Wednesday 22 December 2021

Mypy 0.930 Released

We’ve just uploaded mypy 0.930 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.

New Feature: Explicit Type Aliases (PEP 613)

You can now explicitly mark an assignment as a type alias, by using typing.TypeAlias:

    # On Python 3.9 and earlier, use "from typing_extensions ..."
    from typing import TypeAlias

    A: TypeAlias = int | str

For more context, see the docs or PEP 613.

This was contributed by Shantanu in PR 11305.

New Feature: NotRequired in TypedDicts

You can now define individual TypedDict items as non-required by using typing_extensions.NotRequired. Previously this could only be configured at TypedDict level (by using total=). Example:

    from typing import TypedDict
    from typing_extensions import NotRequired

    class D(TypedDict):
        x: str
        y: NotRequired[int]

    a: D = {"x": "value"}  # OK
    b: D = {"x": "value", y: 0}  # OK
    c: D = {}  # Error: "x" is required

You can also use typing_extensions.Required to mark an item as required in a non-total TypedDict.

For more information, see the draft PEP 655.

This was contributed by David Foster in PR 10370.

New Feature: ParamSpec (Experimental)

Mypy now partially support ParamSpec, which was introduced in Python 3.10. This makes it possible to define type variables that range over the parameter specification of a callable type. In particular, it allows giving precise types for some decorators that change the return type of the decorated function.

Here’s an example copied (and slightly modified) from PEP 612:

    from typing import Awaitable, Callable, ParamSpec, TypeVar

    P = ParamSpec("P")
    R = TypeVar("R")

    def add_logging(f: Callable[P, R]) -> Callable[P, Awaitable[R]]:
        async def inner(*args: P.args, **kwargs: P.kwargs) -> R:
            await log_to_database()
            return f(*args, **kwargs)
        return inner

    def takes_int_str(x: int, y: str) -> int:
        return x + 7

    await takes_int_str(1, "A")  # Accepted
    await takes_int_str("B", 2)  # Rejected

See Python documentation and PEP 612 for additional details. Note that typing.Concatenate and some other use cases are still not supported.

Support for semantic analysis of ParamSpec was contributed by Shantanu. Jukka Lehtosalo implemented type checking of ParamSpec types. Additional fixes were contributed by Nikita Sobolev.

Miscellaneous New Features

  • Define a GitHub Action that makes it easier to run mypy in GitHub CI workflows (Elvis Pranskevichus, PR 11320)
  • Allow mypy to run self check on PyPy 3.8 beta (Ethan Smith, PR 11350)
  • Check that enum values are unique (Nikita Sobolev, PR 11267)
  • Make enum classess with values implicitly final (Nikita Sobolev, PR 11247)
  • Make enum members implicitly final (Nikita Sobolev, PR 10852)
  • Allow NamedTuple to be used as a type that accepts all named tuples, as an experimental extension (Nikita Sobolev, PR 11162)
  • Allow booleans to be narrowed to literal types (Ethan Leba, PR 10389)
  • Add slots=True support for @dataclass (Nikita Sobolev, PR 11483)
  • Add slots=True support for @attr.s (Nikita Sobolev, PR 11489)
  • Support the typing_extensions.OrderedDict alias (Nikita Sobolev, PR 11533)

Usability and Documentation Improvements

  • Add better NamedTuple error messages (Nikita Sobolev, PR 11127)
  • Show all overloads in error message (Akuli, PR 9177)
  • Fix error message for dataclasses.field with positional argument (Hiroshi Ogawa, PR 11180)
  • Fix error message for f-string and str-bytes-safe (Nikita Sobolev, PR 11139)
  • Add documentation for type: ignore (Tushar Sadhwani, PR 11358)
  • Support --exclude more than once on command line (Nipunn Koorapati, PR 11329)
  • Use list[int] instead of List[int] in documentation (PR 11377, PR 11450) (Nick Crews)
  • Remove builtins. from error messages (97littleleaf11, PR 11522)
  • Document explicit type aliases (Nikita Sobolev, PR 11800)
  • Improve documentation of annotating generator functions (Alex Waygood, PR 11623)
  • Improve error message for nested TypedDict (97littleleaf11, PR 11658)
  • Exit gracefully on KeyboardInterrupt (Marc Mueller, PR 10725)


This release includes some optimizations that make mypy a bit faster (in addition to mypyc performance improvements discussed below):

  • Enable --fast-exit by default to speed up mypy (Jukka Lehtosalo, PR 11541)
  • Only read the stdlib versions dictionary once per run (Jukka Lehtosalo, PR 11542)
  • Optimize commonly used function (Jukka Lehtosalo, PR 11543)
  • Minor optimization/cleanup (Jukka Lehtosalo, PR 11544)
  • Reduce the number of executed local imports (Jukka Lehtosalo, PR 11545)

Mypyc Fixes and Improvements

  • Add missing type coercions (Jared Hance, PR 11176)
  • Support --allow-redefinition (Jared Hance, PR 11175)
  • Constant fold integer operations and string concatenation (Jukka Lehtosalo, PR 11194)
  • Use optimized implementation for builtins.sum (Sara Sinback and 97littleleaf11, PR 10268)
  • Move mypyc to console_scripts to fix running on Windows 10 (谭九鼎, PR 11494)
  • Make min(x, y) faster(Chetan Khanna and 97littleleaf11, PR 10265)
  • Make max(x, y) faster (97littleleaf11, PR 11530)
  • Speed up reference counting operations by inlining them in commonly executed blocks (Jukka Lehtosalo, PR 11540)
  • Add support for attrs classes (Chad Dombrova, PR 11328)
  • Port mypyc to Python 3.11 (Victor Stinner, PR 11652)
  • Reduce the amount of debug information included in compiled extension modules (Nehal J Wani, PR 11526)

