Friday 22 January 2021

Mypy 0.800 Released

We’ve just uploaded mypy 0.800 to the Python Package Index (PyPI). Mypy is a static type checker for Python. This release includes new features, bug fixes and library stub (typeshed) updates. 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.

Python 3.9 Support

Mypy 0.800 officially supports the recently released Python 3.9. We now provide compiled binary wheels for Python 3.9, improving type checking speed significantly.

Typing Usability Improvements (PEP 585 and PEP 604)

The necessity to repeatedly import various types and special forms from typing has been a long-term nuisance for users of static type checking and Python.

Two new Python features improve this situation and are now supported by mypy:

  • PEP 585 lets you use list[int] instead of List[int] (no need to import List and other generic collections from typing).
  • PEP 604 lets you write X | Y instead of Union[X, Y], and X | None instead of Optional[X] (no need to import Union or Optional from typing).

Note: Using list[int] requires Python 3.9 and X | Y requires Python 3.10 (alpha) in order to work at runtime. To use them on older versions of Python, use from __future__ import annotations. This allows them to be used in type annotations, but the older variants (or string literal escaping) may be required in non-annotation contexts, such as in type aliases. See the docs for more details.

Here is an example that uses the new features:

    from __future__ import annotations
    def fields(s: str | None) -> list[str]:
        if not s:
            return []
            return s.split(',')

These were implemented by Allan Daemon in PR 9564 and by Marc Mueller in PR 9647.

Improvements to Finding Modules

This release adds several improvements to how mypy finds Python source files to type check.

You can now pass paths to files within namespace packages on the command line, and mypy can better infer their module names. As before, use --namespace-packages to enable namespace packages.

When you use --explicit-package-bases together with --namespace-packages, mypy assumes that only the current directory and directories explicitly specified in MYPYPATH (or mypy_path in the config file) are valid package roots. This can help with situations where the module name of a file is ambiguous. For example, it may not be clear whether src/pkg/ should be treated as src.pkg.mod or pkg.mod, and you can use this option to disambiguate between the two (more information in the docs).

The above improvements were implemented in PR 9742 by Shantanu.

Other related improvements (also implemented by Shantanu):

  • When you run mypy as mypy <directory>, look for source files recursively also inside directories without a (PR 9614)
  • Support namespace packages with -p (PR 9683)
  • Log encountered source files when running mypy with -v (PR 9672)
  • Document the new module finding behavior (PR 9923)

Other Notable Improvements and Bug Fixes

  • Only treat import X as X as a re-export in stubs (Shantanu, PR 9515)
  • Fix package imports with aliases in stubgen (Chad Dombrova, PR 9534)
  • Require first argument of namedtuple() to match the variable name (Momoko Hattori, PR 9577)
  • Add error code for name mismatches in named tuples and TypedDicts to make it easy to disable these error messages (Jukka Lehtosalo, PR 9811)
  • Document local_partial_types config option (Momoko Hattori, PR 9551)
  • Improve ambiguous kwargs checking (Erik Soma, PR 9573)
  • Disable unreachable warnings in boolean operators for type variables with value restrictions (Vincent Barbaresi, PR 9572)
  • Allow assignment to an empty tuple (Tobin Yehle, PR 5617)
  • Use absolute path when checking source duplication error (Yuki Igarashi, PR 9059)
  • Add get_function_signature_hook() to the plugin API (Nikita Sobolev, PR 9102)
  • Speed up type checking of dictionary, set, and list expressions (Hugues, PR 9477)
  • Allow non-types as arguments in Annotated (Patrick Arminio, PR 9625)
  • Add support for next generation attrs API (David Euresti, PR 9396)
  • Fix case folding of missing keys error message for TypedDicts (Marti Raudsepp, PR 9757)
  • Fix generic inheritance of __init__() methods in dataclasses and attrs classes (Nate McMaster, PR 9383, PR 9380)
  • Add more information to error message on too few arguments (Abhinay Pandey, PR 9796)
  • Document PEP 585, 563, 604, and related functionality (Shantanu, PR 9763)

Mypyc Improvements

We use mypyc to compile mypy into fast C extension modules. This release includes many mypyc improvements.

Xuanda Yang finished the migration to use a new, lower-level compiler intermediate representation in his Google Summer of Code project.

New supported Python features:

  • Support the walrus operator (:=) (Michael J. Sullivan, PR 9624)

Performance improvements:

  • Add primitives for list sort and list reverse (Jukka Lehtosalo, PR 9897)
  • Recognize six.moves.xrange again as an alias of range (Jukka Lehtosalo, PR 9896)
  • Speed up some integer primitives (Jukka Lehtosalo, PR 9801)
  • Speed up if x for int values (Jukka Lehtosalo, PR 9854)
  • Implement dict clear primitive (Vasileios Sakkas, PR 9724)
  • Implement list insert primitive (Vasileios Sakkas, PR 9741)
  • Implement float abs primitive (Xuanda Yang, PR 9695)
  • Implement str-to-float primitive (Xuanda Yang, PR 9685)
  • Specialize some calls to frozenset (Michael J. Sullivan, PR 9623)
  • Speed up multiple assignment from tuple (Xuanda Yang, PR 9575)
  • Speed up multiple assignment from sequence (Jukka Lehtosalo, PR 9800)
  • Optimize startswith and endswith (Tomer Chachamu, PR 9557)
  • Add primitives for bitwise ops (Jukka Lehtosalo, PR 9529)
  • Speed up in operations for list/tuple (Johan Dahlin, PR 9004)
  • Add primitives for list, str and tuple slicing (Jukka Lehtosalo, PR 9283)
  • Speed up tuple equality checks (Xuanda Yang, PR 9343)

