes for Crashes

Fix daemon crash on invalid type in TypedDict (Ivan Levkivskyi, PR 17495)
Fix crash and bugs related to partial() (Ivan Levkivskyi, PR 17423)
Fix crash when overriding with unpacked TypedDict (Ivan Levkivskyi, PR 17359)
Fix crash on TypedDict unpacking for ParamSpec (Ivan Levkivskyi, PR 17358)
Fix crash involving recursive union of tuples (Ivan Levkivskyi, PR 17353)
Fix crash on invalid callable property override (Ivan Levkivskyi, PR 17352)
Fix crash on unpacking self in NamedTuple (Ivan Levkivskyi, PR 17351)
Fix crash on recursive alias with an optional type (Ivan Levkivskyi, PR 17350)
Fix crash on type comment inside generic definitions (Bénédikt Tran, PR 16849)

Changes to Documentation

Use inline config in documentation for optional error codes (Shantanu, PR 17374)
Use lower-case generics in documentation (Seo Sanghyeon, PR 17176)
Add documentation for show-error-code-links (GiorgosPapoutsakis, PR 17144)
Update CONTRIBUTING.md to include commands for Windows (GiorgosPapoutsakis, PR 17142)

Other Notable Improvements and Fixes

Fix ParamSpec inference against TypeVarTuple (Ivan Levkivskyi, PR 17431)
Fix explicit type for partial (Ivan Levkivskyi, PR 17424)
Always allow lambda calls (Ivan Levkivskyi, PR 17430)
Fix isinstance checks with PEP 604 unions containing None (Shantanu, PR 17415)
Fix self-referential upper bound in new-style type variables (Ivan Levkivskyi, PR 17407)
Consider overlap between instances and callables (Ivan Levkivskyi, PR 17389)
Allow new-style self-types in classmethods (Ivan Levkivskyi, PR 17381)
Fix isinstance with type aliases to PEP 604 unions (Shantanu, PR 17371)
Properly handle unpacks in overlap checks (Ivan Levkivskyi, PR 17356)
Fix type application for classes with generic constructors (Ivan Levkivskyi, PR 17354)
Update typing_extensions to >=4.6.0 to fix Python 3.12 error (Ben Brown, PR 17312)
Avoid "does not return" error in lambda (Shantanu, PR 17294)
Fix bug with descriptors in non-strict-optional mode (Max Murin, PR 17293)
Don’t leak unreachability from lambda body to surrounding scope (Anders Kaseorg, PR 17287)
Fix issues with non-ASCII characters on Windows (Alexander Leopold Shon, PR 17275)
Fix for type narrowing of negative integer literals (gilesgc, PR 17256)
Fix confusion between .py and .pyi files in mypy daemon (Valentin Stanciu, PR 17245)
Fix type of tuple[X, Y] expression (urnest, PR 17235)
Don't forget that a TypedDict was wrapped in Unpack after a name-defined error occurred (Christoph Tyralla, PR 17226)
Mark annotated argument as having an explicit, not inferred type (bzoracler, PR 17217)
Don't consider Enum private attributes as enum members (Ali Hamdan, PR 17182)
Fix Literal strings containing pipe characters (Jelle Zijlstra, PR 17148)

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:

Alex Waygood
Alexander Leopold Shon
Ali Hamdan
Anders Kaseorg
Ben Brown
Bénédikt Tran
bzoracler
Christoph Tyralla
Christopher Barber
dexterkennedy
gilesgc
GiorgosPapoutsakis
Ivan Levkivskyi
Jelle Zijlstra
Jukka Lehtosalo
Marc Mueller
Matthieu Devlin
Michael R. Crusoe
Nikita Sobolev
Seo Sanghyeon
Shantanu
sobolevn
Steven Troxler
Tadeu Manoel
Tamir Duberstein
Tushar Sadhwani
urnest
Valentin Stanciu

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

The Mypy Blog

Friday, 19 July 2024

Mypy 1.11 Released

Support Python 3.12 Syntax for Generics (PEP 695)

Support for `functools.partial`

Stricter Checks for Untyped Overrides

Type Inference Improvements

Improvements to Detection of Overlapping Overloads

Better Support for Type Hints in Expressions

Mypyc Improvements

Changes to Stubtest

Changes to Stubgen

Miscellaneous New Features

Changes to Error Reporting

Fixes for Crashes

Changes to Documentation

Other Notable Improvements and Fixes

Typeshed Updates

Acknowledgements

Contributors

Friday, 19 July 2024

Mypy 1.11 Released

Support Python 3.12 Syntax for Generics (PEP 695)

Support for functools.partial

Stricter Checks for Untyped Overrides

Type Inference Improvements

Improvements to Detection of Overlapping Overloads

Better Support for Type Hints in Expressions

Mypyc Improvements

Changes to Stubtest

Changes to Stubgen

Miscellaneous New Features

Changes to Error Reporting

Fixes for Crashes

Changes to Documentation

Other Notable Improvements and Fixes

Typeshed Updates

Acknowledgements

Support for `functools.partial`