Other Notable Fixes and Improvements

  • Fix semantic analysis of assignment expressions (Shantanu, PR 11153)
  • Fixes mypy crash on protocol with contravariant variable (Nikita Sobolev, PR 11135)
  • Warn about unused ignores when not all specified error codes are used (Hiroshi Ogawa, PR 11178)
  • Improve type narrowing for walrus operator in conditional statements (Shantanu, PR 11202)
  • Fix literal type compatibility checking special case (Nikita Sobolev, PR 11236)
  • Rename API method builtin_type to named_type (97littleleaf11, PR 11224)
  • Fix narrowing information not propagated in assignment and boolean expressions (Ran Benita, PR 11207)
  • Fix narrowing of a nested union of TypedDicts (Ran Benita, PR 11204)
  • Fix case mismatching modules during namespace package search (Nipunn Koorapati, PR 11261)
  • Disallow invalid identifiers from getting implicit bazel (Nipunn Koorapati, PR 11268)
  • Don’t type check lambdas nested in unchecked functions (Nikita Sobolev, PR 11213)
  • Update None.__bool__ to return Literal[False] (Nikita Sobolev, PR 11290)
  • Relax type checking of % formatting when the right operand is an iterable (97littleleaf11, PR 11319)
  • Fix crash with namespace packages when generating html reports (Nikita Sobolev, PR 11338)
  • Allow slice syntax in Annotated types (Zac Hatfield-Dodds, PR 11345)
  • Show warning if self / cls argument is missing (Nikita Sobolev, PR 11317)
  • Improve subtype checking of TypeGuard types (Nikita Sobolev, PR 11314)
  • Fix type narrowing for overlaping runtime types (Nikita Sobolev, PR 11273)
  • Do not use TypeGuard context for lambda (Nikita Sobolev, PR 11417)
  • Check reachability in module bodies (Nikita Sobolev, PR 11361)
  • Check that __new__ in a metaclass returns a subtype of type (Nikita Sobolev, PR 11420)
  • Improve support of contextlib.asynccontextmanager (Barnaby Shearer, PR 11352)
  • Discard deprecated builtin_type (97littleleaf11, PR 11343)
  • Don't look in user site packages when using a virtual env (Dimitri Merejkowsky, PR 11444)
  • Update stubgen to preserve string literals in annotations (Imad Eddine Rezgui, PR 11292)
  • Fix issue with exporting names in the presence of __getattr__ (Shantanu, PR 11411)
  • Reject duplicate bases when defining TypedDict (97littleleaf11, PR 11485)
  • Fix type inference for index expression with a bounded TypeVar (Ilya Labun, PR 11434)
  • Fix get_dynamic_class_hook in some scenarios (Jade Lin, PR 10904)
  • Check print >> properly (Nikita Sobolev, PR 11576)
  • Correctly handle cls in protocol classmethod (Ilia Novoselov, PR 11119)
  • Use current Python version, rather than hardcoding 3.6, in stubgen (Shantanu, PR 10907)
  • Don't use ModuleType.__getattr__ if we know module symbols (Jukka Lehtosalo, PR 11597)
  • Fix strict equality when using the latest typeshed (Jukka Lehtosalo, PR 11599)
  • Make sure ClassVar does not contain type variables (Nikita Sobolev, PR 11585)
  • Allow overriding attributes with methods (Nikita Sobolev, PR 11561)
  • Don’t generate an error for PEP 593 Annotated with a string literal second argument (Seth Yastrov, PR 10777)
  • Install types to the correct environment (Konstantin Weddige, PR 11457)
  • Support decorators in additional contexts (Nikita Sobolev, PR 11150)
  • Fix crash involving unreachable binary operations (Shantanu, PR 11680)
  • Fix None assignments with mypy daemon cache (Christian Bundy, PR 11574)
  • Handle OSError when accessing the mtime of a mypy cache file (Zac Hatfield-Dodds, PR 11718)
  • Special case some special Enum properties to be non-final (Nikita Sobolev, PR 11713)
  • Fix crash involving undefined cyclic import * (Shantanu, PR 11681)
  • Remove incorrect assumption about file system case sensitivity (Shantanu, PR 11708)
  • Fix compatibility of bool and Literal[True, False] (Nikita Sobolev, PR 11709)
  • Allow subclassing enums with annotations and no values (Nikita Sobolev, PR 11579)

Typeshed Updates

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


Thanks to all mypy contributors who contributed to this release:

  • 97littleleaf11
  • Abdul Rehaman
  • Akuli
  • Alex Waygood
  • Barnaby Shearer
  • Chad Dombrova
  • Chetan Khanna
  • Christian Bundy
  • Daniel Mendelsohn
  • David Foster
  • DetachHead
  • Dimitri Merejkowsky
  • Elisha Hollander
  • Elvis Pranskevichus
  • Ethan Leba
  • Ethan Smith
  • Hiroshi Ogawa
  • Ilia Novoselov
  • Ilya Labun
  • Imad Eddine Rezgui
  • Infinil
  • Jared Hance
  • Ken Jin
  • Konstantin Weddige
  • KotlinIsland
  • Louis Sautier
  • luzpaz
  • Marc Mueller
  • Nehal J Wani
  • Neil Girdhar
  • Nick Crews
  • Nikita Sobolev
  • Nipunn Koorapati
  • Noé Rubinstein
  • parthaxntn
  • Peter Law
  • pranavrajpal
  • Ran Benita
  • Sara Sinback
  • Sebastian Rittau
  • Seth Yastrov
  • Shantanu
  • Taneli Hukkinen
  • Tushar Sadhwani
  • Victor Stinner
  • William Woodruff
  • Zac Hatfield-Dodds
  • 林玮 (Jade Lin)
  • 谭九鼎

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