Bug fixes:

  • Always add implicit None return type to __init__ method (Thomas Johnson, PR 9866)
  • Fix deallocation of deeply nested data structures (Michael J. Sullivan, PR 9839)
  • Fix using package imported inside a function (Jukka Lehtosalo, PR 9782)
  • Fix type of for loop index register in for over range (Jukka Lehtosalo, PR 9634)

Typeshed Updates

Many improvements were made to typeshed — too many to list. Browse the typeshed commit log here.


First of all, we’d like to thank our employer, Dropbox, for funding the mypy core team.

Thanks to all mypy contributors who contributed to this release:

  • Abdullah Selek
  • Abhinay Pandey
  • Adam
  • aghast
  • Akuli
  • Alexander
  • Allan Daemon
  • Aristotelis Mikropoulos
  • Ashley Whetter
  • Brian Mboya
  • Bryan Forbes
  • cdce8p
  • Chad Dombrova
  • David Euresti
  • Denis Laxalde
  • Eisuke Kawashima
  • Erik Soma
  • Ethan Pronovost
  • Florian Bruhin
  • Frank Dana
  • Greg Compestine
  • Guido van Rossum
  • Hugues
  • Jake Bailey
  • Jakub Stasiak
  • Jelle Zijlstra
  • Jeremy Metz
  • Johan Dahlin
  • Jon Shea
  • Jonathan Wong
  • Jürgen Gmach
  • Kamil Turek
  • Krzysztof Przybyła
  • Lawrence Chan
  • Marti Raudsepp
  • Matan Gover
  • Matt Gilson
  • Michael J. Sullivan
  • Momoko Hattori
  • Nate McMaster
  • Nikita Sobolev
  • Nils K
  • Nipunn Koorapati
  • Oleg Höfling
  • Patrick Arminio
  • Rajiv Singh
  • rhkleijn
  • Roland van Laar
  • Shantanu
  • Tobin Yehle
  • Tom Scogland
  • Tomer Chachamu
  • Tripp Horbinski
  • Ville Skyttä
  • Vincent Barbaresi
  • vsakkas
  • Wes Turner
  • willtryagain
  • Xiaodong DENG
  • Xuanda Yang
  • Yash Chhabria
  • Yuki Igarashi

Additional thanks to all contributors to typeshed:

  • Abraham Francis
  • Adam Dangoor
  • Adam Kliś
  • Adam Lichtl
  • Akuli
  • alexander-held
  • an onion
  • Anders Kaseorg
  • Andrew Mitchell
  • Árni Már Jónsson
  • Ash Berlin-Taylor
  • Ashwin Vishnu
  • Avery
  • cdce8p
  • Cebtenzzre
  • Changsheng
  • Christine
  • coiax
  • cptpcrd
  • crusaderky
  • Daniel O'Neel
  • David Caro
  • Dominic Davis-Foster
  • Eric Traut
  • Ethan Pronovost
  • Frank Maximilian
  • Gal Ben David
  • github-actions[bot]
  • Guido van Rossum
  • henribru
  • Hong Xu
  • Hugues
  • Hynek Schlawack
  • jack1142
  • Jake Bailey
  • Jason Fried
  • Jelle Zijlstra
  • Jia Chen
  • Jon Dufresne
  • Jonathan Schoonhoven
  • Jonathan Slenders
  • Joseph Haaga
  • Julien Danjou
  • Jun Jia
  • Jérome Perrin
  • karl ding
  • Katelyn Gigante
  • Kaushal Rohit
  • Kevin Wojniak
  • ky-gog
  • Kyle Fuller
  • kylec1
  • Lam Son Ho
  • Lourens Veen
  • Mahmoud Abduljawad
  • Mariam Maarouf
  • Marti Raudsepp
  • melassa
  • Mickaël Schoentgen
  • Mikhail Sveshnikov
  • Mikołaj Kuranowski
  • Moriyoshi Koizumi
  • Nate McMaster
  • Neel Somani
  • nicolas-harraudeau-sonarsource
  • Nikolaus Waxweiler
  • Nils K
  • Nipunn Koorapati
  • Oleg Höfling
  • Omar Sandoval
  • Paul
  • Pete Scopes
  • Peter Law
  • Philipp Hahn
  • Phillip Huang
  • proost
  • PythonCoderAS
  • Rajiv Bakulesh Shah
  • Ran Benita
  • Raphael Geronimi
  • Rebecca Chen
  • Sam Bull
  • Sebastian Rittau
  • Sergei Lebedev
  • Shantanu
  • Stefano Chiodino
  • Steve Dignam
  • Sténio Jacinto
  • Timur Kushukov
  • Tom Most
  • turettn
  • Unrud
  • Utsav
  • Vasily Zakharov
  • Vincent Barbaresi
  • Vincent Meurisse
  • 愚氓
  • Yuri Khan